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Nutrient Requirements of Horses: Sixth Revised Edition (2007)

Chapter: 8 Feeds and Feed Processing

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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"8 Feeds and Feed Processing." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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8 Feeds and Feed Processing FORAGES to glucose by endogenous enzymes in the small intestine and, together with the free sugars, be absorbed across the Forages for horses are largely represented by the aerial small intestine of the horse and metabolized to yield adeno- portions of pasture grasses, legumes, and forbs. Forage sine triphosphate (ATP). However, the amount of starch di- grasses are divided into two main categories: cool-season gested in the foregut depends upon a number of factors, in- species that are adapted to temperate conditions and warm- cluding the quantity and botanical origin of the starch, season grasses that are adapted to subtropical or tropical en- quantity of amylolytic enzymes present, other feeds fed, and vironments. Forages may either be grazed directly or con- interhorse variation. The cell wall carbohydrates and fruc- served for use when fresh forage is scarce or when pasturing tans cannot be digested by mammalian enzymes in the fore- is not possible. Forages represent a significant portion of the gut (Nilsson, 1988; Åman and Graham, 1990), but are con- diet for all classes of post-weaned horses and indeed may verted via fermentation by the gut microflora to volatile constitute the entire diet for equids in the wild and large fatty acids (VFAs), which are then metabolized to yield ATP. numbers of domesticated horses. The efficiency of energy yield via VFAs is lower than that from glucose absorbed in the small intestine. Furthermore, Chemical Composition the degree to which SC and NSC are degraded depends upon factors such as their chemical composition, physical confor- Forages consist of the leaf, sheath, and stem of the plant mation, and association with poorly degraded noncarbohy- and, depending on the stage of growth, may also include drate compounds. As a consequence, the energy value of flowers and seed-heads. Each of these different plant parts forage is governed to a large degree by the types and relative differs in its chemical composition, and their relative pro- proportions of digestible and fermentable carbohydrate that portions may change substantially during a growing season. it contains. Thus, young plants have a high proportion of leaf; are high in protein, water, and minerals; and are low in fiber and Dietary Fiber–Cell Wall Carbohydrates lignin. As the season progresses, leaf growth slows, stems elongate, reproductive structures develop, photosynthate ac- Plant cell wall carbohydrates are traditionally divided cumulates, and the cell content:cell wall ratio decreases, all into three fractions: cellulose (β-linked polymers of glu- of which serve to change the overall chemical composition cose), hemicelluloses (polymers of arabinose, xylose, glu- and hence nutritive value of the plant. cose, fucose, mannose, and galactose), and pectins (contain- Forages are typically characterized by their high dietary ing β-D 1-4-linked galacturonic acid residues, arabinose, fiber content, which is largely composed of the structural and galactose) (Butler and Bailey, 1973; Åman and Graham, carbohydrates (SC) of the plant cell wall and varying 1990). The sum of these carbohydrates represents the non- amounts of lignin. Forages also contain nonstructural carbo- starch polysaccharide (NSP) fraction of the plant cell wall hydrates (NSC) originating from the cell contents and in- (see Figure 10-1, Chapter 10). The pectins and polymers of clude the simple sugars, glucose fructose, sucrose, and stor- arabinose, galactose, and mannose are frequently readily de- age carbohydrates such as starch or fructan (see Figure 10-1, graded by the hindgut microflora of nonruminants, whereas Chapter 10). Together, the SC and NSC constitute the main those of cellulose and particularly xylan (composed of xy- energy-yielding fractions of forage. Starch can be digested lose residues) are more resistant to breakdown (Graham et 141

142 NUTRIENT REQUIREMENTS OF HORSES al., 1986; Moore-Colyer and Longland, 2000). As the plant Cell Contents matures, these fractions, particularly cellulose, may become The cell contents comprise the NSC (nonstructural car- lignified to varying degrees. Lignin is not a carbohydrate, bohydrate fraction, which is the the sum of simple sugars, but is a generic term applied to a group of heterogeneous fructan, and starch), most of the plant protein, minerals, and compounds derived from the phenylpropanoid pathway. lipids. In immature vegetative tissues, the cell contents may Lignin can become intimately associated with cell wall car- represent some 66 percent of the total DM; however, with bohydrates, rendering them recalcitrant to degradation and increasing forage maturity, the proportion of cell contents thereby reducing the nutritive value of the forage (Hartley decreases and in the mature plant may represent less than 40 and Jones, 1977). percent of the forage DM. Declines in proportions of cell contents are associated with concomitant reductions in di- Cell Wall Content of Pasture Species gestibility, the rate of the decline tending to be greater in legumes than cool-season grasses (Beever et al., 2000). On average over the growing season, cool-season grasses may typically contain 350–650 g cell wall carbohydrates/kg dry matter (DM), of which approximately 50–60 percent Carbohydrates may be cellulose, 30–50 percent hemicellulose (most of which is xylan), and 2–4 percent pectin (Longland et al., Starch is the major storage carbohydrate in most grass 1995). Cool-season grasses typically contain around 5 per- and legume seeds, and the vegetative (i.e., nonreproductive) cent lignin (MAFF, 1992). Legumes, however, often contain tissues of legumes and warm-season grasses (Chatterton et 295–550 g cell wall carbohydrates/kg DM, of which 30–50 al., 1989). Starch production and storage in vegetative tis- percent is cellulose, 25–30 percent hemicellulose (of which sues occurs within the chloroplasts, and is a self-limiting less than half is xylan), and up to 30 percent pectin (Nord- process as the chloroplasts become saturated with starch. kvist and Åman, 1986). Thus, in legumes the proportions of However, the storage carbohydrates in the vegetative struc- hemicellulose and pectins are lower and higher, respectively, tures of many cool-season pasture grasses are fructans, com- compared with grasses. Lignin may account for 40–150 g/kg posed of polymers of fructose and glucose (Ojima and legume DM (Nordkvist, 1987). For routine feed analysis, Isawa, 1968; Smith, 1968; Bender and Smith, 1973), with the residue remaining after treatment with neutral detergent starch as a relatively minor component (Cairns et al., (termed neutral detergent fiber or NDF) is regarded as being 2002a,b; Turner et al., 2002). Photosynthate in excess of im- representative of the cell wall content of gramminaceous mediate plant requirements for growth and metabolism is feeds, being the sum of the lignin, cellulose, and most of the converted to fructan and translocated from the leaf to stem. hemicellulose fractions. Acid detergent insoluble residues Thus, fructan production is not self-limiting, allowing high (acid detergent fiber or ADF) comprise the cellulose and levels of fructan to accumulate. As a consequence, the aver- lignin fractions (see Chapter 10 for further detail). Warm- age NSC content of warm-season grasses tends to be sub- season grasses mature more rapidly and tend to have a stantially lower than that of fructan-accumulating cool- greater proportion of cell wall and be more highly lignified season grasses when grown under the same conditions, with than their cool-season counterparts, and may exceed 800 g 129 and 209 g NSC/kg DM, respectively, observed by Chat- NDF/kg DM (Buxton et al., 1995) and 150 g lignin/kg DM terton et al. (1989) when studying more than 180 accessions (Woolfolk et al., 1975). Cell wall composition changes as of warm- and cool-season grasses grown under laboratory the plant matures, with young vegetative growth being rela- conditions. tively high in readily degradable hemicelluloses, with com- The highest total NSC contents reported were 654 and paratively little cellulose and lignin (Dulphy et al., 1997b). 771 g/kg DM for Phalaris aquatica and Bromus carinatus As the plant matures and produces more stem, the propor- spp., respectively, both of which contained 450 g fructan/kg tions of cellulose and lignin increase and those of hemicel- DM (Chatterton et al., 1989). However, levels of fructan lulose decrease, with concomitant reductions in ruminal in measured in 10 mixed grass and legume horse pastures in vitro degradability as the season progresses (Givens et al., Germany varied from 10–74 g/kg DM (Vervuert et al., 2005), 1992). Various environmental factors affect cell wall con- whereas orchardgrass in the Mediterranean accumulated up tent: high temperatures tend to increase cell wall content to 400 g fructan/kg DM in the stem bases (Volaire and (Deinum et al., 1968) and lignification in legumes and in Lelieve, 1997). In a study of total water-soluble carbohydrate cool- and warm-season grasses (Wilson et al., 1991; Buxton (WSC, the sum of simple sugars and fructan) and fructan et al., 1995). As a consequence, forages grown at high tem- contents of various ryegrass species and varieties in fertilized peratures tend to be of reduced feeding value compared to field plots over three growing seasons in the United King- those grown at lower temperatures. For example, ruminal dom, WSC contents of vegetative tissues ranged from less NDF digestion was reduced by 13 and 19 percent, respec- than 100 g/kg DM to more than 385 g/kg DM with fructan tively, for leaf and stems of Bermudagrass grown at 32°C vs. contents of 75–279 g/kg DM; the remainder of the WSC 22°C (Buxton et al., 1995). fraction was largely composed of sucrose, fructose, and glu-

FEEDS AND FEED PROCESSING 143 cose (Longland et al., 2006). Thus, levels of WSC and fruc- NSC accumulation are superimposed upon these general- tan in pasture grasses can vary widely. Although high levels ized profiles. Furthermore, where fluctuations in tempera- of NSC may render a forage high in digestible energy (DE), ture and/or light intensity are minimal, e.g., under tropical there is evidence to suggest that overconsumption of starch conditions or mid-season in more temperate regions, the (Potter et al., 1992a) or fructan (Pollit et al., 2003) may elicit above generalized profiles of NSC accumulation may be the onset of metabolic disorders in horses, such as colic and less apparent or may not emerge (Van Soest et al., 1978). laminitis, and high intakes of simple sugars may be involved However, in general, environmental influences that both re- in the development of insulin resistance in horses (Hoffman duce photosynthetic activity ( e.g., cloud cover) and enhance et al., 2003). Pastures sown with forage species that accumu- plant growth (e.g., warm, moist, fertile soils) result in low- late reduced concentrations of NSC should be used for ered accumulation of NSC in vegetative tissues, whereas equids susceptible to these conditions. conditions that enhance photosynthesis but reduce growth In addition to plant species, various other factors affect (e.g., high light intensity coupled with cool temperatures) the amounts of NSC that accumulate in forage. Plant tissues allow elevated levels of NSC, particularly fructan, to accu- differentially accumulate fructan, with bases of plant parts mulate. It is of note that high levels of pasture NSC (starch, accumulating more fructans than the apices, (Williams et al., fructan, or total WSC) have been linked with the onset of 1993), and stems more than leaves (Waite and Boyd, 1953a). disorders such as laminitis. Thus, stems of ryegrass, timothy, fescue, and orchardgrass contained 1.5- to 10-fold the amount of fructan found in the Protein leaves of the same plant (Waite and Boyd, 1953b), and, as a consequence, the NSC content of a plant is influenced by the Protein in the vegetative tissues of forages is largely con- leaf:stem ratio. Environmental effects also influence centrated in the leaf, with much lower levels being found in herbage NSC content. Under conditions of reduced light, ei- the stem; thus, alfalfa and timothy leaves contained some 2- ther through shading or cloud cover (Ciavarella et al., 2000) to 3-fold as much protein as the corresponding stem tissue or with application of fertilizer nitrogen (Jacobs et al., 1989; (Collins, 1988). In addition to plant part, the protein content O’Keily et al., 2002), NSC accumulation decreased, of forages also varies greatly with species and differing envi- whereas drought resulted in an increase in NSC and fructan ronments. However, when averaged across a growing season, (Volaire and Lelievre, 1997). Furthermore, cool tempera- legumes are higher in protein than grasses, with cool-season tures had a profound effect on NSC accumulation, resulting grasses generally higher in protein than warm-season grasses in 2- to 3-fold higher levels of total NSC in both warm- (166 (Whitman et al., 1951). Stage of growth also had a profound vs. 92 g/kg DM) and cool-season (312 vs. 107 g/kg DM) influence on forage protein content; protein level declined grasses, respectively, when grown at night/day temperatures with plant maturity (Green et al., 1971). In North Dakota, the of 10/15°C as opposed to 15/25°C (Chatterton et al., 1989). average crude protein (CP) content of a number of cool- The NSC levels in plants are in constant flux, being a season grasses, was 220 g CP/kg DM in April, but declined function of photosynthetic activity on the one hand and uti- to less than 70 g/kg by October. The corresponding values for lization, translocation, and storage for growth and develop- a range of warm-season grasses were 150 and < 50 g CP ment on the other. Thus, due to the combined effects of light (Whitman et al., 1951). Likewise, the CP content of alfalfa at and temperature on NSC accumulation, there are diurnal the bud stage can be in the region of 250 g CP/kg DM, variations in NSC content, such that levels of NSC tend to whereas at the mid-bloom stage, it may be 150 g CP/kg DM rise during the morning to reach maxima in the afternoon, (Hunt, 1995). Ear emergence (heading) has been identified as with levels declining overnight until the following daylight a critical point after which there is a rapid decline in protein (Bowden et al., 1968; Holt and Hilst, 1969). Similar diurnal content and a concomitant increase in that of fiber. Although patterns of NSC accumulation have also been reported for the timing of 50 percent heading is relatively constant within legumes, with starch increasing 2-fold to 200 g/kg DM from a species, the rate of protein decline varies considerably be- 9 am–3 pm in alfalfa (Lechtenberg et al., 1971). There are tween plant species at a given growth stage (Harkess and also seasonal variations in NSC of grasses and legumes. Alexander, 1969). Management practices also affect the pro- Waite and Boyd (1953b), studying perennial ryegrass, timo- tein content of pastures. Frequent defoliation through clip- thy, orchardgrass, and fescue, reported that NSC was high- ping or grazing results in higher protein contents, because the est in late spring, declining mid-season and rising to inter- herbage is maintained at a younger stage of growth. Clipping mediate levels in early autumn. Decreases in storage Coastal Bermudagrass once every 2 weeks resulted in swards carbohydrates in legumes also occur with increasing matu- that contained 88 percent leaf, 210 g CP/kg DM, and 90 g rity (Demarquilly, 1981). Although these generalized pat- lignin/kg DM. The corresponding values for the swards terns of diurnal and seasonal variation largely hold true clipped every 8 weeks were 51, 120, and 120, respectively under standardized conditions, they are subject to consider- (Burton et al., 1995). Fertilizer application also affects the CP able change in the field, as the effects of varying light inten- content of grass. The average CP content of cool-season pas- sity, temperature, fertilizer, water status, and other factors on tures managed for various DM yields rose from approxi-

144 NUTRIENT REQUIREMENTS OF HORSES mately 250 g CP/kg to 300 g/kg when fertilized with 100 or Summary 400 kg nitrogen/hectare (Humphreys et al., 2001), and the Clearly, overall forage quality is affected by plant protein content of Coastal Bermudagrass averaged over a sea- species, maturity, and environmental effects. In general, fac- son was reported to rise from 98 to 155 g CP/kg when re- tors that reduce plant growth and maintain swards at an im- ceiving no fertilizer or 400 lbs/acre, respectively (Evers, mature growth stage will enhance forage digestibility 1998). Likewise, application of fertilizer increased the con- through elevated cell content:cell wall ratios. Conversely, centration of all protein fractions in Bermudagrass, stargrass, those factors (e.g., warm temperatures, strong light) that in- and bahiagrass (Johnson et al., 2001). crease rates of growth and development will result in a de- cline in quality through decreased leaf:stem ratios and asso- Lipid ciated declines in cell contents and increased cell wall content and lignification. Thus, the quality of warm-season The lipid content of fresh grass, alfalfa, and clovers is forages is generally lower than that of cool-season species. usually low, in the range of 10–50 g/kg DM (MAFF, 1992). A mean difference of around 15 units of total digestible nu- trients (TDN) for ruminants between warm-season and cool- Minerals season grasses was recorded (McDowell, 1972, in Van Soest et al., 1978). There are wide interspecific variations in forage mineral contents. Thus, when grown on the same soil, legumes are generally higher in calcium, potassium, magnesium, copper, Fresh Pastures for Horses zinc, iron, and cobalt than grasses, whereas grasses tend to be higher in manganese and molybdenum. In a study of 17 The feed value of pastures for horses is a function of pas- North American grass species grown on the same soil and ture intake and forage nutrient composition, digestibility, sampled at similar growth stages, copper ranged from and bioavailability. Although horses are preferential grazers, 4.5–21.1 ppm and manganese from 96–815 ppm (Beeson et it has been found that some wild ponies in the United King- al., 1947). In addition to interspecies differences in forage dom and feral horses in the United States will ingest up to mineral content, a number of environmental influences, such 20 percent of their daily intake from browse species, even at as season, plant maturity, geographical location, soil type, times of peak grass growth (Hansen, 1976; Putman et al., pH, and fertilizer history, can result in broad intraspecific 1987). However, such access to browse species is often lim- variations in plant mineral status (Stout et al., 1977). There ited for pastured domestic equids: these animals rely on is often a rapid uptake of minerals during early growth that grass and legume swards, which will be the focus in the rest is reduced by the end of the season. The most common ele- of this section. ments affected by plant growth stage are copper, zinc, iron, cobalt, and molybdenum (Underwood, 1981). During over- Pasture Intakes wintering of forages, leaves, which are often higher in trace elements than the stem, are lost, with a concommitant re- There is scant information on the voluntary dry matter in- duction in minerals with a decrease in the leaf:stem ratio. take (VDMI) of fresh forage by horses, largely due to the Soil pH can have a profound effect on forage mineral status, difficulties in measuring intakes by grazing animals. Meth- with zinc, copper, cobalt, and manganese concentrations in ods used include subtraction of harvested residual herbage clover and ryegrass decreasing with increasing soil pH from calculated herbage mass allowance (Duren et al., (Mitchell, 1957). However, expected average mineral con- 1989), marker studies such as use of n-alkanes (Nash, 2001; tents of forage foliage have been suggested to be approxi- Grace and Brody, 2001), measurement of fecal outputs and mately (in g/kg DM): calcium (5–20), phosphorus (2–5), known organic matter (OM) or DM digestibilities (Mescho- sodium (0.1–0.3), magnesium (1.2–8), potassium (15–40), nia et al., 1998; Grace et al., 2002a), “cut and carry” tech- and sulfur (2–3.5) (McDonald et al., 1996; Barker and niques to housed animals (Chenost and Martin-Rosset, Collins, 2003). Average values for forage micromineral con- 1985), change in body weight (BW) after accounting for in- tent (ppm) have been given as: iron (50–1,000), manganese sensible weight loss and excretory outputs (Ince et al., (30–300), boron (10–50), copper (5–15), zinc (10–100), 2005), or through determination of bite size, number, and molybdenum (1–100), and nickel (0.2–2) (Barker and duration of feedings (Duren et al., 1989). Collins, 2003). Estimates of VDMI for grazing horses generally range from 1.5–3.1 percent of BW (Table 8-1). Average daily DM intakes were highest for lactating mares, which on average Vitamins consumed 2.8 percent of BW whereas the remaining cate- Forages vary widely in vitamin content. However, im- gories of horse ingested approximately 2 percent of their mature, leafy forage should normally provide sufficient BW. This concurs with Dulphy et al. (1997b), who con- carotene and B vitamins for horses (Evans, 1973). cluded that average DM intakes of fresh forage were in the region of 2 percent of BW.

FEEDS AND FEED PROCESSING 145 TABLE 8-1 Estimated Voluntary Fresh Matter Intake (VFMI) and Voluntary Dry Matter Intake (VDMI) of Fresh Herbagea VFMI VDMI VDMI (g FM/ (g DM/ (g DM/ Type of Horse Type of Forage kg BW) kg BW) kg BW0.75) Source Mature trotters Fresh cut grass (4 species) 98.6 (DM) Chenost and Martin-Rosset and alfalfa 51.4 (DOM) (1985) Mature light horses Fresh pasture (first cut) 18 Dulphy et al. (1997a) Mature light horses Fresh pasture (second cut) 20 Dulphy et al. (1997a) Nonlactating, heavy breed Natural grassland 175.2 (DM) Fleurance et al. (2001) mares 166 (OM) Thoroughbred yearlings Perennial 20 85 Grace et al. (2002a) Lactating Thoroughbred mares ryegrass/white clover mix 24 118 Grace et al. (2002b) Weanling Thoroughbreds 18 76.3 Grace et al. (2003) Lactating Thoroughbred mares Orchardgrass and perennial 117–122 24–26 Marlow et al. (1983) ryegrass early vegetative stage Lactating Thoroughbred mares Orchardgrass and perennial 84–100 26–31 Marlow et al. (1983) ryegrass mid bloom Nonlactating Thoroughbred Orchardgrass/perennial 73–77 15–22 Marlow et al. (1983) mares (n = 2) ryegrass Weanling stock horses 20–30 McMeniman (2003) 1–2-year-old selle Francais 82 (OM) Meschonia et al. (2000) Draught breed mares Semi-natural, medium- 101–215 Menard et al. (2002) quality grassland aDOM = digestible organic matter, OM = organic matter, DM = dry matter A number of authors have reported that increased lignified vascular and schlerenchyma tissues (Akin and Bur- herbage DM intakes are coincident with increased herbage dick, 1975). Furthermore, plant maturity is an important de- quality, in terms of digestibility (Moffitt et al., 1987), high terminant of forage digestibility, with digestibility declining sugar content (Rogalski, 1984), or inferred increased CP and with reductions in protein content, and of increases in fiber reduced cell wall contents arising from fertilizer nitrogen associated with the decreases in leaf:stem ratios that occur application (Benyovsky et al., 1998). However, other studies during reproductive development and stem elongation. reported decreased intake of higher-quality pastures. For ex- Warm-season species tend to mature more rapidly than their ample, young Thoroughbred fillies on improved pastures cool-season counterparts, with concomitant declines in di- were more active and spent less time grazing than those gestibility. Nutrient digestibilities of various forages at dif- grazing less nutritious swards (Nash and Thompson, 2001). ferent growth stages are summarized in Table 8-2. Thus, the Thus, the VDMI response of growing horses to increasing DM digestibility of bluegrass/alfalfa swards by horses was pasture quality may be positive or negative. This has led to 73 percent for young pasture 11 cm tall and 52 percent for the suggestion that control of VDMI may be a function of older, more mature swards 47 cm high with a higher pro- DE intake in these animals (Hoskin and Gee, 2004). Thus, portion of stem than the younger material (McMeniman, animals with previously insufficient or marginal DE intakes 2003). The DM digestibilities of semi-natural grassland of may show enhanced VDMI of high-quality pastures, medium quality were 61 percent in May and 53 percent in whereas those with previously adequate DE intakes may re- July (Menard et al., 2002). The energy value of pastures at duce their VDMI to maintain constant DE intakes. different seasons has been determined by a number of re- searchers—the DE of leafy spring ryegrass/white clover swards and spring/summer pastures were respectively 2.88 Pasture Digestibility (Hunt, 1995) and 2.82 Mcal/kg DM (Gallagher and Mc- Digestibility of fresh pasture varies with plant species Meniman, 1988; Martin, 1993), declining to 1.9 and 1.8 and type. In general, cool-season grasses are more digestible Mcal/kg DM, when the pasture had a reduced leaf:stem ratio than warm-season grasses. This is purported to be due to the with the advancing seasons. Similarly the DE of alfalfa de- higher proportions of easily degraded mesophyll cells in clined from 2.87 for immature material to 2.4 Mcal/kg DM cool-season grasses compared with their warm-season coun- when in full bloom (Hunt, 1995). Digestibility of forage terparts, which have higher proportions of more recalcitrant protein is associated with protein content (Evans, 1973),

146 NUTRIENT REQUIREMENTS OF HORSES TABLE 8-2 Contents of Digestible Energy and Protein and Apparent Dry Matter and Protein Digestibilitiesa of Various Fresh Forages by Horses DE CP (Mcal/ Content Type of Forage kg DM) (g/kg DM) DMDb CPDc Sourced Semi-natural medium-quality grassland –May 61 Menard et al. (2002) –June 57 –July 53 –September 55 –October 56 Pasture spring/summer 2.3–2.82 103–130 Gallagher and McMeniman Pasture/winter 1.8–2.5 121–151 (1988); Martin (1993) Ryegrass 60 Hussein et al. (2001) Tall fescue 66 Orchardgrass 71 Spring ryegrass/white clover 2.6–2.83 148–220 Martin (1993) Spring ryegrass/white clover (leafy) 2.88 220 Summer ryegrass/white clover (leafy) 2.5 150 Hunt (1995) Late summer, stemmy ryegrass/white clover 1.9 100 Ryegrass/white clover/autumn 2.6 250 Ryegrass/white clover/winter (leafy) 2.7 250 Pre-bloom red clover 2.64 230 Full-bloom red clover 2.4 180 Hunt (1995) Immature alfalfa 2.87 250 Pre-bloom alfalfa 2.75 220 Early-bloom alfalfa 2.64 200 Mid-bloom alfalfa 2.51 160 Young pitted bluegrass and alfalfa, 11 cm high 73 McMeniman (2003) Mid-season pitted bluegrass and alfalfa, 23 cm high 68 Late-season pitted bluegrass and alfalfa, 47 cm high 52 aDigestibilitiesare expressed as percentages. bDMD = dry matter digestibility. cCPD = crude protein digestibility. dAll studies used mature horses except McMeniman (2003), which used young horses. with CP digestibility declining with increased plant maturity Meniman, 1988; Martin, 1993). A number of authors have and reduced protein levels. concluded that providing that there is sufficient pasture of a DE no less than 2.4 Mcal/kg DM and 105 g CP/kg DM, lac- tating mares consuming pasture DM at a level equivalent to Feeding Value of Pastures 2.4 percent of their BW per day would not require diet sup- Ad libitum access to pasture can support nonproductive plementation for these nutrients (Martin-Rosset et al., 1986; adult horses. At certain times of year, this access can cause Doreau et al., 1988; Martin, 1993). However, high-quality, them to exceed their requirement for energy and protein, re- lush pastures of cocksfoot orchardgrass and perennial rye- sulting in obesity (Elphinstone, 1981; Hughes and Gal- grass, although able to support nonproductive mares, were lacher, 1993; Marlow et al., 1983). unable to sustain lactating Thoroughbred mares. The high In a 2-year study on pregnant mares at pasture, it was moisture content of the pastures was thought to prevent the found that although pasture nutrient content fluctuated lactating mares from ingesting sufficient dry matter, and throughout the seasons to well above and somewhat below they avidly ate all high-DM forages offered to them. It is of the calculated requirements of the mares, their protein and note that these mares were able to maintain body condition energy requirements were met, as they gained weight in two when the same pastures were of lower moisture content successive seasons during lactation (Gallagher and Mc- (Marlow et al., 1983). Keenan (1986) also reported on the

FEEDS AND FEED PROCESSING 147 need to provide high-fiber supplements to horses consum- Pasture can meet or often exceed the overall energy and ing irrigated, low-DM, low-fiber pastures to prevent bark- protein needs of many types of horses. However, intensively chewing and other undesirable behaviors. exercised horses or those working for long hours may not be In two separate studies, Bigot et al. (1987) and Staun et able to meet their nutritional needs entirely from pasture, al. (1989) independently fed groups of young horses either due to increased nutrient demands and reduced time avail- a high or low plane of nutrition during the winter, then able for grazing. Furthermore, diets high in indigestible grazed the two groups of horses together each summer, for fiber increase gut fill and, as a consequence, may increase 3–3.5 years. Although growth was slower in the animals fed BW, which may be a disadvantage in certain disciplines less intensively in winter, they exhibited compensatory such as racing (Caroll and Huntingdon, 1988) or may be re- growth such that there was no difference in body weight be- garded as unsightly in the show horse. Although appropriate tween the different groups by the time the animals were 3 gut fill is regarded as beneficial for the endurance horse, as years old, demonstrating that pasture was not only able to large amounts of forage in the hindgut may act as a reservoir support growth, but also was able to redress deficits arising for water and electrolytes (Duren, 1998), the presence of a from suboptimal feeding. However, it is of note that com- visibly distended “hay-belly” is not desirable. pensatory growth may be associated with an increased risk of developmental orthopedic disease (DOD). In areas with Pastures for Horses an extended growing season, young horses can maintain steadier growth rates on pasture. For example, the growth Pasture species do not grow throughout the year. Their rates of young Thoroughbreds raised entirely on ryegrass- growth depends on soil temperature, moisture, and fertility. based pastures in New Zealand (Brown-Douglas, 2003) Growth is also affected by management practice; overgrazing were similar to those reported elsewhere for this type of will reduce yields and encourage growth of unpalatable horse fed cereal concentrates and hay in addition to pasture species and weeds. Ideally, horse pastures should contain a (Hintz, 1979; Pagan, 1996). mix of grasses and legume species that are adapted to the Pastures can only supply the nutrient requirements of prevailing soil and climatic conditions and that can also with- horses if they are stocked appropriately, where due account is stand close grazing and wear. It is outside the scope of this taken of (1) the quality and yields of the forages, and (2) the text to provide a comprehensive review of all forage species size, workload, and physiological status of the animal. Annual used for horse pastures, soonly some of the more commonly DM yields from natural unimproved pastures can be less than used forage species are described here. Suitable cool-season 1 ton/hectare (t/ha), whereas those from well-managed im- species (those that start to grow when temperatures reach proved pastures can be 15 t/ha or more (Morrison et al., 1980; 7°C, with optimal growth between 16–24°C) include or- Hopkins et al., 1990). Blue couch, white clover, and lotonon- chardgrass (Dactylis glomerata), toxin or endophyte-free tall sis (Lotononsis bainessi) pastures that provided 2.15 Mcal/kg fescue (Festuca arundinacea), and Kentucky bluegrass (Poa DM and 145 g CP/kg DM met the energy and protein re- pratensis), which can all resist tight grazing and treading. quirements of breeding, lactating, and re-breeding Thorough- Other suitable, perennial cool-season species include endo- bred mares when stocked at density of 1.6 ha per animal (Gal- phyte-free perennial ryegrass (Lolium perenne) varieties, lagher and McMeniman, 1988). Grace et al. (2002a,b, 2003) which are palatable, high yielding, and of high nutritive estimated that 0.2, 0.25, and 0.5 ha per animal was sufficient value. Timothy (Phleum pretense) is also useful. It is a tradi- for weanlings, yearlings, and lactating mares, respectively, tional species for horses and is late flowering, producing grazing ryegrass pastures yielding approximately 2 t DM/ha, leafy herbage mid-summer when such forage may be scarce. but no allowance was made for herbage wastage in these cal- However, timothy does not tolerate very close grazing, and culations. However, Hunt (1995) regarded 1 ha as sufficient to late summer and autumn production may be low. Meadow support one-and-a-half 500-kg mares plus foals in New fescue (Festuca pratensis), smooth bromegrass (Bromus in- Zealand. Elphinstone (1981) concluded that 1-ha pastures of ermis), Matua prairiegrass (Bromus wildenowii), and low- kikuyu, blue couch, and Rhodes grass met the nutritional re- alkaloid varieties of reed canary grass (Phalaris arundi- quirements for a mature horse at maintenance or a working naceae) are also used for horse pastures. Fine-leaved fescues horse in spring and autumn. If pasture is the sole feed for such as creeping red fescue (Festuca rubra) are good for pro- horses, then knowledge of annual DM yield is desirable to en- duction of a wear-resistant turf and useful in areas of high able efficient stocking and utilization of the pastures. traffic, such as exercise areas or around water sources or Many well-managed pastures can yield at least 5 t DM/ha gateways, but are of mediocre grazing value. Inclusion of per year. Coupled with conservation of grass in times of legumes in horse pastures as minority species provides a plenty, if well managed, an area of 1 ha of such pasture good source of dietary protein and calcium. Furthermore, should be able to provide sufficient annual forage DM for a through their ability to “fix” atmospheric nitrogen, legumes 500-kg horse requiring 10 kg DM/d, after accounting for enhance the nitrogen status of the soil, reducing the need for pasture wastage due to treading and defecation and for additional nitrogen fertilizer. Legume species that proliferate losses during herbage conservation. by rhizomes and stolons are suitable for horse pastures as

148 NUTRIENT REQUIREMENTS OF HORSES they are less susceptible to aggressive defoliation and wear. producing overgrown herbage unpalatable even to ruminants. White clover (Trifolium repens) is commonly found in pas- In the absence of other species of grazing livestock, strip graz- tures grazed by horses, being generally tolerant of close graz- ing or division of grazing into three or four smaller paddocks ing and mixing well with most grasses. With proper manage- will allow short-term rotation (often either 2 weeks grazing ment, white clover can persist in swards for many years. and 4 weeks rest, or 1 week grazing and 3 weeks rest) Alfalfa (Medicago sativa L.) is often recommended for horse (Matches, 1992; Emmick and Fox, 1993; Henning, 1994). pastures because of its high quality, yield, and compatibility Coupled with daily removal of manure, this practice will help with grasses; varieties that have their crowns beneath the soil to prevent the development of roughs and lawns, and it can do are better able to tolerate damage by horses than standard much to increase the yield and quality of available grazing cultivars (Jordan et al., 1995). Red clover (Trifolium (Odberg and Francis-Smith, 1977). To prevent pasture deteri- pratense) is less tolerant of close grazing than white clover oration, soils should be analyzed to enable appropriate main- and generally requires reseeding every 2–3 years. Other tenance levels of fertilizer to be applied. If lawns have devel- legumes for horse pastures include bird’s-foot trefoil (Lotus oped, they can be preferentially fertilized to maintain the corniculatus) and sainfoin (Onobrychis viciifolia), although nutrient status of the sward. these often need careful establishment and grazing manage- Where growth of grass is excess to immediate require- ment if they are to persist in the sward. Both annual or peren- ments, the grass can be either stockpiled for winter feed or nial species of lespedeza (Kummerowia spp.) may also be conserved. Stockpiling is the practice of allowing pastures to used in horse pasture seed mixtures. grow from mid-late summer onwards to produce stands of In hot, dry environments when most cool-season forages fairly mature sward. When autumn pastures have become de- lie dormant, warm-season grasses (which start growth at pleted, animals can be strip-grazed on stockpiled pastures. 15°C with temperature optima of 32–35°C) can grow, pro- Strip-grazing is necessary to maximize the use of the stock- viding a good source of forage in the summer months. piled fodder and to prevent excessive wastage via trampling Warm-season perennial grasses suitable for horses include and defecation on fresh sward, rendering it unpalatable. Bermudagrass (Cynodon dactylon), which tolerates tight Stockpiling of permanent pastures can only be considered for grazing and wear as does bahiagrass (Paspalum notatum), free-draining soils or in areas subject to only moderate winter which will also grow in soils of low to mediocre fertility. rainfall. Where these conditions are not met, the pasture may Dallisgrass (Paspalum dilatatum) is also suitable, but re- become irrecoverably damaged, necessitating complete re- quires more fertile soils with higher water content than bahi- seeding. Stockpiling can be an economical alternative to for- agrass. Various bluestem species (Andropogon spp.), such as age preservation, but most domesticated equines receive some Caucasian or big bluestem, may also be used. The warm- form of conserved forage for at least a portion of the year. season annual species are generally of higher nutritive value than the warm-season perennials. Species used for horse Conserved Forage for Horses pastures include pearl millet (Pennisetum glaucum) and crabgrass (Digitaria spp.). During cool periods, warm- Forages may be conserved for use when there is little season grasses are dormant. In areas that will allow their fresh forage available or when animals are confined to stalls. growth, use of both cool- and warm-season species can Such conservation is achieved by drying, ensiling, or apply- greatly extend the grazing season, as the majority of the ing preservatives. After drying, the forage may be chopped, cool-season grass production occurs in spring and autumn, ground and pelleted, cubed, or wafered, whereas ensiled for- whereas that of warm season species occurs in the summer. ages are either bulk-stored in bunkers (clamps) or silos, or In suitable areas, this strategy allows grazing for much of the baled and wrapped in plastic film. The feed quality of con- year, and annual herbage production can be further extended served forage can be no greater than the original sward, and by seeding dormant warm-season perennial pastures with thus stage of plant maturity at time of harvest is an impor- cool-season annual species such as annual ryegrass or small- tant factor influencing the feeding value of the final product. grain species, e.g., wheat, rye, or oats. Young leafy swards have the highest feed quality but low Horses are spot grazers. They overgraze some areas of a yields. As plants mature, their DM increases. The leaf:stem pasture and defecate in others, which, if left unmanaged, re- ratio decreases with concomitant increases and decreases sults in overgrazed “lawns” and rank overgrowth of herbage in fiber and protein, respectively. Preservation of high- in latrine areas, termed “roughs” (Odberg and Francis-Smith, moisture, young, leafy herbage as hay can be problematical 1976). Long-term rotational or co-grazing of large horse pas- in areas where good haymaking weather is uncertain; under tures with sheep or cattle is beneficial, as the ruminants will those circumstances, such herbage is often ensiled. remove the roughs and may help break the lifecycle of equine parasites. This practice helps prevent the pasture from be- Hay coming “horse sick,” where overgrazed lawns become subject to ingress of weed species, some of which may be toxic ( e.g., The ultimate aim of haymaking is to produce a palatable, Seneccio spp.) and the roughs grow unchecked, eventually hygienic product that retains much of the nutrient quality of

FEEDS AND FEED PROCESSING 149 the original sward. This is achieved by minimizing nutrient the amount of loss increasing with numbers of showers, losses in the field and curing the herbage to a stage that pre- amounts of rain, and the DM of the crop. Leaching has been vents molding and reduces nutrient losses and deterioration reported to remove 20–40 percent of the DM, 20 percent of during storage. CP, 35 percent of NFC, and 30 and 65 percent, respectively, There are inevitable biological and physical losses of DM of phosphorus and potash (Shepherd et al., 1954). The aver- and nutrients during haymaking, associated with drying, bal- age WSC contents of hays made from four cool-season ing, and subsequent storage and usage. Drying of forage be- species, which had received a light rain shower during dry- gins immediately after it is cut, but the rate of drying depends ing, averaged half those of the original swards and losses of upon the differential in water vapor pressure between the sur- CP were up to 27 percent (Ince et al., 2006). Losses of NSC rounding air and in the surface tissues of the plant. When leached from alfalfa and red-clover hays exposed to heavy plant and air vapor pressure are equal, no further drying oc- rain during curing resulted in a 45 percent increase in the curs (Technical Committee, 1964). Vapor pressure is affected proportion of fiber relative to hays protected from rain by temperature, air movement, solute concentration, and (Collins, 1983). Rain not only leaches nutrients, but also in- water movement within the plant tissues. Hays made and creases leaf shatter due to the extra mechanical operations stored in humid environments cannot achieve a DM as high required to dry the hay. as those made and stored under more arid conditions. High-moisture hays may be effectively preserved through Different plant parts dry at different rates, with leaves artificial drying in barns. Nutrient losses from hays dried in tending to dry more rapidly than the thicker stem. This can barns without added heat are usually lower than for hays lead to leaf shatter during mechanical handling and loss of dried in the field; addition of heat to 60°C to accelerate the the more nutritious leaf material (Shepherd et al., 1954; drying process reduces DM losses still further through rapid Rotz and Muck, 1994). Leaf shatter may account for DM inhibition of enzyme activity (Butler and Bailey, 1973). losses from grass and legume hays of 2–5 and 3–35 percent, However, such heating can result in the formation of Mail- respectively. Use of mower/conditioners in which the stems lard products (protein/carbohydrate complexes), which are are crushed and cuticular wax is scarified during cutting can resistant to proteases, thus reducing protein digestibility help reduce losses due to leaf shatter by reducing the dis- (Goering and Van Soest, 1967). parity in drying rates by various plant parts (Rotz and Muck, High-moisture (250 g/kg) hays can also be preserved via 1994; Rotz, 1995). application of microbial inhibitors such as organic acids, Plant respiration continues post-cutting and is responsible buffered acids, or ammonia. Propionic acid or acid mixtures for loss of nutrients, particularly the NSC fractions (Wylam, applied to hay at rates of 1–2:100 (volume:weight) generally 1953; Raguse and Smith, 1965), which account for the ma- reduce mold growth and heating. Short-term (30-day) jority of respiratory DM losses during field curing of hay. studies with growing horses fed hay preserved with a Thus, the average WSC content of fresh, cool-season pasture propionate/acetic acid mixture showed no difference in ac- grasses was 16 percent, whereas that of sun-cured hay was 10 ceptance or growth rate compared with those fed untreated percent (MAFF, 1992), and the NSC contents of fresh forage hay, with no apparent ill effects, whereas in another study, legume pasture vs. legume hays were 15 and 11 percent, re- horses preferred untreated alfalfa hay to that which had been spectively (Dairy One, 2003). In a summary of literature, treated with a similar propionate/acetic acid preparation Shephard et al. (1954) reported that such DM losses ranged (Lawrence et al., 1987, 2000). However, the expense in- from 4–16 percent, higher losses being associated with curred with barn drying and the corrosiveness and difficul- slower drying rates. Losses of CP and changes in constituent ties in working with acid or alkaline preservatives have lim- nitrogen fractions can occur during haymaking. Proteolysis ited their use in hay preservation (Collins and Owens, 2003). during drying causes a rise in nonprotein nitrogen (NPN) and Forages conserved as hay by desiccation should contain changes in amino-acid profiles (Brady, 1960), whereas the no more than 200 g moisture/kg, to prevent proliferation of nitrate content is largely unaffected (Butler and Bailey, microorganisms, heating (due to microbial respiration), and 1973). Other nutrient losses include some organic acids subsequent reduction of nutritive quality (Collins and (James, 1953) and vitamins. Upward of 80 percent of Owens, 2003). However, under conditions of high (90–100 carotene (the precursor of vitamin A) may be lost (Butler and percent) relative humidity that favor microbial growth, hay Bailey, 1973), and vitamin E (alpha tocopherol) content also stored at 200 g moisture/kg can become moldy. Under these declines during haymaking (Miller, 1958). As the fiber frac- circumstances, baling at lower moisture contents is neces- tions are relatively unaffected by plant respiration, their pro- sary. For low-density bales, recommended moisture contents portions increase during herbage drying. Once herbage mois- are around 180 g/kg; for more densely packed bales, drier ture has been reduced to 400 g moisture/kg, plant respiration hay of 120–140 g moisture/kg is suggested to improve ceases, and further nutrient losses in the field are largely due preservation (Rotz and Muck, 1994; Rotz, 1995). There are to weathering and handling. usually minimal losses of DM or nutrients in hays stored at Hay yield and quality are reduced if exposed to rain dur- less than 150 g moisture/kg (Czerkawski, 1967). However, ing curing, through leaching of soluble components, with in hays of greater moisture content, varying losses occur

150 NUTRIENT REQUIREMENTS OF HORSES during storage, mainly associated with microbial respiration However, studies with ruminants have reported OM di- (and heating), which, in turn, is affected by ambient condi- gestibilities of 73 and 65 percent for the original sward and tions. It has been estimated that on average, during storage, hay, respectively (Shepperson, 1960), and well-cured alfalfa hay of 150 g moisture/kg will lose 5 percent of DM, and this hay was reported to contain 15 percent less CP and be 10 loss increases by 1 percent for every further 10 g/kg increase percent less digestible than the original standing crop in moisture up to 200 g moisture/kg (Collins and Owens, (Collins, 1990). Field losses of TDN in well-cured hays and 2003). Thus, DM losses were small for hays of 880–930 g those exposed to rain were, respectively, 26 and 42 percent DM/kg stored at temperatures of 7°C or less, but losses of (Shepherd et al., 1954.). Similarly, alfalfa and red-clover 8 percent were recorded for hays of 820 g DM/kg stored at hays exposed to rain were 12 percent less digestible relative 36°C for 9 months, largely accounted for by losses in NSC to hays protected from rain (Collins, 1983). Furthermore, ru- (Czerkawski, 1967). Mineral content, lignin, and fiber frac- minant digestibility of Bermudagrass hays was little affected tions remained relatively stable during bale storage. DM by heating up to 48°C, but declined by 14 percent when in- losses during storage of large round bales of alfalfa hay (av- ternal bale temperatures exceeded 60°C (Coblentz et al., erage DM 810 g/kg), which were either stored in a barn or 1998). It is likely that reductions in the feeding value of such outside with or without a cover, were 2.5, 6, and 15 percent, hays would also be seen in horses. respectively (Belyea et al., 1985). Losses (biological and There may also be further DM losses of hay at feed-out. mechanical) of ryegrass hay stored on the ground over 7 Weathered or otherwise unpalatable hay may be rejected, re- months were 27 percent, whereas those for hays stored in a sulting in considerable wastage. When fed to heifers, losses barn were 2.3 percent (Nelson et al., 1983). Total DM field from large round bales of alfalfa hay stored indoors or out- and storage losses of conserved grass-legume mixes have side with or without cover were 12, 14, and 25 percent, re- been reported to range from 15–30 percent (Hoglund, 1964). spectively, resulting in total losses of 15, 20, and 40 percent Large reductions in DM and quality occur during storage of the original DM (Belyea et al., 1985). In the uncovered of hays containing more than 200 g moisture/kg, with ex- bales stored outside, rain weathered the outer portions cessive heating (due to microbial respiration) playing a sig- equating to approximately 40 percent of the original hay nificant role in the deterioration of high-moisture hays. DM. At feed-out, the animals wasted much of this material Thus, hays of 840 g DM/kg heated little during storage and in their search for more palatable fodder in the center of the microbial contamination was low, whereas hays of 750 g bale (Belyea et al., 1985). Likewise, feed refusals of rye- DM/kg heated to more than 45°C and became obviously grass hay stored on the ground were 22 percent, compared moldy with Aspergillus spp. Hays with DM contents of less with 1.2 percent of barn-stored hay; the sum of storage than 600 g/kg reached temperatures of 60–65°C and con- losses and animal wastage of these ryegrass hays equated to tained thermophilic fungi (Gregory et al., 1963). Storage of 49 and 3.5 percent of the original harvested DM for the hays of less than 700 g DM/kg may result in a charred prod- ground and barn-stored hays, respectively (Nelson et al., uct of lowered nutritive value, and extreme heating can re- 1983). sult in spontaneous combustion (Browne, 1933). Studies on Hays of high nutritive value are characterized by a large alfalfa hay baled at either 700 or 800 g DM/kg indicated that proportion of leaf, and high protein and low NDF contents. forage quality deterioration (increased NDF and unavailable DM yields of such “prime” quality hay are low, and a com- nitrogen content) was greater for the higher moisture hays, promise between the nutritive value of the hay and DM yield deterioration being highly correlated with accumulated days is usually made to produce hays of various feed value or of heating to > 30°C. The greatest changes in forage quality “grades.” With the possible exception of lactating mares and occurred between days 4 and 11 post-baling, but in the growing youngstock, many horses are unlikely to require the higher moisture hays, microbial activity was prolonged and very high CP contents of “prime” hay. If such high-protein caused further deterioration between days 11 and 22. There- forage is fed in conjunction with sufficient energy, the pro- after, there was little change in nutritive value (Coblentz et tein excess to requirements will be excreted, contributing to al., 1996). Likewise, in Bermudagrass hays baled at mois- the environmental burden of excretory nitrogen. Further- tures ranging from 208–325 g/kg, there was a positive linear more, excess protein intakes may have detrimental acido- relationship between hay moisture content and spontaneous genic effects in sports horses (Graham-Thiers et al., 1991). heating that, in turn, was significantly correlated with in- Horses can be fed well-preserved hays with lower nutrient creased deterioration of nitrogen fractions (Coblentz et al., value, which can be supplemented as necessary during times 2000). Hays visibly contaminated with molds should not be of increased nutrient demand. Threshed mature hay, in fed to horses. In addition to the reduction in feeding value, which the seed has been removed, may also be fed to horses, such hays may contain mycotoxins, and inhalation of fungal but it is of low nutritional value (Schurg et al., 1978; Hyslop spores from molded forage can lead to respiratory compro- et al., 1998b). mise in both horses and their human attendants. It is of note, however, that the visual appearance of con- There is a paucity of information on comparisons of the served forages is an imprecise indicator of their potential nu- nutritive value of hay and the sward of origin in equines.

FEEDS AND FEED PROCESSING 151 tritive value, which can only be properly assessed via feed lationships between NDF content and intakes did not account analysis. for all of the variation, and it is likely that VDMI of hay is controlled by a number of interacting factors. Hay Intakes Effect of Particle Size Contrary to fresh pasture intakes, higher voluntary dry mater intakes (VDMI) generally occur with alfalfa hay com- Grass hay is normally fed to horses in the long form, and pared with long-grass hays (Cymbaluk, 1990a; Dulphy et al., there was no difference in intakes of the same hay when 1997b; Crozier et al., 1997; LaCasha et al., 1999). The range chopped (Hyslop et al., 1998a; Morrow et al., 1999). How- of VDMI of long hays reported by a number of studies is de- ever, when hays are presented in a dense form such as tailed in Table 8-3. Intakes typically averaged between 2 and wafers or pellets, intakes have been reported to be greater 2.4 percent of BW for grass and alfalfa hays, respectively. than when hay of similar composition was fed loose, such The mechanisms that control VDMI of hay in horses are that horses ate 0.17 and 0.24 more wafered and pelleted al- unclear: it has been proposed that VDMI of hay is governed falfa hay, respectively, than when it was fed in the loose by energy requirement (Aiken et al., 1989), dry matter di- form (Haenlein et al., 1966). Likewise, the VDMI of horses gestibility (Crozier et al., 1997), and cell wall content (St. fed long-stem, threshed ryegrass hay was significantly in- Lawrence et al., 2001). However, Martin-Rosset and Ver- creased when the same material was pelleted, cubed, or bri- morel (1991) found no consistent relationship between quetted (Schurg et al., 1978). Reported daily VDMI of VDMI and energy requirement, and Dulphy et al. (1997a) horses fed loose alfalfa hays range from 73–122 g DM/kg failed to find a relationship between NDF, CF or CP, and BW0.75 (Haenlein et al., 1966; Cymbaluk and Christiensen, VDMI of hay. Nevertheless, St. Lawrence et al. (2001) re- 1986; Cymbaluk, 1990a; Crozier et al., 1997; Dulphy et al., ported a significant relationship between the NDF content of 1997a,b), compared to 88.3–139 g DM/kg BW0.75 for four cool-season grass hays and VDMI (p < 0.001), and they wafers, cubes, or pellets (Haenlein et al., 1966; Cymbaluk were able to use this relationship to predict intakes of differ- and Christiensen, 1986; Cymbaluk, 1990a; Todd et al., ent hays by horses. Similarly, Reinowski and Coleman 1995). Furthermore, VDMI of horses fed pelleted high-fiber (2003) found a significant correlation between warm-season complete diets was some 2.6-fold that of the same complete grass hay NDF content and VDMI. However, the reported re- feed offered as a chaff, when pellets were fed as the first TABLE 8-3 Estimated Voluntary Dry Matter Intake (VDMI) of Various Hays by Horses and Ponies VDMI VDMI Type of Horse Type of Hay (g DM/kg BW) (g DM/kg BW0.75) Source Yearling Bermudagrass 21–25 111 Aiken et al. (1989); La Casha et al. (1999) Matua 28 128 LaCasha et al. (1999) Alfalfa 31 142 LaCasha et al. (1999) Mature horses Alfalfa 17.2–31 73–142 Crozier et al. (1997); Dulphy et al. (1997a,b); Haenlein et al. (1966); LaCasha et al. (1999) Altai wildrye 81.2 Cymbaluk (1990a) Bermudagrass 95.6 Aiken et al. (1989) Bluestem 22 115 Reinowski and Coleman (2003) Bromegrass 114 Cymbaluk (1990a) Crested wheatgrass 85 Cymbaluk (1990a) Eastern gammagrass 19 Reinowski and Coleman (2003) India grass 22 Reinowski and Coleman (2003) Kentucky bluegrass 82.3 Cymbaluk (1990a) Oat hay 81.2 Cymbaluk (1990a) Reed canarygrass 99.4 Cymbaluk (1990a) Tall fescue 111 Crozier et al. (1997) Timothy 29.2 Reinowski and Coleman (2003) Early cut grass 18.2 87 Dulphy et al. (1997a, 1997b) Re-growth grass 20 96 Dulphy et al. (1997a, 1997b) Mature ponies High-quality grass hay 110 Hyslop et al. (1998a) Poor-quality grass hay 113 Hyslop et al. (1998a) Mature threshed grass hay 95.6 Hyslop et al. (1998b) Meadow hay 26 153 Pearson and Merrit (1991) Mature meadow hay 14.7 63 Moore-Colyer and Longland (2000)

152 NUTRIENT REQUIREMENTS OF HORSES feed in a crossover design experiment. The pellets were tainly, grinding and pelleting hay caused a significant de- more rapidly consumed than the chaff, largely due to faster crease in both the rate and extent of in situ NDF degradation bite rates and decreased chewing, and thus the chaff-fed an- (Drougoul et al., 2000a,b). However, the apparent DM, CP, imals spent more time feeding (Argo et al., 2002). It is of ADF, and cell wall constituent digestibilities of long-stem note that rapid intakes of pelleted diets may elicit the onset threshed ryegrass hay were reported to be 58, 60, 34, and 40 of undesirable behaviors such as wood-chewing (Haenlein percent, respectively, when fed to horses at 1.5 percent of et al., 1966), and it is a common perception that addition of BW (Schurg et al., 1978). However, when the same material chopped conserved forage to sweet feeds, grains, or pelleted was cubed, briquetted, or pelleted, DMD was significantly feeds will reduce the rate of meal intake. However, although reduced (P < 0.05). Furthermore, the ADF digestibility Ellis et al. (2005) recorded a 120 percent increase in feed in- of the pellets was 25 percent, being significantly lower take time by horses when chopped Lucerne straw was added (P < 0.05) than for the long-stem (34 percent), cubed (34 to pelleted feeds, there was no effect on speed of consump- percent), or briquetted (31 percent) material. tion when similar types of chaff were fed with oats (Harris et al., 2005; Brussow et al., 2005). Cereal Straw Voluntary dry matter intakes of horses fed cereal straws Digestibility of Hay ad libitum are generally reported as being lower than for The digestibility of hay depends upon the plant species, grass or legume hays (see Chapter 11). Thus, pooled data maturity at harvest, leaf:stem ratio, speed of drying, and from a number of studies showed that VDMI of cereal straw conditions under which it has been stored. Nutrient di- averaged 54 and 64 percent of the intakes of alfalfa and gestibilities of several long-grass and legume hays by horses grass hays, respectively (Dulphy et al., 1997b). However, if reported in the literature are shown in Table 8-4. It is of note oat (Hyslop and Calder, 2001) or wheat (Hansen et al., that artificially, rapidly dehydrated grass had higher di- 1992) straw was fed with alfalfa (1:1), VDMI was similar to gestibilities of DM, CP, NDF, and gross energy (GE) than a 100 percent alfalfa diet. when the same crop was field-cured as hay (Hyslop et al., The calculated DE value of wheat straw was low, at only 1998a). The DE of grass hays averaged 1.79 Mcal/kg DM, 43 and 57 percent of alfalfa and Midwest prairie hays, re- the range being 1.40–2.13 Mcal/kg DM, with the DE of al- spectively, with DMD and fiber digestibilities of 23 and 11 falfa hay averaging 2.42. The digestibility of CP in grass percent (Hansen et al., 1992). The DM, CP, and fiber di- hays averaged 53 percent (range 20–74), whereas that for al- gestibilities of oat straw calculated by difference were falfa was 74 (range 64–83 percent). In a survey of the litera- 32–50 percent with a DE of 1.97 Mcal/kg DM (Hyslop and ture of the digestibility of grass hays by horses, Chenost and Calder, 2001). Treatment can improve the nutrient di- Martin-Rosset (1985) found digestibilities of OM, CP, and gestibility of straw. When wheat straw was ammoniated, crude fiber (CF) to average 56, 72, and 38 percent, respec- there was a significant (P < 0.05) increase in both the di- tively, for hays with a CP content of greater than 120 g/kg. gestibility of DM (23 percent untreated vs. 44 percent am- These authors found corresponding values for hays contain- moniated) and NDF (11 vs. 36 percent) (Hansen et al., ing 80–120 g CP/kg to be 49, 55, and 44. For low-protein 1992). Low intakes of straw may be related to an increased hays of less than 80 g CP/kg, the digestibilities for OM, CP, mean gastrointestinal particle retention time (Pearson and and CF were 44, 27, and 47 percent, respectively. Although Merrit, 1991; Pearson et al., 2001) and are associated with densification of forages appears to increase intakes, total extended feeding and chewing times, presumably due, at tract nutrient digestibility does not usually appear to be af- least in part, to the recalcitrant nature of the lignified cell fected. Thus, processing alfalfa hay into pellets, cubes, or by walls of the stem. chopping had little effect on total tract digestibility of DM, CF, CP, or minerals (Jackson et al., 1985; Pagan and Jack- Problems with Feeding Hay and Straw son, 1991a; Todd et al., 1995). Likewise, similar apparent digestibilities of DM, GE, and NDF of a chaffed or pelleted Alfalfa hay is high in minerals, especially calcium and high-fiber diet fed to ponies were observed by Argo et al., magnesium, and their possible role in the formation of en- (2002) and the nutritive value (digestibility and mean reten- teroliths is discussed in Chapter 12. Furthermore, some al- tion time) of long hay or long silage was similar to that of falfa hay harvests can be contaminated with blister beetles the corresponding short chopped feed (Morrow et al., 1999). that produce cantharidin, a highly toxic chemical that can be No differences in digestibility of OM, NDF, or CP were ob- fatal to horses (Schoeb and Panciera, 1978). The generally served by Drogoul et al. (2000a) when pelleted or chopped low intake of cereal straws precludes their use as the sole hay was fed at similar intakes to ponies. The pelleted hay ra- diet unless they are being fed to occupy animals undergoing tions were retained in the large intestine longer, which when a weight reduction program. Some cereal straws contain combining evidence of similar digestibility, suggests pellet- high levels of silica, which has been implicated in the for- ing slowed the rate of fiber degradation in the hindgut. Cer- mation of urinary calculi (Nash, 1999). Forages high in sil-

FEEDS AND FEED PROCESSING 153 TABLE 8-4 Apparent Dry Matter, Organic Matter, Energy, Protein, and Fiber Digestibilitiesa of Various Hays in Horses DE (Mcal/ NDFD/ Type of Horse Type of Hay DMDb kg DM) OMDc CPDd GEDe (ADFD) f Source Mature mares Elephant grass 43 45 25 41 40 Almeida et al. (1999) Alfalfa 55 57 71 53 36 Coastgrass 50 51 56 48 63 Yearling Quarter Matua bromegrass 51 64 74 47 LaCasha et al. (1999) horse Coastal Bermudagrass 46 60 64 52 Alfalfa 63 74 83 24 Mature cross-bred Altai wildrye 47 2.03 54 44 50 Cymbaluk (1990a) geldings Bromegrass 48 2.12 51 45 44 Crested wheatgrass 42 1.82 29 40 41 Kentucky bluegrass 45 1.95 59 44 51 Oat hay 48 2.08 60 47 44 Reed canarygrass 38 1.58 52 38 34 Mature Arabians Alfalfa 58 73 47 Crozier et al. (1997) Tall fescue 48 67 44 Caucasian bluestem 44 43 41 Mature ponies Alfalfa 62 2.58 77 41a Cymbaluk and Christiensen Oat 55 2.33 68 37a (1986) Bromegrass 51 2.13 67 39a Slough 43 1.75 57 42a Mature ponies Chopped alfalfa 53 2.25 51 64 49 29 Hyslop and Calder (2001) Meadow hay 36–44 1.4 36–40 20–29 33–33 40–41g Moore-Colyer and Longland (2000) McClean et al. (2000); Hale and Moore-Colyer (2001) Mature ponies Dehydrated grass 47 2.3 50 62 50 51 Hyslop et al. (1998a) Mature meadow hay 0.41 2.17 44 48 45 40 Mature ponies Mature threshed grass hay 30 1.49 30 31 33 28 Hyslop et al. (1998b) Mature horses Alfalfa 56 75 Van der Noot and Timothy orchardgrass 49 54 Gilbreath (1970) Bromegrass 49 60 Mature horses Hay > 120 g CP/kg 56 68–75 Hintz (1969); Van der Noot Hay 81–119 g CP/kg 49 54 and Gilbreath (1970) in Hay < 80 g CP/kg 38–49 10–43 Chenost and Martin-Rosset (1985) aDigestibilities are expressed as percentages. bDMD = dry matter digestibility. cOMD = organic matter digestibility. dCPD = crude protein digestibility. eGED = gross energy digestibility. fNDFD/ADFD = NDF or ADF digestibility. gTotal nonstarch polysaccharides (NSP) digestibility. ica are known to be of reduced digestibility for a number of was found between development of colic and either type of livestock species (Laca et al., 2001). dried forage or frequency of feeding (Traub-Dargatz et al., Although there have been a number of studies on the role 2001). However, other studies have reported forage type, of dried forage in the elicitation of colic, the evidence is con- form, or a change in dietary forage to influence colic risk tradictory. Thus, in a study in the United States of 21,800 (Tinker et al., 1997; Cohen et al., 1999; Hudson et al., 2001; horses across 28 states on 1,026 premises, no association Hillyer et al., 2002; Little and Blickslager, 2002). Bearing in

154 NUTRIENT REQUIREMENTS OF HORSES mind such equivocal findings, it may be prudent to introduce relatively low in aeroallergens (Vandenput et al., 1997). In a new feeding regimens, forage types, or batches gradually survey of 99 silages (mean DM 631g/kg ± 137) and 53 hays over time. fed to a total of 323 horses, Coenen et al. (2003) did not de- Other problems associated with feeding hay include the tect any problems of intake or nutrient digestibility associ- elicitation of respiratory compromise. Even well-made and ated with feeding silage, and although average fecal DM stored hay, which to the naked eye appears “clean,” contains was significantly (P < 0.05) lower in the silage-fed horses considerable numbers of respirable particles (such as dust compared to those given hay, the differences were small and fungal spores), which can lead to the onset of allergic (188 vs. 203 g/kg, respectively). Furthermore fecal pH was respiratory disease in horses (Clarke, 1992; Robinson et al., significantly (P < 0.05) higher in the horses fed the silage 1995; Robinson, 2001). In an effort to prevent such prob- diets compared with those fed hay (pH 6.78 vs. 6.62). The lems, it is common practice to soak hay in water to reduce authors concluded that grass silage was suitable forage for the number of respirable particles; however, if soaked for horses, although silages can be relatively high in nonprotein more than 10 minutes, this operation results in the loss of nitrogen, which is of little nutritional value to horses (see considerable amounts of minerals, particularly sodium, Chapter 4). potassium, and phosphorus, and extended periods of soak- ing reduces WSC content (Moore-Colyer, 1996; Blackman Silage Intakes and Moore-Colyer, 1998), substantially reducing the nutri- ent content of the hay. Hay contaminated with nonforage High-DM grass silage (haylage) was well accepted by plant species can also present a hazard to equine health. ponies. Daily VDMI ranged from 61–98 g DM/kg BW0.75, During forage harvesting, poisonous weeds, which may nor- equivalent to 1.47–2.2 percent of BW (Morrow et al., 1999; mally be avoided in the fresh state by grazing equids, can Moore-Colyer and Longland, 2000; Bergero et al., 2002), as become incorporated into the swath. In some cases, drying shown in Table 8-5. It is of note that the highest haylage in- or wilting of such weeds renders them palatable to horses takes were by ponies in medium work compared with those without affecting their toxicity. One such example is the he- at maintenance or in light work (Bergero et al., 2002), which patotoxic weed, ragwort (Sennecio spp.); the repeated in- may suggest that nutrient requirements were influencing gestion of even small amounts of ragwort can prove fatal intakes. However, comparatively low-DM clamp (bunker) (Knottenbelt, 2000). It is therefore essential that fields des- silage was not so palatable, and VDMI was 38.8 g DM/kg tined for conservation as hay or silage are free of such BW0.75 d (0.92 percent BW/d) (Moore-Colyer and Long- weeds. land, 2000). McLean et al. (1995) reported that intakes of low-DM clamp grass silage were less than 50 percent of in- takes of hay, as were intakes of 32 percent DM maize (corn) Ensiled Forages silage (Martin-Rosset and Dulphy, 1987; Martin-Rosset et Forages may be conserved as silage when weather condi- al., 1987). However, low-DM cannot be the sole determinant tions are not sufficiently reliable for hay production. During for reduced intakes of ensiled forages by horses, as intakes ensiling, forage is preserved by anaerobic fermentation of of red-clover silage were 31 and 20 percent higher than for the NSC fraction to organic acids, resulting in a decline in hay and haylage, respectively, despite the red-clover silage pH usually from about pH 6 to 4.5. The acidity prevents the having a DM content of 268 g/kg vs. 852 and 371 g DM/kg growth of spoilage microorganisms. As a consequence of for hay and haylage, respectively (Hale and Moore-Colyer, fermentation of NSC fractions during ensilage, ensiled for- 2001). High-DM alfalfa silage was also found to be highly ages are low in NSC compared to the original sward. Silage palatable for ponies at 76.8 g DM/kg BW0.75 d (1.9 percent is classified according to DM content: low-DM silages are of BW/d) (Murray, 2004). With the exception of the low- approximately 30 percent DM, wilted silages are commonly DM grass and maize silages, it was concluded that ad libi- 30–40 percent DM, and high-DM silages (often referred to tum access to grass and legume haylage could lead to obe- as haylage) are usually between 40–65 percent DM. sity in ponies, and therefore intakes of such forages should There are few reports on feeding ensiled forages to be restricted. horses, as they have historically been regarded as unsuitable for horses due to (1) their acidity and perceived laxative ef- Digestibility of Ensiled Forages fects (Pillner, 1992) and (2) questionable hygienic quality, as silage can occasionally contain Listeria spp. or Clostrid- The digestibility of ensiled grass were substantially ium botulinum (the causal agent of botulism), to which higher than grass hays (Table 8-6). Thus, percentage di- horses are highly susceptible (Ricketts et al., 1984). How- gestibilities of DM, CP, and fiber for haylage and clamp ever, with the advent of high-DM-baled silage, there is often silage (in parenthesis) averaged 58 (66), 61 (67), and 60 insufficient moisture for the proliferation of Clostridia spp. (76), respectively, with the corresponding values for the and feeding haylage to horses is becoming increasingly pop- grass hay being 37, 25, and 41 (Morrow et al., 1999; Moore- ular. Especially as compared to hay, well-preserved silage is Colyer and Longland, 2000; Bergero et al., 2002). Thus the

FEEDS AND FEED PROCESSING 155 TABLE 8-5 Voluntary Dry Matter Intakes (VDMI) of Ensiled Forages by Ponies and Horses VDMI VDMI VDMI (g DM/kg (g DM/kg Type of Horse Type of Forage DM % (kg/d) BW/d) BW0.75 d) Source Ponies Meadow hay 92 4.95 14.7 62.9 Moore-Colyer and Longland (2000) Haylage 67 6.3 18.4 79.2 Big bale silage 50 5.96 17.3 74.6 Clamp silage 34 2.95 9.17 38.8 Meadow hay 85 5.5 Hale and Moore-Colyer (2001) Ponies Big bale grass silage 37 6.1 Big bale red-clover silage 27 7.2 Ponies Long or short chop grass silage 36 4.4 14.6 61 Morrow et al. (1999) Ponies at maintenance Grass haylage 55 6.5 85 Bergero et al. (2002) Ponies in light worka Grass haylage 63 6.7 20.5 87 Ponies in medium workb Grass haylage 65 7.6 22.3 98 Horses Maize silage 31 40.6 Martin-Rosset and Dulphy (1987) Ponies Alfalfa silage 30 19.3 76.8 Murray (2004) a10-min walk, 20-min trot, and 5-min gallop per day. b40-min walk, 80-min trot, and 20-min gallop per day. DE and digestible crude protein (DCP) values of the hay theoretical DE and DCP maintenance requirements of were 1.38 Mcal/kg DM and DCP 6 g/kg DM, respectively, ponies fed the haylages were exceeded by up to 1.7- and 2.2- while the corresponding average values for the haylages fold for DE and DCP, respectively, but the DE and DCP in- were 2.28 Mcal/kg DM and 54 g DCP/kg DM (Moore- takes of ponies fed a poor-quality hay were only 0.9 and 0.2 Colyer and Longland, 2000). These authors reported that the of their requirements. Although the clamp silage was of high TABLE 8-6 Apparent Dry Matter, Organic Matter, Protein, and Energy Digestibility of Ensiled Forages by Ponies TNSPD/ Horse Type Type of Forage DMDa OMDb CPDc GEDd CFDe NDFD f Source Welsh ponies Grass hay 39 40 20 33 41 Moore-Colyer and Welsh ponies Grass haylage 57 57 48 52 45 Longland (2000) Welsh ponies Big bale silage 61 62 66 55 67 Welsh ponies Clamp silage 67 67 68 55 76 65 Ponies at maintenance Haylage 58 57 77 52 48 Bergero et al. (2002) Ponies in light workg Haylage 63% DM 55 55 57 50 45 Ponies in medium workh Haylage 65% DM 49 49 60 50 33 44 Ponies Hay 36 36 29 Hale and Moore- Ponies Big bale grass silage 69 70 68 Colyer (2001) Ponies Big bale red-clover silage 74 74 80 Ponies Long or short chop grass silage 67 74 62 Morrow et al. (1999) Ponies Alfalfa silage 62 60 76 59 55/49 Murray (2004) NOTE: Digestibilities are expressed as percentages. aDMD = dry matter digestibility. bOMD = organic matter digestibility. cCPD = crude protein digestibility. dGED = gross energy digestibility. eCFD = crude fiber digestibility. fTNSPD/NDFD = total nonstarch polysaccharides (NSP) digestibility or neutral detergent fiber (NDF) digestibility. g10-min walk, 20-min trot, and 5-min gallop per day. h40-min walk, 80-min trot, and 20-min gallop per day.

156 NUTRIENT REQUIREMENTS OF HORSES nutritive value with a DE of 2.87 Mcal/kg DM and 104 g idly growing plants. When the plant tissues are stressed, DCP/kg DM, intakes were only just sufficient to meet the such as through frost, drought, or mechanical injury, free maintenance energy requirement of the ponies. The di- cyanide can be released. gestibility of red-clover silage was significantly greater than Some varieties of white clover are also cyanogenic, and either grass silage or hay with a DE value of approximately these varieties should be avoided in horse pastures (Clark et 3 Mcal/kg DM and 154 g DCP/kg DM. Intakes of red-clover al., 1990). Acute ingestion of significant amounts of cyanide silage by ponies exceeded their theoretical energy and pro- can lead to rapid death due to respiratory paralysis, and con- tein maintenance requirements by 2.7- and 5.5-fold, respec- sumption of low levels over a prolonged period can cause tively (Hale and Moore-Colyer, 2001). There is clearly a abortion in mares, a staggering gait, cystitis, and weight loss need to restrict intakes of such highly nutritious forms of (Adams et al., 1969; Turner and Szczawinski, 1991). Alsike conserved forage. In a survey of 239 horses fed high-DM clover (Trifolium hybridum) contains hepatotoxic alkaloids. (average 63 percent DM, 12.4 percent CP) grass haylage and Consumption by horses may result in photosensitization of 147 fed hay (85 percent DM, 10 percent CP), Coenen et al. unpigmented skin, loss of condition, neurological distur- (2003) reported that grass silage yielded sufficient nutrients bance, and eventual hepatic failure (Cooper and Johnson, to meet the requirements of most classes of horses except 1984; Cheeke and Schull, 1985). There is a widely held be- lactating mares. lief that horses ingesting fresh or conserved forages con- taining high levels of nitrates can suffer from nitrate poison- ing. High levels of herbage nitrate can be caused by Potential Problems of Feeding Ensiled Forage excessive application of nitrogen fertilizer or when plants The hygienic quality of ensiled products for horses is have been subject to cool temperatures, low light intensity, paramount, as horses do not possess the ability of ruminants or water stress (Allison, 1998). Pasture species known to ac- to metabolize certain toxins. Care should be exercised to cumulate nitrates include barley, bromegrass, corn, Johnson ensure that no soil or animal carcasses are ensiled with the grass, oat hay, orchardgrass, rape, sorghum, sweet clover, forage as this can lead to proliferation of the bacterium and wheat. Nitrates remain stable in well-cured hay, but in Clostridium botulinum, the causal agent of botulism that can moist hay, microbial action can convert nitrate to the more be fatal in horses (Ricketts et al., 1984). Furthermore, en- toxic nitrite. Nitrates, however, are substantially reduced in siled products that have been subject to aerobic spoilage ensiled forages. As nitrates tend to accumulate in plant (molding) should not be fed. bases, harvesting herbage with a raised cutting height may help reduce the nitrate content of conserved forages. Alfalfa hay grown in soils high in selenium may accrue selenium Anti-Quality Factors in Forages levels up to 50 ppm (Davies et al., 2004). Ingestion of such Although all of the pasture species described in Chapter hay in horses can lead to selenium toxicosis. 11 have been fed to horses, some may contain substances that are injurious to health and performance. These anti- Contamination by Insects quality factors may be inherent to the forage, or they may be the result of microbial or insect contamination of fresh or Alfalfa hay can contain blister beetles (Epicauta vittat conserved forages. In many cases, only very small amounts and E. pennsylvanica), which produce cantharidin that is of anti-quality factors are required to exert an unfavorable highly toxic to horses and can be fatal (Schoeb and Panciera, effect on horse health. Some of these anti-quality factors are 1978). described below. Microbial Contamination Inherent Plant Anti-Nutrients Bacteria High oxalate and phytate levels in some warm-season grasses including kikuyu grass or setaria may cause calcium The bacterium Clostridium botulinum, the casual agent and phosphorus deficiency in horses, leading to osteodys- of botulism, can be found in ensiled forages that are con- trophia fibrosa (ODF) (Elphinstone, 1981; Williams, 1987), taminated with soil or small animal carcasses. Botulism in and calcium and phosphorus supplements should be pro- horses is frequently fatal (Ricketts et al., 1984; Hunter et al., vided for growing or lactating mares grazing such pastures 2002). Listeria monocytogenes, the causal agent of listerio- (McKenzie et al., 1981), as detailed more fully in Chapter sis in horses, may also be found in poorly preserved silages 14. Sorghum, Sudan grass, hybrid Johnson grass, and and was associated with listeriosis in a number of Icelandic sorghum-Sudan grass hybrids are not recommended for ponies that had consumed spoiled grass silage contaminated horses as they contain varying levels of cyanogenic glyco- with the bacterium (Gudmundsdottir et al., 2004). sides, with particularly high levels observed in young rap-

FEEDS AND FEED PROCESSING 157 Fungi taining to effects in horses. Certain strains of Stachybotrys spp. that are common saprophytes of improperly dried hay Contamination of forages by fungi depends upon a num- and straw produce trichothecene satratoxins, which cause ber of factors, including the health and physiological status stachybotrytoxicosis, a fatal hemorrhagic disorder in horses of the plant, harvesting methods, weather conditions during (Forgacs, 1965, 1972). Cereal straws may be contaminated harvest, speed of stabilizing the conserved forage, and am- with the tricothecene deoxynivalenol (DON), known to bient conditions during transport and storage. Fungi may be cause a range of severe disorders in many livestock species, surface contaminants of herbage or may invade plant tissues although the reported effects of this mycotoxin on horses is and cause disease (plant pathogens) or reside within the an area of debate. Reports range from no apparent effect on plant without causing any immediate, overt negative effects barren mares or geldings when fed over 40 days (Johnson et (endophytes) (Bacon and White, 2000). Contamination of al., 1997) through reduced intake (Raymond et al., 2003) to forages with any of the above classes of fungi can reduce being implicated in colic cases in a retrospective study by both their palatability and intake. It may result in some form North Carolina State University. However, the presence of of mycotoxicosis, the degree of which depends upon the other mycotoxins in the feed of the latter two studies make amount and duration of mycotoxin ingestion. The cumula- it difficult to ascribe a strict cause-and-effect relationship tive effects of ingestion of low levels of mycotoxins may between DON and unfavourable effects on horses. possibly contribute to a gradual deterioration of equine Properly ensiled forages (whereby near-anaerobic condi- health and performance. As a general rule, feeding forages tions have been achieved coupled with a rapid decline in pH) contaminated with fungi should be avoided. should be relatively free of fungal contaminants. However, Fresh pastures in Europe have been found contaminated where conditions are aerobic and pH reduction is compro- with Claviceps, Pythomyces, Neotyphodium, and Rhizocto- mised, fungal contamination may occur, with the resultant nia spp. (Le Bars and Le Bars, 1996). Claviceps spp. production of a number of mycotoxins (Dutton et al., 1984). produce sclerotia, termed ergots, and infect a number of Patulin was found in the majority of corn silages tested in a gramminaceous forages, including species of ryegrass, study in France (Escoula, 1977) and ochratoxin A and cit- canarygrass, dallisgrass, and various native species. Ergots rinin (nephrotoxic in many species) have been found in corn produce a range of toxic alkaloids, such as ergotamine, silages and hays (Carlton and Tuite, 1977). Penicillium which may result in death within hours of ingestion roqueforti varieties that can produce patulin, botryo- (Wilcox, 1899). Ergots are found on seed heads, and thus in- diploidin, and penitrem A were the main contaminant of Eu- gestion can be avoided by ensuring pastures are regularly ropean grass, sugar-beet, and corn silages (Nout et al., 1993). clipped or tightly grazed to prevent seed-head formation. Although no reference to the activity of such mycotoxins in Fusarium spp. are frequently associated with fresh pastures horses has been found, they have been reported to be injuri- and, under appropriate environmental conditions, may pro- ous to cattle. Aspergillus, Penicillium, and Fusarium spp. duce tricothecenes zearolone and/or fumonisins (Scudamore have all been found present in silage 2–3 months old, and Livesey, 1998; Yiannikouris and Jouany, 2002), the latter whereas Byssochlamys nivea, a patulin producer, appeared being known to cause blind staggers (equine leuco- after 6 months of silage storage (Le Bars and Le Bars, 1996). encephalomalacia) and death in horses. Red clover or alfalfa Improperly conserved yellow sweet-clover (Melilotus offici- infected with Rhizoctonia leguminicola may contain nalis) or white sweet-clover (Melilotus alba) hay, haylage, or slaframine, resulting in excessive salivation and increased silage may be contaminated with Penicillium spp., which can water consumption by horses. Slaframine breaks down with convert nontoxic coumarin to the toxin dicoumarol. Symp- storage; levels were reported to decline from 100 mg/kg to toms of dicoumarol poisoning include nasal bleeding, joint 7 mg/kg after 10 months (Hagler and Behlow, 1981). swelling, lameness, and respiratory difficulties (Hendrix, Hay produced and stored under humid conditions may 2003). Dicoumarol levels are often higher in large round contain a wide range of fungal contaminants in addition to bales where the hay tends to be of lower dry matter than in Fusarium spp. Thus, Asperigillus, Stachybotrys, and Peni- small, rectangular bales. Forages may harbor a number of cillium spp. may all be found in insufficiently dried hay or fungal contaminants at any one time, and mycotoxins have straw, and, as a result, these conserved forages may contain been reported to act in synergy in some species. Thus, an array of mycotoxins including aflatoxins and patulin, in fusaric acid enhanced fumonisin activity in poultry (D’Mello addition to those produced by Fusarium spp. (Le Bars, 1976; and MacDonald, 1997), and diacetoxyscirpenol acted in syn- Clevstroem et al., 1981; Scudamore and Livesey, 1998). ergy with trichothecenes in pigs (Smith et al., 1997). It is Aflatoxins cause reduced feed intakes, weight loss, hepatic conceivable that similar synergistic mechanisms between disease, and brain, kidney, and heart damage in horses mycotoxins may also occur in horses. (Hintz, 1990). Patulin has been shown to be carcinogenic in laboratory animals and symptoms may include neurological disorders (Riley, 1998), but there is little information per-

158 NUTRIENT REQUIREMENTS OF HORSES Endophyte Contamination 1986; McGreevy et al., 1995; Redbo et al., 1998), and be- havioral problems (Gillham et al., 1994). It is known that The toxic endophyte Neotyphodium coenophialum (for- oversupply of starch or other rapidly fermentable carbohy- merly Acremonium coenophialum), an inhabitant of tall fes- drates can result in lactate acidosis, hindgut dysfunction, cue in U.S. pastures, is a common problem, due to produc- colic, and laminitis in horses (Garner et al., 1977; Pollit et tion of the alkaloid ergovaline, causing late abortion, al., 2003). Potter et al. (1992a) recommended a maximum prolonged gestation, dystocia, and agalactia in broodmares of 4.0 g starch/kg BW/meal to prevent such disorders, and reduced growth in young horses (Hoveland, 1992; whereas Meyer et al. (1993) were more conservative in Aiken et al., 1993). In the United States, the threshold levels their recommendation of a maximum of 2 g starch/kg of ergovaline in horse feeds are 0.3–0.5 mg/kg. Varieties of BW/meal to prevent hindgut dysfunction. Even the latter tall fescue are now available that are inhabited by nontoxic recommendation for maximum amounts of starch to be fed strains of endophyte; however, tall fescue containing the per meal may be somewhat high for some equids, as even toxic form of the endophytes may be more hardy and can 2.1 g starch/kg BW/meal from a hay-cube:rolled-barley diet reestablish in pastures. Perennial ryegrass infected with the (50:50) was sufficient to elicit unfavorable changes in in- endophyte Neotyphodium lolii produces the toxic alkaloid tracecal fermentation in ponies (McClean et al., 2000). lolitrem B, which causes ryegrass staggers (Stynes and However, the undesirable effects of high starch intakes (3.4 g Bird, 1983). Although there is no threshold set for lolitrem starch/kg BW/meal) on hindgut function were reduced B concentrations in horse fodder, two groups of horses were when the diet contained an NDF:starch ratio of 1:1 (Med- observed to suffer ataxia, tremors, and paralysis when ina et al., 2002). Keenan (1986) reported that young horses fed ryegrass hay containing 5–6 mg lolitrem B/kg (Sloet van that grazed on lush, low-fiber pastures indulged in substan- Oldruitenborgh-Oosterbaan et al., 1999), and ponies fed a tial bark-stripping. Lactating mares grazing low-DM, low- sole diet of ryegrass seed cleanings containing 5.3 mg fiber pastures avidly consumed supplemental high-fiber lolitrem B/kg were symptomatic of ryegrass staggers. feedstuffs (Marlow et al., 1983). The incidence of horses Good-quality hay should be free of mold, dust, and performing oral stereotypies, such as wood-chewing, was weeds. Contamination of hay and straw by fungi can be re- reduced when long hay, as opposed to a pelleted diet, was duced by drying to at least 85 percent DM. Where such con- fed (Willard et al., 1977) and increasing the amount of hay ditions cannot be achieved, the use of antifungal agents such and decreasing the amount of sweet feed resulted in de- as propionic acid to reduce the pH to levels below that tol- creased incidence of crib-biting (Gillham et al., 1994). Ex- erated by most contaminating fungi can be employed. Dur- tended feeding time and greater buffering of gastric con- ing ensilage, ensuring anaerobic conditions and enhancing a tents (due to increased saliva production) have been rapid decline in pH immediately post-harvest by the use of implicated in the reduction of undesirable oral behaviors various biological inoculants and/or enzyme preparations through increased provision of long forage. Certainly, sur- can help reduce levels of fungal contaminants. Other bio- veys have shown that the risk of crib-biting is increased by logical strategies for reducing the effects of mycotoxins in- low-forage or high-starch diets (McGreevy et al., 1995; clude use of bacterial strains that limit mycotoxin bioavail- Redbo et al., 1998), and foals fed concentrates at weaning ability (El-Nezami et al., 1988; Yoon and Baeck, 1999) or were four times more likely to crib-bite than those that were those that metabolize or biotransform them (Nakazato et al., not (Waters et al., 2002; Bachmann et al., 2003 ). Bach- 1990). These latter methods are as yet somewhat slow and mann et al. (2003) concluded that management strategies to inefficient, and methods for inoculating feedstuffs with non- prevent development of stereotypic behaviours in horses in- mycotoxin-producing strains to outcompete those that pro- cluded providing diets high in fiber with minimal amounts duce toxins have been areas of investigation (Cotty and of concentrates. Reduced gastric pH, elicited by a combi- Bhatnagar, 1994). Use of forage varieties bred for resistance nation of high-concentrate/low-forage diets and long peri- to fungal contaminants is a more direct and potentially effi- ods of fasting between feeds, may be related to the inci- cient approach to reducing the incidence of mycotoxicosis dence of gastric ulcers in horses. Endoscopic examination from forages (Yiannikouris and Jouany, 2002). of racehorses in training receiving high levels of cereals in- dicated that more than 80 percent of the animals had sig- Forage/Fiber Requirement nificant gastric ulceration (Hammond et al., 1986). How- ever, when such racehorses were turned out to pasture for 1 There are no reports of trials that prove a direct require- month or more, the incidence of ulcers decreased to 52 per- ment for forage/fiber in horse diets. Nevertheless, fibrous cent (Murray et al., 1989). Gastric ulcers are rare in horses forages clearly form the basis of the diet of wild equids, and maintained solely on pasture, and the gastric pH of horses there is a large body of circumstantial evidence that sug- with ad libitum access to timothy hay was significantly gests that insufficient dietary fiber in equid diets can lead to higher than in horses that had been fasted (Murray and hindgut acidosis (Medina et al., 2002), colic ( Tinker et al., Schusser, 1989). Such evidence, although not conclusive, 1997), gastric ulcers (Murray and Schusser, 1989), in- strongly suggests that diets high in dietary fiber are benefi- creased risk of crib-biting and wood-chewing (Keenan, cial to horses. The NRC (1989) recommended that equid

FEEDS AND FEED PROCESSING 159 diets should contain no less than 1 percent of BW as forage (byproducts or co-products) of these processes may be in- (DM) per day. Indeed, the necessity for feeding fibrous for- corporated into feeds for horses as energy sources. Hominy ages is implicit in redressing the energy deficit in diets of feed is produced during the manufacture of corn grits or less than 2 g starch/kg BW/meal, as exclusive use of oil corn meal for human use and is sometimes called corn grits and/or protein in this regard would be unpalatable on the byproduct. Hominy feed usually contains at least 4 percent one hand and impractical, energetically inefficient, and en- fat and is relatively low in fiber. Protein quantity and quality vironmentally unsound on the other. are relatively low, but the energy value is at least compara- ble to barley or oats. Corn gluten feed, corn gluten meal, and corn distillers’ dried grains are the residues from processing GRAINS AND GRAIN BYPRODUCTS or distilling. These products are much higher in protein and A wide variety of grains and grain byproducts may be fiber than corn, and much lower in starch. Distillers’ dried used as feeds for horses, usually to increase the energy den- grains have been shown to be palatable to horses (Pagan and sity of horse diets. Grains and their byproducts vary in pro- Jackson, 1991b). tein content and quality, but are usually low in sodium and On a volume basis and on a weight basis, oats are much calcium. The calcium:phosphorus ratio of grains and grain less energy-dense than corn. The bushel (bu) weight of corn byproducts may be 1:3 or wider. Approximately 60 percent will be greater than 50 lb/bu, whereas the bushel weight of of the phosphorus in oats, corn, and barley is contained in oats will often be less than 40 lb/bu. Bushel weight can be phytate (Ravindran, 1996). These characteristics do not re- affected by the variation in the ratio of hull to kernel. When late directly to the value of grains as energy supplements, but the proportion of hull increases, the bushel weight de- they affect the quantity and type of other feeds that must be creases. In addition, the oat hull contains high levels of cel- included in the diet to provide a balanced ration. Variation in lulose and xylan, which are much less digestible than the the nutrient content of grains can arise from differences in starch in the kernel. Therefore, as the proportion of hull in- soil fertility and growing conditions. In general, the compo- creases, the energy value of the oats decreases. Oats contain sition of grains is less variable than the composition of for- more fiber and less starch than corn. However, oat starch has ages. However, because processing methods can vary from been reported to be more readily digested than corn starch in mill to mill, the nutrient composition of a byproduct feed is the small intestine (Radicke et al., 1991). Therefore, if subject to much greater variation than the composition of a grains are fed at high intakes, the use of oats may result in whole grain. Nevertheless, byproduct feeds are often eco- less overflow of starch to the large intestine. Oats also tend nomically priced, and they can have many nutritional char- to be higher in protein, lysine, and fat than corn, but like acteristics that make them effective as horse feeds. corn, oats have an inverted calcium:phosphorus ratio. Oats In the United States, grains may be marketed on the basis are palatable to horses, but the rate of consumption of oats of grade. The grading criteria vary for each grain, but char- is often much less than for textured sweet feed or pelleted acteristics such as bushel weight, percentage of damaged feed (Harbor et al., 2003). White oats are the most common kernels, and the presence of foreign material are considered. type grown in the United States, but red oats and gray oats A higher grade is associated with higher bushel weight, are also available. High fat (12–15 percent oil) varieties of fewer damaged kernels, and less foreign material. oats have been produced, termed “naked oats,” which thresh free of the hull and are higher in energy than traditional oats (Valentine, 1999). Oats are often fed whole, crimped, or Common Grains and Grain Byproducts Fed to Horses rolled. Oats may be cleaned to remove loose hulls and con- Corn (maize) is often an economical energy source for taminating material. When the hull is removed from the oat, horse feeds. Dent corn is the most common type of corn the remaining product is referred to as the groat. Oat groats used in animal feeds, whereas sweet corn and popcorn are are used primarily in the production of human foods, but usually grown for human consumption. Corn is frequently some may be incorporated into horse feeds. They are higher fed cracked, rolled, or flaked, and is highly palatable. Corn in starch, fat, and energy than whole oats. Byproducts of oat is lower in CP and lysine, but higher in DE, than oats or bar- milling that are available for animal feeds include feeding ley (Table 16-6). Corn has a high bushel weight, and it is oat meal, oat mill byproduct (oat mill feed), and oat hulls. higher in starch and lower in fiber than barley or oats (Table Oat hulls are high in NDF and ADF and can be used to 2-2). The small intestinal digestibility of the starch in whole increase the fiber content of a diet, but are only marginally corn is lower than for the starch in whole oats (Radicke et digestible, so they provide little energy to horses. Feeding al., 1991), although some types of processing will improve oat meal is relatively low in fiber and contains pieces of the small intestinal availability of corn starch. The effects of oat groats and some oat flour. Oat mill feed is a byproduct starch source and grain processing on starch digestibility are intermediate to feeding oat meal and oat hulls in fiber con- discussed in detail later in this chapter. tent and contains some pieces of oat groats as well as some Corn is often processed to produce ingredients used in oat hulls. the manufacture of food for human consumption, such as Barley is an important energy feed worldwide. It is the corn oil, corn starch, and corn syrup. Many of the residues basic unit of energy for the French horse feeding standards.

160 NUTRIENT REQUIREMENTS OF HORSES The DE, CP, starch, and lysine content of barley is interme- is usually incorporated into horse feeds because it is palat- diate to oats and corn (Table 16-6 and Table 2-2). Using able, helps to reduce dust, and may also aid in preventing ileally fistulated animals, Meyer et al. (1993) reported that sifting of ingredients in a mixed feed. Typical inclusion rates the preileal digestibility of starch in rolled barley was ap- of molasses vary, but are usually less than 10 percent of a proximately 21.4 percent, compared to 85.2 percent for oats. concentrate. On a DM basis, molasses contains 4 to 6 per- In addition, de Fombelle et al. (2004) reported that small in- cent potassium (Table 16-6) and 62–90 percent NFC (Table testinal digestion of barley starch was lower than oat starch 2-2). Under some circumstances molasses may be blended (87.4 percent vs. 99.8 percent, respectively, in horses fed a with other liquids; for example, a low level of fat may be high-starch diet). De Fombelle et al. (2004) used a mobile added to molasses that will be incorporated in textured feeds nylon bag technique and the substrates were ground, which (sweet feeds) to enhance the appearance of the feed and de- may have resulted in the high small intestinal starch di- crease caking in cold weather. gestibilities. Barley is the primary grain used in the brewing Another byproduct of the sugar industry is beet pulp. industry and brewers’ dried grains are often incorporated Dried beet pulp is available with or without added molasses. into animal feeds. As with distillers’ dried grains, brewers’ The addition of molasses increases the sugar and potassium dried grains tend to be higher in protein, higher in fiber, and content of beet pulp. Beet pulp has a higher water holding lower in starch than whole grains. Hordenine (N,N- capacity than hay cubes or soy hulls (Moore-Colyer et al., dimethyltyramine) may be found in some barley ingredients, 2002) and may be fed moistened. Several research studies particularly in barley sprouts (Schubert et al., 1988). Horde- have fed diets containing sugar-beet pulp to horses (Harris nine originating from reed canarygrass or barley sprouts has and Rodiek, 1993; Crandell et al., 1999; Warren et al., 1999; been found in urine samples collected for drug-testing pur- Palmgren-Karlsson, 2002). Beet pulp is usually used as a poses in horses (Irvine, 1988; Sams, 1997). component of mixed concentrate feeds; however, Harris and Most wheat grown in the United States is used for human Rodiek (1993) fed diets consisting of 45 percent beet pulp consumption, so the inclusion of wheat grain in horse diets and 55 percent alfalfa pellets without negative effects, and is somewhat uncommon. However, wheat byproduct feeds Warren et al. (1999) fed horses a diet containing 55 percent are often used in horse feeds, wheat middlings (midds) beet pulp. being one of the most common. Because most of the flour Moore-Colyer et al. (2002) reported a total tract disap- has been removed, wheat midds are higher in fiber and pro- pearance of 85 percent for unmolassed sugar-beet pulp using tein, but lower in energy than wheat grain (Table 16-6). a mobile nylon bag technique in ponies. This estimate of dry Wheat midds may contain more than 1 percent phosphorus matter digestibility for beet pulp compares well with the re- (Table 16-6). Approximately 80 percent of the phosphorus sults of Harris and Rodiek (1993). In their study, dry matter in wheat midds is found as phytate (Ravindran, 1996). Be- digestibility of a beet pulp/alfalfa pellet diet was 72 percent cause of the high phosphorus content, calcium supplemen- compared to 63 percent for the alfalfa pellets alone. Palm- tation is usually necessary when wheat midds constitute a gren-Karlsson (2002) found that replacing 40 percent of the significant portion of the diet. Due to their fine texture, oats in a hay-oat diet with a combination of molassed beet wheat middlings are not easily fed alone; however, they are pulp and dried brewers’ grains (86:14 ratio) did not affect commonly used in pelleted feeds. Wheat bran is higher in the energy digestibility of the diet. These studies suggest ADF and NDF than wheat midds, and it is also high in phos- that the energy value of beet pulp is higher than alfalfa pel- phorus. Other wheat byproducts that may be incorporated in lets and possibly as high as oats. Pagan (1998) reported a horse feeds include wheat mill run, wheat shorts, and red DE value of 2.8 Mcal/kg for beet pulp on an as-fed basis. dog. Wheat mill run is a combination of bran and midds. However, the beet pulp used in nutrition studies with horses Wheat shorts and red dog contain slightly more flour than has been very diverse. For example, Moore and Colyer et al. wheat midds. (2002) studied the digestibility of sugar-beet pulp contain- Other grains that can be fed to horses include rye and ing 54.7 percent NDF and 7.8 percent CP (DM basis), while sorghum grain. Sorghum grain is very small in size, and it is Palmgren-Karlsson (2002) fed beet pulp containing 23.6 commonly processed prior to incorporation in horse diets. percent NDF and 11.5 percent CP. In Table 16-6 the DE con- The nutrient composition of sorghum is similar to corn, al- tent (2.8 Mcal DE/kg DM) of beet pulp was calculated using though it may be slightly lower in energy value. Rye is lower an equation that was derived from feeding experiments that in palatability than most other cereal grains, but the chemi- primarily evaluated forages and grains. It is possible that this cal composition is similar to oats. Rye may be contaminated equation underestimates the DE content of beet pulp. The with ergot (see section Mycotoxins in Grains). nonstarch polysaccharide and NDF fractions of beet pulp appear to be more digestible than those found in hay (Moore-Colyer et al., 2002). Other Byproduct Feeds Citrus pulp has also been fed to horses (Ott et al., 1979b). Molasses is a byproduct of sugar manufacturing and is Palatability was low when citrus pulp was incorporated at a usually produced from sugar cane or sugar beets. Molasses rate of 30 percent of a coarse concentrate but was considered

FEEDS AND FEED PROCESSING 161 adequate when the citrus pulp constituted 15 percent of a toxin contamination of grains (CAST, 2003), although no pelleted concentrate. Citrus pulp is somewhat more readily single combination of conditions can define mycotoxin pro- fermented in vitro than beet pulp (Sunvold et al., 1995). Ott duction potential. It is possible that the concentration of et al. (1979b) suggested that the DE content of citrus pulp some mycotoxins in grains may increase during storage was slightly higher than the DE content of pulverized oats. (Bacon and Nelson, 1994). Temperature and moisture con- Soybean hulls (soyhulls) are a byproduct of the soybean ditions during harvesting and storage can promote myco- processing industry. Soyhulls contain 53–70 percent NDF toxin production along with insect infestation and mechani- and less than 3 percent starch (Table 2-2). Coverdale et al. cal damage to grain kernels. Mycotoxins may depress feed (2004) used soyhulls to replace up to 75 percent of the diet intake, impair growth, or cause disease in animals; however, of mature horses receiving alfalfa/bromegrass hay and found the signs are not consistent among species. Many factors no effect on apparent total-tract dry matter digestibility. The may influence the susceptibility of an animal to a specific NDF and ADF concentration in the soyhulls (60.6 and 43.7 mycotoxin, including animal species, age, general health, percent, respectively) was similar to the NDF and ADF con- and immune status (Hollinger and Ekperigin, 1999). centration in the hay (58.1 and 39.1 percent, respectively). The Fusarium spp. produce a variety of mycotoxins in- Soyhulls containing 59.1 percent NDF and 43.8 percent cluding zearalenone, deoxynivalenol (vomitoxin), T-2 toxin, ADF had a higher total-tract dry matter digestibility than and fumonisin B1, B2, and B3. Fusarium spp. can affect the hay cubes containing 62.3 percent NDF and 35.5 percent plant during growth, causing stalk and ear rot in corn, and ADF (Moore-Colyer et al., 2002). Using cecal inocula, Bush scab or head blight in other plants (Whitlow and Hagler, et al. (2001) reported that the in vitro dry matter disappear- 2002). Zearalenone, deoxynivalenol, and T-2 toxin can be ance of soyhulls was higher than for low-quality alfalfa, but found in a variety of feedstuffs, whereas the fumonisins are lower than oats (Bush et al., 2001). Pagan (1998) reported a usually associated with corn or corn byproducts. There is DE value for soyhulls of 2.6 Mcal/kg (as-fed basis), which great variation in the susceptibility of different animal would be similar to immature legume hay. As with beet species to the Fusarium spp. mycotoxins. Swine are affected pulp, the equation for estimating DE may underestimate the by concentrations of deoxynivalenol and zearalenone as low DE content of soyhulls. The feeding value of almond hulls as 1 ppm and 0.1 ppm, respectively (Whitlow and Hagler, has also been investigated (Clutter and Rodiek, 1991). Al- 2002). By comparison, feed consumption was not affected mond hulls (22 percent ADF) were well accepted by mature when horses were fed barley containing more than 36 ppm horses and increased dry matter digestibility of an alfalfa deoxynivalenol (Johnson et al., 1997). Other studies have re- and oat hay ration. ported that grain intake by horses was affected when the Although rice is not commonly incorporated into horse grain contained a mix of mycotoxins (15 ppm deoxyni- diets in the United States, rice bran may be used. Rice bran valenol and 2 ppm zearalenone, or 11 ppm deoxynivalenol contains approximately the same amount of NDF as oats, but and 0.8 ppm zearalenone), but no effects on animal health it is relatively high in crude fat (19–28 percent: DePeters et were noted (Raymond et al., 2003, 2005). al., 2000). Rice bran contains a moderate amount of nonfiber Equine leucoencephalomalacia, often referred to as blind carbohydrates, including starch (DePeters et al., 2000). Un- staggers, or moldy corn poisoning has been reported to occur less it has been combined with other ingredients, rice bran in horses fed diets containing corn or corn byproducts con- may have an inverted calcium:phosphorus ratio. Rice bran taminated with Fusarium verticilliodes (syn. moniliforme) can contain a naturally occurring lipase that can be inacti- (Vesonder et al., 1989; Wilson et al., 1990a.). Toxicosis is be- vated by some types of processing. Rice bran that is pro- lieved to result from the consumption of high levels of fu- cessed soon after milling in order to inactivate this lipase is monisin B1 (Wilson et al., 1990b; Ross et al., 1991), although referred to as stabilized rice bran. Stabilization reduces the fumonisin B2 and B3 occur in close association with fumon- potential for rancidification. According to AAFCO (2005), isin B1 (Murphy et al., 1993). Horses affected by leucoen- the free fatty acid content of crude fat in rice bran must be cephalomalacia may exhibit facial paralysis and ataxia. Mor- less than 4 percent. Like rice bran, whole soybeans are a high- bidity and mortality are high (Ross et al., 1991). Equine fat feed ingredient that may be incorporated into horse feeds. leucoencephalomalacia has been produced experimentally Whole soybeans may be roasted or extruded prior to incor- by feeding corn screenings known to be contaminated with poration to inactivate naturally occurring trypsin inhibitors. fumonisin B1 and B2 (Wilson et al., 1992; Ross et al., 1993). Ross et al. (1993) found that susceptibility to toxicosis was variable among ponies and suggested that animals with im- Mycotoxins in Grains paired liver function had increased susceptibility. The Food Under some conditions, cereal grains or cereal grain and Drug Administration recommends 5 ppm as the upper byproducts may contain mycotoxins. The concentrations limit for fumonisin in corn and corn byproducts intended for and types of mycotoxins found in grains can vary greatly. equine consumption, with the stipulation that the contami- An interaction between environmental temperature and nated feed will not constitute more than 20 percent of the diet moisture is an important determinant of preharvest myco- (FDA, 2001a).

162 NUTRIENT REQUIREMENTS OF HORSES Fumonisin contamination of corn is variable and may be Another contaminant that may be found in some cereal influenced by growing conditions. Surveys in 1995 and grains is ergot. An ergot is a small brown mass (sclerotium) 1996 found that 6.9 and 3.9 percent, respectively, of samples that is produced by molds in the Claviceps genus. Ergots from the preharvest U.S. corn crop contained more than contain ergot alkaloids that may cause neurological, be- 5 ppm fumonisin (APHIS, 1995, 1996). Fumonisin contam- havioral, vascular, and reproductive effects in animals. In ination of corn is more likely to occur when the crop has horses, agalactia, dystocia, and placental abnormalities have been subjected to stress from weather or insect damage. The been reported in mares consuming oats contaminated with fumonisin-producing molds may proliferate during storage ergot (Riet-Correa et al., 1998; Copetti et al., 2002). if moisture content is favorable (Bacon and Nelson, 1994). Corn screenings have been reported to be higher in fumo- FATS AND OILS nisin than whole corn grain (Murphy et al., 1993). The FDA Center for Veterinary Medicine does not recommend that The digestion and metabolism of fats by horses have corn screenings be used in feeds for horses (FDA, 2001b). been discussed at length in Chapter 3. In general, fats and In 1998, the National Animal Health Monitoring System of oils do not contain any appreciable amounts of protein or the U.S. Department of Agriculture collected samples from minerals and are usually included in horse diets for their en- more than 900 equine operations to assess the incidence of ergy value and associated metabolic benefits. Depending fumonisin contamination of equine feeds. Approximately 95 upon the extent of processing, some vegetable oils may con- percent of all samples contained less than 2 ppm fumo- tain some natural antioxidants. nisin, and slightly less than 1 percent contained more than 5 Many different types of fats and oils are available for in- ppm fumonisin. Samples that were obtained from horse corporation into equine diets. Animal fats include tallow, operations that used homegrown grain had a greater inci- lard, and rendered fat. Vegetable fats include corn oil, soy- dence of high concentrations of fumonisin than samples ob- bean (soya) oil, sunflower oil, canola oil, and rice oil. tained from operations that used grain obtained from a retail Blends of animal and vegetable fats have also been fed to supplier. horses (Bowman et al., 1979; McCann et al., 1987). Animal The mycotoxins produced by Aspergillus spp. include fats are typically higher in saturated fatty acids than veg- aflatoxin B1, B2, G1, and G2. Aflatoxins are not unique to etable oils, which are predominantly unsaturated fatty acids. cereal grains and may be found in peanuts, cottonseed, and An exception would be coconut oil, which is low in unsatu- other feeds. Aflatoxins are considered to be carcinogenic rated fats but high in medium-chain saturated fatty acids. (FDA, 1994). In horses, aflatoxins have been associated Corn oil is typically high in linoleic acid and low in linolenic with liver disease and death (Angsubhakorn et al., 1981; acid, whereas linseed oil is comparatively lower in linoleic Vesonder et al., 1991). The toxic level of aflatoxin in horse acid and higher in linolenic acid. The fatty acid composition feeds has not been clearly established. Angsubhakorn et al. of various fats and oils is shown in Table 8-7. (1981) reported that horses consuming feed containing more Fats and oils are usually incorporated into horse feeds for than 200 ppb aflatoxin B1 became ill soon after the feed was their nutritional characteristics, but they are also used to af- introduced to the diet, and that several horses died. Vesonder fect the physical characteristics of the feed. Addition of fats et al. (1991) reported that three horses that died had con- and oils may decrease the dustiness of a feed and decrease sumed corn containing more than 100 ppb aflatoxin. Hasso caking in sweet feeds. Addition of fat may also enhance (2003) attributed a bout of soft feces observed in Arabian mixing qualities. However, inclusion of high levels of fat in horses in Iraq to aflatoxin contamination of the barley in the pelleted feeds may negatively affect pellet quality. In addi- diet. The barley contained 12.5 ppb aflatoxin, and horses re- tion, fats may be susceptible to oxidation, resulting in ran- ceived 5–6 kg/d. No effects on feed or water intake were cidity. Unsaturated fats are usually more susceptible to oxi- noted. The FDA has suggested action levels for aflatoxin dation than saturated fats. To minimize oxidation and concentration in specific feeds used in certain circumstances increase shelf life, antioxidants may be added to feeds con- for cattle, swine, and poultry, but not for horses (FDA, taining high levels of fat. The use of antioxidants in horse 1994). An action level of 20 ppb has been given for animal diets is reviewed in Chapter 9. feeds and feed ingredients used for animal species and uses Different fat sources vary in their palatability to horses. not specifically designated by other guidelines. An action In a study conducted on the acceptance of various plant and level is not a formal tolerance level but has been described animal lipid sources when incorporated at 15 percent of the as a level that a qualified expert witness would consider in- concentrate ration, corn oil was found to be most palatable, jurious to health during testimony in a federal court (Price et followed by corn oil blended with other oils of plant or ani- al., 1993). mal origin. Compared to corn oil, peanut and safflower oils Mycotoxins associated with Penicillium spp. include were moderately palatable, whereas cottonseed oil and tal- ochratoxins and citrinin. Ochratoxins are believed to nega- low were largely unpalatable (Holland et al., 1998). In fur- tively affect other species, but little documentation of effects ther studies, 10 percent coconut and soya oils percent were on horses can be found. well accepted by horses (Pagan et al., 1993).

FEEDS AND FEED PROCESSING 163 TABLE 8-7 Fatty Acid Composition of Some Fats and Oils Available for Use in Equine Feeds Selected Fatty Acids (% of total fatty acids)a International Other Type of Oil Feed Number C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 Fatty Acidsb Animal Fats and Fish Oils Tallow 4-08-127 3.0 24.5 3.7 19.3 40.9 3.2 0.7 4.9 Lard 4-04-790 1.3 23.8 2.7 13.5 41.2 10.2 1.0 6.3 Herring 7-08-048 7.2 11.7 9.6 0.8 12.0 1.1 0.8 56.8 Menhaden 7-08-049 8.0 15.1 10.5 3.8 14.5 2.2 1.5 44.5 Vegetable Oils Canola (rapeseed) 4-06-144 — 4.8 0.5 1.6 53.8 22.1 11.1 6.1 Corn 4-07-882 0 10.9 — 1.8 24.2 58.0 0.7 4.4 Cottonseed 4-20-836 0.8 22.7 0.8 2.3 17.0 51.5 0.2 4.7 Linseed — — 5.3 — 4.1 20.2 12.7 53.3 4.4 Olive — 0 11.0 0.8 2.2 72.5 7.9 0.6 5.0 Palm — 1.0 43.5 0.3 4.3 36.6 9.1 0.2 5.0 Peanut 4-03-658 0.1 9.5 0.1 2.2 44.8 32.0 — 11.3 Safflower 4-20-256 0.1 6.2 0.4 2.2 11.7 74.1 0.4 4.9 Sesame — — 8.9 0.2 4.8 39.3 41.3 0.3 5.2 Soybean 4-07-983 0.1 10.3 0.2 3.8 22.8 51.0 6.8 5.0 Sunflower 4-20-833 — 5.4 0.2 3.5 45.3 39.8 0.2 5.6 aValues from the U.S. Department of Agriculture Food Composition Standard Release 12 (1998) and Pearl (1995) of the Fats and Protein Research Foundation. bOther fatty acids are predominantly polyunsaturated fatty acids greater than 18 carbons long. PROTEIN SUPPLEMENTS cottonseed meal, making them acceptable for inclusion in horse feeds. Others protein supplements used in horse feeds The quality of protein supplements fed to horses is a include sunflower meal, peanut meal, lupin seed meal, function of both the amino acid profile and digestibility of beans, linseed meal, brewers’ dried grains, and distillers’ the protein source. Protein supplements are either from plant grains (Frape, 1998). or animal sources. Those of animal origin are superior in Particular attention should be paid to the lysine level in terms of their amino acid profile to those of plant origin. the protein source especially for growing horses and lactat- However, animal protein sources are more expensive and ing mares. Deficient lysine levels will limit growth in young often unpalatable compared to protein supplements of plant horses (Ott et al., 1979a) and may affect milk protein qual- origin. It is of note that in some countries, such as the United ity thereby, ultimately affecting the growth and development Kingdom, feeding animal protein products to horses is of the nursing foal (Glade and Luba, 1990). Table 8-8 gives banned (DEFRA, 2006). amino acid profiles of some common feedstuffs for horses. Many protein supplements such as canola meal, soybean Dietary supplement with urea has been shown to increase meal, brewers’ dried grains, fishmeal, linseed meal, and cot- blood and urine concentrations of urea but has improved ni- tonseed meal, have been fed to adult horses with no ill ef- trogen balance when protein in the diet was deficient (Slade fects (Slade et al., 1970; Hintz and Schryver, 1972; Reitnour et al., 1970; Godbee and Slade, 1981; Martin et al., 1991, and Salsbury, 1976; Gibbs et al., 1996; Martin et al., 1996). 1996). However, it should be noted that large quantities of However, protein sources such as milk byproducts and soy- urea in the diet can result in death. Feeding 450 g of urea re- bean meal, as well as canola meal, have proved superior to sulted in ammonia toxicity (Hintz et al., 1970) and death of other protein sources for growing horses based on greater the horses in the study. Inclusion of urea in the diet does not ADG (Borton et al., 1973; Prior et al., 1974; Ott et al., improve the quality (i.e., the amino acid profile) of the diet 1979a; Cymbaluk, 1990b). This is probably due to a better and should be avoided in the diets of growing horses and amino acid profile in these protein sources, as well as a po- lactating mares. tentially superior digestibility in the foregut of the horse. Soybeans and some varieties of peas contain a trypsin in- hibitor that can interfere with protein digestion, and cotton- MINERAL AND VITAMIN SUPPLEMENTS seed meal contains gossypol, which has been suggested to Mineral Supplements bind iron and interfere with protein digestion. During pro- cessing, heating can destroy the trypsin inhibitors in soy- Supplemental minerals differ in chemical form, concen- beans and peas and inactivate the toxins from gossypol in tration, and bioavailability. Supplemental minerals are gen-

164 NUTRIENT REQUIREMENTS OF HORSES TABLE 8-8 Amino Acid Contents of Some Horse Feed Ingredients and Forages (percentage DM basis) DM CP Arg His Ile Leu Lys Met Cys Phe Tyr Thr Feed Name Feed Description IFNa % % (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Alfalfa meal dehydrated, 17% CP 1-00-023 92 17 0.77 0.41 0.74 1.32 0.77 0.27 0.22 0.92 0.60 0.77 meal dehydrated, 20% CP 1-00-024 92 19.6 1.0 0.98 0.98 1.54 0.99 0.37 0.33 1.02 0.66 0.90 Bakery waste dried bakery product 4-00-466 91 10.8 0.52 0.26 0.46 0.88 0.33 0.20 0.22 0.55 0.42 0.36 Barley grain, two row 4-00-572 89 11.3 0.61 0.30 0.43 0.88 0.44 0.22 0.33 0.62 0.32 0.39 grain, six row 4-00-574 89 10.5 0.53 0.24 0.40 0.77 0.44 0.19 0.22 0.54 0.35 0.37 grain, hulless 4-00-552 88 14.9 0.61 0.25 0.45 0.88 0.44 0.18 0.22 0.67 0.44 0.44 Beet, sugar pulp, dried 4-00-669 91 8.6 0.35 0.25 0.4 0.55 0.55 0.08 0.11 0.33 0.44 0.41 Brewers’ grains dried 5-02-141 92 26.5 1.68 0.58 1.12 2.31 1.31 0.49 0.55 1.42 0.97 1.04 Buckwheat, common grain 4-00-994 88 11.1 1.10 0.28 0.44 0.66 0.66 0.21 0.22 0.51 0.34 0.45 Canola (rapeseed) meal, sol. extr. 5-06-145 90 35.6 2.42 1.06 1.57 2.86 2.31 0.81 0.99 1.47 1.24 1.74 Casein dried 5-01-162 91 88.7 3.59 3.10 5.13 9.68 8.14 3.00 0.44 4.84 5.25 4.38 Cassava (tapioca or manioc) meal 4-01-152 88 3.3 0.20 0.09 0.21 0.22 0.11 0.04 0.11 0.16 0.04 0.12 Citrus pulp dried 4-01-237 85 6.9 0.23 0.12 0.19 0.34 0.18 0.07 0.01 0.23 — 0.26 Coconut (copra) meal, sol. extr. 5-01-573 92 21.9 2.40 0.43 0.83 1.54 0.66 0.38 0.33 0.92 0.63 0.73 Corn, yellow distillers’ grain 5-02-842 94 24.8 0.99 0.69 1.00 2.86 0.77 0.47 0.33 1.09 0.90 0.68 distillers’ grain with solubles 5-02-843 93 27.7 1.24 0.77 1.13 2.86 0.66 0.55 0.55 1.47 0.91 1.03 distillers’ solubles 5-02-844 92 26.7 0.99 0.73 1.33 2.53 0.88 0.56 0.55 1.33 0.88 1.13 gluten feed 5-02-903 90 21.5 1.14 0.74 0.73 2.20 0.66 0.39 0.55 0.78 0.63 0.81 gluten meal, 60% CP 5-28-242 90 60.2 2.12 1.41 2.73 11.0 1.10 1.57 1.31 3.88 3.56 2.29 grain 4-02-935 89 8.3 0.41 0.25 0.31 1.10 0.33 0.19 0.22 0.43 0.27 0.38 grits byproduct (hominy feed) 4-03-011 90 10.3 0.61 0.31 0.40 1.10 0.44 0.19 0.22 0.47 0.44 0.44 grain, steam-flaked 4-02-854 88 9.4 0.44 0.29 0.31 1.04 0.30 0.19 0.22 0.43 — 0.34 Cottonseed meal, mech. extr. 41% CP 5-01-617 92 42.4 4.68 1.22 1.42 2.75 1.87 0.74 0.77 2.17 1.35 1.47 meal, sol. extr. 41% CP 5-07-872 90 41.4 5.01 1.29 1.43 2.75 1.87 0.74 0.77 2.32 1.34 1.49 Fababean (broadbean) seeds 5-09-262 87 25.4 2.51 0.74 1.43 2.10 1.76 0.22 0.33 1.13 0.96 0.98 Flax (linseed) meal, sol. extr. 5-02-048 90 33.6 3.26 0.75 1.72 2.31 1.32 0.65 0.66 1.73 1.13 1.27 Lentil seeds 5-02-506 89 24.4 2.26 0.86 1.10 1.98 1.87 0.20 0.33 1.42 0.77 0.92 Lupin (sweet white) seeds 5-27-717 89 34.9 3.71 0.85 1.54 2.64 1.65 0.30 0.55 1.34 1.48 1.32 Milk (cattle) dried 5-01-175 96 34.6 1.28 1.15 2.06 4.07 3.19 1.01 0.33 1.96 2.06 1.78 Millet (proso) grain 4-03-120 90 11.1 0.45 0.22 0.51 1.32 0.22 0.34 0.22 0.62 0.34 0.44 Molasses beet-sugar 4-00-668 77.9 8.5 0.42 0.14 0.38 0.31 0.09 0.02 0.07 0.23 — 0.14 sugarcane 4-00-696 74.3 5.8 0.28 0.09 0.26 0.21 0.06 0.01 0.05 0.16 — 0.09 Oat grain 4-03-309 89 11.5 0.96 0.34 0.53 0.99 0.44 0.24 0.44 0.72 0.45 0.48 grain, naked 4-25-101 86 17.1 0.85 0.31 0.53 0.99 0.55 0.22 0.33 0.66 0.46 0.44 groat 4-03-331 90 13.9 0.94 0.26 0.61 1.10 0.55 0.22 022 0.72 0.56 0.48 Pea seeds 5-03-600 89 22.8 2.06 0.59 0.95 1.65 1.65 0.23 0.33 1.09 0.78 0.85 Peanut (groundnut) meal, mech. extr. 5-03-649 92 43.2 5.27 1.11 1.55 3.08 1.65 0.55 0.66 2.22 1.91 1.28 meal, sol. extr. 5-03-650 92 49.1 5.60 1.17 1.96 3.10 1.87 0.57 0.77 2.58 1.98 1.39

FEEDS AND FEED PROCESSING 165 TABLE 8-8 continued DM CP Arg His Ile Leu Lys Met Cys Phe Tyr Thr Feed Name Feed Description IFNa % % (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Potato protein concentrate 5-25-392 91 73.8 4.18 1.88 4.49 8.36 6.38 1.85 1.32 5.48 4.69 4.73 Rice bran 4-03-928 90 13.3 1.10 0.37 0.48 0.99 0.66 0.29 0.33 0.61 0.44 0.52 Safflower meal, sol. extr. 5-04-110 92 23.4 2.24 0.65 0.73 1.65 0.77 0.37 0.44 1.17 0.84 0.71 meal without hulls, sol. extr. 5-07-959 92 42.5 3.95 1.17 1.86 2.86 1.32 0.72 0.77 2.20 1.19 1.41 Sesame meal, mech. extr. 5-04-220 93 42.6 5.35 1.08 1.61 2.97 1.10 1.26 0.88 1.95 1.67 1.58 Sorghum grain 4-20-893 88 9.2 0.42 0.25 0.41 1.32 0.22 0.19 0.22 0.54 0.38 0.34 Soybean meal, sol. extr. 5-04-604 89 43.8 3.55 1.29 2.20 3.74 3.08 0.67 0.77 2.40 1.86 1.90 meal without hulls 5-04-612 90 47.5 3.83 1.41 2.38 4.07 3.30 0.74 0.77 2.63 2.00 2.04 protein concentrate — 90 64.0 6.37 1.98 3.63 5.83 4.62 0.99 1.10 3.74 2.75 3.08 protein isolate 5-08-038 92 85.8 7.56 2.48 4.68 7.26 5.83 1.11 1.32 4.77 3.41 3.47 seeds, heat processed 5-04-597 90 35.2 2.86 1.06 1.78 3.08 2.42 0.58 0.66 2.01 1.45 1.55 Sunflower meal, sol. extr. 5-09-340 90 26.8 2.62 0.73 1.42 2.09 1.10 0.65 0.55 1.35 0.84 1.14 meal without hulls, sol. extr. 5-04-739 93 42.2 3.22 1.01 1.58 2.53 1.32 0.90 0.77 1.83 1.13 1.46 Triticale grain 4-20-362 90 12.5 0.63 0.28 0.43 0.88 0.44 0.22 0.33 0.54 0.35 0.39 Wheat bran 4-05-190 89 15.7 1.18 0.48 0.54 1.10 0.66 0.27 0.33 0.68 0.47 0.57 grain, hard red spring 4-05-258 88 14.1 0.74 0.37 0.52 0.99 0.44 0.25 0.33 0.73 0.44 0.45 grain, hard red winter 4-05-268 88 13.5 0.66 0.35 0.44 0.99 0.33 0.22 0.33 0.66 0.41 0.40 grain, soft red winter 4-05-294 88 11.5 0.55 0.22 0.50 0.99 0.44 0.24 0.33 0.69 0.40 0.42 grain, soft white winter 4-05-337 89 11.8 0.61 0.30 0.48 0.88 0.33 0.22 0.33 0.60 0.39 0.38 middlings, 9.5% fiber 4-05-205 89 15.9 1.07 0.48 0.58 1.21 0.66 0.28 0.33 0.77 0.31 0.56 red dog, 4% fiber 4-05-203 88 15.3 1.06 0.45 0.60 1.21 0.66 0.25 0.44 0.72 0.50 0.55 shorts, 7% fiber 4-05-201 88 16.0 1.12 0.47 0.64 1.10 0.77 0.27 0.33 0.77 0.56 0.62 Whey dried 4-01-182 96 12.1 0.29 0.25 0.68 1.21 0.99 0.18 0.33 0.42 0.27 0.79 low lactose, dried 4-01-186 96 17.6 0.58 0.36 1.28 1.76 1.65 0.42 0.55 0.69 0.57 1.28 permeate, dried — 96 3.8 0.07 0.06 0.18 0.22 0.22 0.03 0.00 0.06 — 0.15 Yeast, brewers’ dehydrated 7-05-527 93 45.9 2.42 1.20 2.37 3.41 3.52 0.81 0.55 1.91 1.71 2.42 Forages Cool-season grasses pasture intensively managed 2-02-260 20 26.5 0.30 0.13 0.23 0.43 0.24 0.09 0.07 0.32 — 0.24 hay, all samples 1-02-250 88 10.6 0.04 0.01 0.03 0.06 0.03 0.01 0.01 0.04 — 0.04 hay, immature 1-02-212 84 18.0 0.12 0.05 0.10 0.20 0.11 0.04 0.041 0.12 — 0.12 hay, mid maturity 1-02-243 84 13.3 0.07 0.03 0.06 0.11 0.06 0.03 0.02 0.07 — 0.06 hay, mature 1-02-244 84 10.8 0.04 0.01 0.03 0.07 0.04 0.01 0.01 0.04 — 0.04 silage, all samples 3-02-222 37 12.8 0.05 0.03 0.06 0.10 0.05 0.02 0.01 0.08 — 0.06 silage, immature 3-02-217 36 16.8 0.09 0.05 0.10 0.17 0.09 0.03 0.02 0.12 — 0.09 silage, mid-maturity 3-02-218 42 16.8 0.09 0.05 0.10 0.17 0.09 0.03 0.02 0.12 — 0.09 silage, mature 3-02-219 39 12.7 0.05 0.03 0.06 0.10 0.06 0.02 0.01 0.07 — 0.06 Grass-legume mixtures hay, immature 1-02-275 84 18.4 0.14 0.06 0.12 0.22 0.132 0.05 0.04 0.14 — 0.12 predominantly hay, mid-maturity 1-02-277 87 17.4 0.12 0.05 0.11 0.19 0.12 0.04 0.04 0.12 — 0.11 grass (17–22% hay, mature 1-02-280 85 13.3 0.07 0.03 0.06 0.11 0.07 0.02 0.02 0.07 — 0.06 hemicellulose) silage, immature 3-02-302 47 18.0 0.10 0.05 0.11 0.19 0.11 0.04 0.03 0.14 — 0.11 silage, mid-maturity 3-02-265 45 17.6 0.10 0.05 0.11 0.19 0.11 0.04 0.03 0.14 — 0.11 silage, mature 3-02-266 39 15.4 0.08 0.04 0.09 0.09 0.09 0.03 0.02 0.10 — 0.08 Mixed grass and legume (12–15% hay, immature 1-02-275 83 19.7 0.18 0.07 0.15 0.28 0.18 0.06 0.06 0.17 — 0.16 hemicellulose) hay, mid-maturity 1-02-277 85 18.4 0.15 0.06 0.12 0.23 0.08 0.05 0.04 0.14 — 0.80 hay, mature 1-02-280 90 18.2 0.14 0.06 0.12 0.22 0.14 0.05 0.04 0.14 — 0.79

166 NUTRIENT REQUIREMENTS OF HORSES TABLE 8-8 continued DM CP Arg His Ile Leu Lys Met Cys Phe Tyr Thr Feed Name Feed Description IFNa % % (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) silage, immature 3-02-302 46 20.3 0.14 0.06 0.16 0.26 0.16 0.06 0.04 0.18 — 0.77 silage, mid-maturity 3-02-265 44 19.1 0.13 0.06 0.14 0.24 0.15 0.05 0.03 0.16 — 0.76 silage, mature 3-02-266 43 17.4 0.10 0.05 0.11 0.19 0.12 0.04 0.02 0.13 — 0.63 Predominantly legume (10–13.5% hay, immature 1-02-275 84 20.5 0.21 0.08 0.18 0.32 0.21 0.06 0.06 0.20 — 0.20 hemicellulose) hay, mid-maturity 1-02-277 84 19.1 0.18 0.07 0.15 0.26 0.17 0.06 0.05 0.17 — 0.15 hay, mature 1-02-280 84 17.2 0.14 0.06 0.12 0.21 0.14 0.04 0.04 0.13 — 0.12 silage, immature 3-02-302 43 20.0 0.15 0.07 0.15 0.24 0.17 0.05 0.03 0.17 — 0.15 silage, mid-maturity 3-02-265 43 19.0 0.14 0.06 0.15 0.24 0.16 0.05 0.03 0.16 — 0.14 silage, mature 3-02-266 43 18.3 0.12 0.06 0.13 0.21 0.13 0.04 0.03 0.14 — 0.12 Legumes, forage pasture intensively managed 2-29-431 21 26.5 0.38 0.14 0.30 0.52 0.34 0.11 0.10 0.08 — 0.31 hay, all samples 1-20-648 88 20.2 0.20 0.08 0.17 0.30 0.21 0.06 0.06 0.19 — 0.18 hay immature 1-07-792 84 22.8 0.27 0.10 0.10 0.38 0.27 0.08 0.07 0.24 — 0.22 hay, mid-maturity 1-07-788 84 20.8 0.21 0.08 0.18 0.31 0.21 0.07 0.06 0.20 — 0.18 hay, mature 1-07-789 84 17.8 0.16 0.06 0.13 0.23 0.16 0.05 0.05 0.16 — 0.14 silage, all samples 3-07-796 39 20.1 0.16 0.07 0.16 0.25 0.18 0.06 0.03 0.17 — 0.15 silage, immature 3-07-795 41 23.2 0.21 0.09 0.20 0.32 0.24 0.07 0.04 0.22 — 0.18 silage mid-maturity 3-07-797 43 21.9 0.185 0.08 0.18 0.30 0.21 0.06 0.04 0.20 — 0.19 silage, mature 3-07-798 43 20.3 0.16 0.07 0.16 0.26 0.19 0.06 0.03 0.16 — 0.16 Bermudagrass (Cynodon dactylon) coastal hay, early heading 1-20-900 87 10.4 0.04 0.02 0.03 0.07 0.04 0.01 0.01 0.04 — 0.04 Oat silage, headed 3-21-843 35 13.0 0.04 0.03 0.09 0.11 0.06 0.03 0.01 0.08 — 0.02 hay 1-09-099 92 9.1 0.20 0.18 0.50 0.61 0.32 0.17 0.07 0.42 — 0.38 Wheat straw 1-05-175 93 4.8 0.05 0.08 0.08 0.16 0.16 0.06 0.05 0.09 — 0.15 silage, early head 3-21-865 33 12.0 0.25 0.45 0.50 0.83 0.53 0.22 0.08 0.53 — 0.52 Corn, yellow silage, normal 3-28-248 35 8.8 0.17 0.15 0.29 0.76 0.22 0.13 0.12 0.34 — 0.28 aNOTE: IFN = International Feed Number First digit is class of feed: 1, dry forages and roughages; 2, pasture, range plants, and forages fed green; 3, silages; 4, energy feeds; 5, protein supplements; 6, minerals; 7, vitamins; 8, additives; the other five digits are the International Feed Number. erally in the form of salts (e.g., carbonates, oxides, sulfates) forms commonly used in feeding horses are shown in the or in a complex with a chelating agent (e.g., polysaccha- Table 16-7. rides, proteinates, amino acids). Mineral chelates are de- Bioavailability, defined as the degree to which an in- fined as metal complexes in which the metal is held in the gested nutrient in a particular source is absorbed in a form complex through more than one point of attachment to the that can be utilized in metabolism by the animal (Ammer- ligand (chelating agent), with the metal atom occupying a man et al., 1995), varies between different chemical forms central position in the complex (McDowell, 2003). The con- of mineral. Unfortunately, information regarding bioavail- centration of a mineral for any given form depends upon the ability of different forms of minerals specific to horses is relative mass of the element, compared to the compound in lacking. However, it is a commonly held view that chelated which it is bound. For example the molecular weight of minerals are more bioavailable to horses than nonchelated cupric oxide (CuO) is 79.5454 (i.e., Cu = 63.546 + O = forms. Chelated minerals are chemically altered and bound 15.9994). Therefore, copper oxide contains 80 percent Cu to proteins or amino acids and do not have to compete for (i.e., 63.546/79.5454). In comparison, cupric sulfate ligands in the stomach to be absorbed; thus, in theory they (CuSO4и5 H2O) only contains 25 percent Cu (63.546/ should be more bioavailable than those that are not chelated 249.68). Mineral concentrations of supplemental mineral (Baker et al., 2005). However, the limited studies on the rel-

FEEDS AND FEED PROCESSING 167 ative bioavailability of chelated minerals in horses have pro- dling, and consumer acceptance (Fahrenholz, 1994; Van Der duced equivocal results, thus making it difficult to general- Poel et al., 1995; Behnke, 1996; Hancock and Behnke, ize on the merits or otherwise of using chelated mineral sup- 2001). Processing alters the size, density, and texture of feed, plements in horse feeds. Wagner et al. (2005) reported no which can positively influence animal utilization and pur- difference in absorption and retention of copper, manganese, chaser acceptance. Processing may improve the intake and and zinc when fed as the oxide, sulfate, or organic-chelate digestibility of rations, and deleterious compounds may be form. Likewise, Highfill et al. (2005) reported calcium from denatured or inactivated. Hygienic specifications may be im- a calcium-amino acid proteinate was no more available than proved by reducing bacterial and/or fungal infestations. In the calcium from calcium carbonate. Baker et al. (2005) re- addition, processing allows more varieties of feedstuffs to be ported mature horses did not appear to digest and retain or- incorporated into rations, and reduces the level of dust. Diet ganically chelated copper and zinc sources as efficiently as uniformity, feed handling, and storage may be enhanced. inorganic sources. In comparison, Baker et al. (2003) re- Animal feed processing systems use equipment that ported yearling geldings fed a supplemental organic copper shapes and thermally treats feed. Processing methods that and zinc source had a greater apparent copper digestibility, are common to horse rations include chopping, cubing, higher daily copper balance, and higher daily zinc balance wafering, and pelleting of forages and complete feeds. as compared to those fed a diet supplemented with an inor- Grains, grain byproducts, and complete feeds are routinely ganic source. Wagner et al. (2005) suggested that growing pelleted; extruded; compressed by rolling, flaking, or crimp- horses may find more benefit from chelated sources than ing; or treated with steam and high temperatures that alter mature, idle horses. their physical and chemical properties. Feeds may be In the absence of further information on mineral bioavail- cleaned, screened, or sifted to remove various sized parti- ability specific to horses, estimates for some nonruminant cles. Supplements and additives are commonly processed to (e.g., swine) and ruminant species have been summarized produce finely ground powders, crumbles, bricks, blocks, (McDowell, 2003) and may provide a rough estimate of gels, or emulsified liquids. There are other processing meth- bioavailability differences between mineral sources. ods inherent to the preparation of individual feedstuffs prior to mixing such as culturing of biological materials, curing forages for preservation, extracting oils or other compounds Vitamin Supplements intended for other uses, or removing or denaturing undesir- Supplemental vitamins differ in chemical form, vitamin able compounds that might affect the health or performance activity, and stability. Several vitamins (A, D, E, and K) are of a horse. susceptible to destruction by various factors associated with The Official Publication of the Association of American feed manufacturing (e.g., heat, moisture, pH, light, pro- Feed Control Officials (AAFCO, 2005) serves as one source oxidants). Vitamin stability of supplemental forms is often of information and recognized standard in the United States improved by synthesis of stable derivatives (e.g., esterifica- for feed manufacturing regulations, guidelines, and feed tion of retinol to retinyl-palmitate), addition of antioxidants terms and ingredients. Some of the terminology more com- (e.g., ethoxyquin, butylated hydroxytoluene), fat coating, in- monly used in formulation of horse diets is defined in Ap- corporation into gelatin-carbohydrate matrix (e.g., beadlets, pendix 8-1. spray-dried powder), and absorbance and adsorbance of vi- There are many reported advantages for processing feeds tamins to various carrier substances (e.g., adsorbance to sil- intended for animal consumption. A review of research con- ica) (McDowell, 2003). Several detailed reviews on vitamin ducted by Hancock and Behnke (2001) identified several stability and factors influencing stability have been pub- benefits with processing swine feeds. Pelleting can increase lished (Roche Vitamins Inc., 2000; BASF Corp., 2001). rates of gain, increase palatability, decrease feed wastage, Table 8-9 lists different forms of vitamins, vitamin activity and decrease environmental wastes as compared with meal of each form, physical form, applications, and information feeding. Grinding complete diets can increase feed utiliza- regarding vitamin stability. More specific information re- tion. Differences in the particle size of ground feed can in- garding numerous factors and the magnitude of their effect fluence gain efficiency, digestibility, and feed intake. How- on stability is available (Roche Vitamins Inc., 2000; BASF ever, the review also pointed out the complexity of diet Corp., 2001). processing technology and the need for evaluating the con- ditions under which each trial was conducted. Animal re- sponse can be significantly influenced by differences in feed FEED PROCESSING AND MANUFACTURING ingredients, intakes, and processing technologies. Rations fed to horses routinely contain feed ingredients The effect of processing feed in ruminant diets also has re- that are processed post-harvesting. Processing can affect the ceived considerable attention. Research areas include the in- physical, chemical, and microbiological properties of the fluence of ration particle size on ruminal function, ruminally feedstuff to be processed and may improve animal perfor- protected amino acids, and a significant amount of interest mance, feed manufacturing and storage, ease of feed han- with processing and carbohydrate utilization (NRC, 2000,

168 NUTRIENT REQUIREMENTS OF HORSES TABLE 8-9 Supplemental Vitamin Sources: Chemical Form, Vitamin Activity, Physical Form, and Applicationsa Vitamin Vitamin Chemical Form Activity/ Physical Form Applications A retinyl-acetate 2.9 IU/µg beadlets dry feeds spray- or drum-dried powders dry feeds and water-dispersible vita- min products retinyl-palmitate 1.82 IU/µg liquid water-soluble emulsions liquid feed supplements D cholecalciferol 40 IU/µg beadlets with vitamin A dry feeds spray- or drum-dried powders dry feeds, water-dispersible vitamin liquid concentrates products liquid feed supplements E dl-α-tocopheryl acetate (all-rac) 1 IU/mg adsorbate powders, dry dilutions, oils dry feeds spray-dried coated powders dry feeds, water-dispersible vitamin products d-α-tocopheryl acetate (RRR) 1.36 IU/mg liquid concentrates liquid feed supplements K menadione sodium bisulfite 500 g/kg dry dilutions dry feeds menadione dimethyl-pyrimidinol bisulfite 454 g/kg water-dispersible powders water-dispersible vitamin products menadione sodium bisulfite complex 330 g/kg Thiamin thiamin mononitrate 920 g/kg crystalline; fine powder dry feeds thiamin hydrochloride 890 g/kg crystalline; dry dilution dry feeds and water-dispersible vita- min products Riboflavin crystalline riboflavin 1 g/g spray-dried powder dry feeds Niacin crystalline nicotinic acid 1 g/g crystalline and dried dilutions dry feeds crystalline nicotinamide 1 g/g Folic acid crystalline folic acid 1 g/g crystalline and dried dilutions dry feeds Biotin crystalline d-biotin 1 g/g crystalline and dried dilutions dry feeds Vitamin C L-ascorbic acid (100% crystalline) 1 g/g crystalline water-dispersible vitamin products L-ascorbic acid (50%; fat coated) 500 g/kg fat coated dry feeds L-ascorbic acid (97.5%; ethylcellulose- 975 g/kg ethylcellulose-coated dry feeds coated) ascorbyl palmitate 400 g/kg ascorbic acid ester dry feeds calcium ascorbyl-2-monophosphate 350 g/kg ascorbic acid ester dry feeds aAdapted from Roche Vitamins Inc. (2000); McDowell (2000). 2001). In a review of research with feedlot cattle, Owens et al. cessing of horse feeds compared to the effects of processing (1997) stressed the need to carefully analyze the conditions of feedstuffs for other species of livestock. under which specific trials were conducted. The effects of Most of the investigations and reports have centered on processing on animal performance differed among trials, comparative utilization of feeds processed by different meth- probably due to differences in the grain processing methods, ods or on the effect of processing on ingestive behavior. grain choices, and characteristics of additional ingredients. Reviewing research in other species is beneficial to de- Total Tract Digestibility velop a more comprehensive knowledge of how processing affects different production parameters, as results can be ex- The past NRC committee reported that processing in- trapolated in areas where specific research in the equine is creased the total tract digestibility of oats and barley by only lacking. However, the level of accuracy of predicting re- a small percentage (2–5 percent) with increased benefits for sponses may be low because of differences in physiology grains with harder seed coats (NRC, 1989). The suggestion and management of horses. that processing of softer seed-coated grains, such as oats, There are fewer research trials reported and there is less has little effect on total tract digestibility has been confirmed depth of information available, about the effects of feed pro- by several trials.

FEEDS AND FEED PROCESSING 169 Coleman et al. (1985) reported similar dry matter and en- Starch is highly digestible, with total tract digestibility rang- ergy digestibility for oats fed whole, rolled, or pelleted when ing from 87 percent to nearly 100 percent (Potter et al., mature horses were fed the grains at two different intake lev- 1992a). Starch not digested in the small intestine is readily els with an alfalfa cube forage. Lopez et al. (1988) found no digested by microbial digestion in the hindgut. Large differences in digestibility of energy, CP, ether extract, ADF, amounts of starch bypassing the small intestine is thought to or NDF for rations containing alfalfa hay cubes with whole, increase digestive upset because of adverse changes in the cut and vacuum-cleaned, or cut and recombined oats. microbial population and dysfunction of the hindgut. Similar to the results with processed oats, Coleman et al. The rate and site of digestion of starch is influenced by (1985) found no improvement in total tract digestibility of many factors, including intake level, morphology and pro- DM or energy when whole barley was rolled and fed to ma- cessing of starch, intake of the total ration, forage intake, ture horses. McLean et al. (1999) found no differences in rate of passage through the small intestine, level of amy- total tract apparent digestibility of DM, OM, GE, starch, CP, lase, and individual differences between horses (Potter et ADF, or NDF when ponies were fed rolled, micronized, or al., 1992a; Kienzle et al., 1992, 1997; Meyer et al., 1993; extruded barley. Kienzle, 1994; Cuddeford, 1999; Hussein and Vogedes, 2003; Pelleting a diet of rolled barley, alfalfa hay, and wheat Jose-Cunilleras et al., 2004). bran did not affect CF, CP, and nitrogen-free extract (NFE) Processing is but one of the potential influences on the digestibility of growing-horse rations (Hintz and Loy, 1966). site of starch digestion, and the interaction of associated fac- Pipkin et al. (1991) noted similar digestibility of rations fed tors increases the difficulty of independently assessing the as a complete, wafered form or as a textured grain mix and effect of processing. Regardless, processing can affect the long-stem forage. Raina and Raghavan (1985) reported de- site of starch digestion. Comparisons of whole, crushed, creased DM and CF digestibility of a ground concentrate fed ground, or popped corn suggest that grinding increases with chopped hay as compared to the same grain mix in pel- small intestinal starch digestibility about 15 percent over di- leted or unprocessed form. They postulated the decrease in gestibility of whole or cracked corn (Table 8-10) (Meyer et digestibility might be due to an increased rate of flow in the al., 1993). In addition, the percent preileal starch digestibil- hindgut associated with the smaller particle size of the ity was higher for popped corn than for whole, crushed, or ground mix. ground corn. Digestibility estimates were obtained from five There are reported increases in the total tract digestibility ponies fitted with permanent fistulas at the end of the je- of ether extract. Hintz and Loy (1966) reported a 6 percent junum. Chromic oxide was used as a marker. The ponies increase in the total tract digestibility of ether extract when were fed at 12-hour intervals, and the grains were fed with a complete ration was fed in a pelleted vs. a nonpelleted green meal. Starch intake per meal and digestibility are pro- form. Pagan and Jackson (1991a) noted that pelleting alfalfa vided in Table 8-10. hay decreased total tract digestibility of fat. Diets in both The increase in preileal starch digestibility of processed studies contained low amounts of fat with values reported corn may parallel the degree of molecular disruption of the between 2.8 to 4 percent ether extract. starch molecule. Popping is thought to destroy the structure Even though most of these reports suggest little benefit of of starch in corn to a greater level than grinding or crushing, processing oats, barley, or alfalfa forage on total tract di- and this greater level of destruction enhances solubility of gestibility, these methods may have more significance if starch molecules and absorption in the small intestine of the feeding horses with poor dentition or horses with limited di- horse (Potter et al., 1992a; Meyer et al., 1993; Kienzle, gestive capacities, such as young horses that are meal-fed 1994). The disruption of the structure of starch is more ex- large amounts of grains. It is reasonable also to assume that tensive with heat- and steam-processed corn than with coarsely processing hard-seed coat grains such as corn has more benefit. The unique arrangement of the horse’s digestive tract may limit the significance of measurements such as total TABLE 8-10 Comparison of Small Intestinal Starch tract digestion. As the digestive processes of the stomach Digestibility of Processed Corna,b and small intestine are significantly different than digestion Whole Crushed Ground Popped in the hindgut, site of digestion may be of more importance Corn Corn Corn Corn when addressing the relative benefit of processing. Most no- Starch intake tably, recent research interests have focused on site of di- (g/kg BW/meal) gestion of starch. morning:evening 1.9:1.0 1.9:1.1 2.1:2.0 1.3:1.5 Preileal starch digestibility (%) 28.9c 29.9c 45.6d 90.1e Starch Digestibility aAdapted from Meyer et al. (1993). Many horses are fed meals containing significant bDigestibility reflective of total ration of grain fed with green meal. amounts of starch in combination with hay or green forage. c,d,eValues with different superscripts reported different at P < 0.05.

170 NUTRIENT REQUIREMENTS OF HORSES ground corn and more extensive with ground corn than TABLE 8-11 Comparison of Small Intestinal Starch whole or broken corn (Kienzle et al., 1997). The proposed Digestibility of Processed Oatsa,b relationship between processing methods, disruption of the Whole Oats Rolled Oats Ground Oats structure of starch, and the potential effects on digestibility Starch intake and animal performance receives considerable support with (g/kg BW/meal) research conducted on other species (Gray, 1992; NRC, morning:evening 2.0:1.0 1.8:0.9 1.8:1.8 2001; Zinn et al., 2002; Zarkadas and Wiseman, 2002). Preileal starch Gelatinization refers to the irreversible swelling and the digestibility (%) 83.50c 85.23c 98.05d destruction of the internal crystalline structure of starch aAdapted from Kienzle et al. (1992). granules brought about by thermal processing (Selmi et al., bDigestibility reflective of total ration of grain fed with grass meal. 2000; Zinn et al., 2002; Vervuert et al., 2004). The ability of c,dValues with different superscripts were reported to be significantly dif- thermal, high-pressure processing methods to gelatinize ferent (P < 0.05). starch in cereal grains has been well established (Selmi et al., 2000). Processing methods, such as steam flaking of corn and micronizing of barley and wheat, can increase the degree of gelatinization of starch in cereal grains. Increases small intestinal starch digestibility of oats, whether in solubility and availability of starch to enzymatic and mi- processed or not, is supported by examination of oat starch crobial digestion should increase feed value; however, re- granules retrieved from intestinal chyme. The structure of search findings in other species provide differing results the starch granules in both whole and rolled oats appears to with diet digestibility and animal performance. be greatly altered by enzymatic digestion, possibly a result Work with growing-finishing beef cattle suggests that of the normal morphology of the starch molecule in oats and steam flaking sorghum improves the feeding value by 12–15 the effect of normal chewing on the disruption of the oat percent above dry rolling (Swingle et al., 1999). Similarly, seed coat (Kienzle et al., 1997). Zinn et al. (2002) suggested steam flaking corn increases the Studies have been conducted to compare digestibility of net energy for maintenance and gain in growing cattle by 14 different grains that are similarly processed. Preileal starch percent and 17 percent, respectively, over dry rolling. Stud- digestibility of rolled oats has been reported to be over three ies with mature dairy cows suggest less consistent improve- times that of rolled barley (Table 8-12) (Meyer, 1993). Ob- ments in feed utilization (NRC, 2001), as does research with servations of the morphology of starch granules suggest that growing swine (Hongtrakul et al., 1998; Zarkadas and Wise- the susceptibility to enzymatic digestion of starch molecules man, 2001, 2002). in rolled barley is somewhat intermediate between starch in Associative interactions such as intake level, degree of processed corn and rolled oats (Kienzle et al., 1997). retrogradation, differences in starch morphology between Potter et al. (1992a) reported little difference in the pre- grain species and varieties, animal differences, and process- cecal starch digestibility of rolled corn, oats, barley, or ing methods will influence the results of research relating sorghum when fed in low amounts to ponies (Table 8-13). processing, starch morphology, and animal performance. The Four ileally cannulated ponies were fed at 12-hour intervals, degree of structural breakdown of a starch molecule may and rations included chopped Bermudagrass hay with the vary with different sources of a specific grain processed sim- grains. The prececal starch digestibility of rolled barley was ilarly because of differences in equipment and conditions at numerically higher; however, this value may have been in- mills, and differences in nutrient content and harvesting fluenced by an abnormally high digestibility of rolled barley methods of the grain (Kienzle et al., 1997; Zinn et al., 2002). by one of the ponies used in the experiment (Potter, 1992a). Meyer et al. (1993) reported that ground corn milled at dif- When combining results with oats and sorghum, Potter et ferent locations to similar maximally allowed size (maxi- al. (1992a) reported higher prececal starch digestion with mum particle size < 2 mm) had enough differences in the de- micronizing as compared with crimping (61 percent vs. 42 gree of starch alteration to influence trial results. Similar to corn, grinding can increase preileal starch di- gestibility of oats over values obtained with whole or rolled TABLE 8-12 Comparison of Small Intestinal Starch oats (Table 8-11) (Kienzle et al., 1992). Estimates in Table Digestibility of Processed Oats and Barleya,b 8-11 were obtained from six horses with cannulas in the ter- Rolled Oats Rolled Barley minal jejunum. Chromic oxide was used as a marker. The Starch intake (g/kg BW/meal) horses were fed at 12-hour intervals. Grass meal was added morning:evening 1.8:0.9 2.0:2.0 to both meals when ground oats were fed, and to the evening Preileal starch digestibility (%) 85.2c 21.4d meal only when the whole and rolled oats were fed. aAdapted from Meyer et al. (1993). The degree of small intestinal digestibility of whole or bDigestibility reflective of total ration of grain fed with green meal. coarsely processed oats appears to be greater than corn c,d Values with different superscripts were reported to be significantly dif- (Kienzle et al., 1992; Meyer et al., 1993). The relatively high ferent (P < 0.05).

FEEDS AND FEED PROCESSING 171 TABLE 8-13 Comparison of Small Intestinal Starch Even though results appear to be specific to intake levels, Digestibility of Grainsa,b,c types of processing, and types of grains, digestibility trials Rolled Rolled Rolled Rolled show that processing can influence the degree of small in- Corn Oats Barley Sorghum Mean testinal starch digestibility. Indirect measures of prececal starch digestibility support results identified in digestion tri- Starch intake (g/kg BW/meald) 1.3 1.2 1.5 1.2 1.3 als. Intracecal lactate concentrations following meals of rolled, micronized, or extruded barley have been reported to Prececal starch be higher when ponies were fed rolled barley as compared digestibility (%) 80.9 81.0 95.9 80.3 85.1 to the other two processed barley meals (McLean et al., Total tract starch 2000). Steam flaked corn fed to mature horses produced a digestibility (%) 98.9 98.9 98.9 97.6 98.6 greater glycemic response and higher peak plasma glucose aAdapted from Arnold (1982) as reported in Potter et al. (1992a). concentration than did ground or cracked corn (Hoekstra et bDigestibility reflective of total ration of grain fed with chopped Bermuda- al., 1999). Other research conducted to evaluate the effect of grass hay. cNo significant differences observed between grains. processing on glycemic and insulinemic responses in horses dMeals fed twice daily. has not elicited as clear a response. Horses fed finely ground, steamed, micronized, steam flaked, or popped corn had similar responses in post-prandial blood glucose and in- sulin concentrations regardless of processing treatment percent). Four ileally cannulated horses were fed at 12-hour (Vervuert et al., 2004). intervals, and rations consisted of a 50:50 ratio of grain and Similarly, Vervuert et al. (2003) found little effect on Bermudagrass hay. An interaction between grain type and post-prandial plasma glucose or insulin response when oats processing method was noted (Table 8-14), with micronized were processed by grinding, steam flaking, or popping. As- oats having higher prececal starch digestibility than crimped sociative interactions such as individual horse responses, sorghum. differences in the degree of starch gelatinization of similar Small intestinal starch absorption is not only energeti- grains processed similarly, and intake levels may partially cally efficient compared with microbial digestion in the explain the differences in results from various researchers. cecum and large intestine, but also it guards against diges- Additionally, Kronfeld et al. (2004) reinforced the need to tive disorders resulting from soluble carbohydrate digestion use glycemic indices cautiously as large errors in their mean by microbes in the cecum and large intestine (Lewis, 1995; values and the nonlinearity of glucose-insulin regulatory Longland, 2001). It is apparent that processing grains can system limited glycemic indices’ use. significantly influence the relative amounts of starch di- Processing also influences the rate of starch digestion. gested in the small intestine. However, as influencing factors McLean et al. (1998) compared the rate of starch degradation such as differences in the degree of starch disruption of sim- of whole, micronized, and extruded barley by placing incuba- ilarly processed grains, differences in intake, and differences tion bags in situ of cecally fistulated ponies. Little differences between horses affect results, application of results from in degradation of dry matter or starch were evident after 40 single trials should be guarded, and differences in animal re- hours of incubation. However, there was a reported increased sponse should be expected. rate of degradation of micronized barley as compared with whole barley during the first 20 hours of incubation. TABLE 8-14 Comparison of Small Intestinal Starch Protein Digestibility Digestibility of Grains Fed at Moderate Intakesa,b The lack of research relating processing to protein di- Crimped Micronized Crimped Micronized gestibility in horses makes recommendations difficult. As Oats Oats Sorghum Sorghum Mean mentioned in the previous section on total tract nutrient di- Starch intake gestibility, most reports suggest little to no influence of pro- (g/kg BW/meal) 2.6 2.4 2.9 2.8 2.7 cessing on protein digestibility. Horses are capable of both Prececal enzymatic digestion of protein leading to absorption of starch digest- amino acids in the small intestine and fermentative digestion ibility (%) 48.0 62.3c 36.0d 59.0 51.3 of protein leading to absorption of ammonia in the large in- Total tract testine. This capability, with the differences in the absorbed starch products of digestion, limits the usefulness of total tract pro- digestibility 94.4 93.8 94.0 94.5 94.1 tein digestibility as a measurement to determine the relative aAdapted from Householder (1978) as reported in Potter et al. (1992a). value of different processing methods. bDigestibilityreflective of total ration of grain fed with Bermudagrass hay. In a summary of several trials conducted in their labora- c,dValues with different superscripts reported different at P < 0.1. tory, Potter et al. (1992b) identified compensatory digestion

172 NUTRIENT REQUIREMENTS OF HORSES of protein similar to that found with digestion of energy. volatility and solubility (Hardy et al., 1999; Mavromichalis They concluded that large intestine true digestibility of pro- and Baker, 2000). tein from several different feedstuffs was 80–90 percent, while small intestine true digestibility of protein varied from SUMMARY 45–80 percent. Prececal digestibility differed with inclu- sions of different feedstuffs and levels of intake. Specific horse diets vary greatly in form, ranging from One of the trials compared mixed diets of grass hay and rations consisting solely of nonharvested forages to those either oats or sorghum where the grains were fed crimped or containing feeds processed by a variety of methods. Feed micronized (Table 8-15). Four ileally cannulated horses processing provides many potential advantages to the feed were fed at 12-hour intervals, and rations consisted of a milling industry. Formulations may include a greater variety 50:50 ratio of grain and Bermudagrass hay. Total tract ap- of ingredients, feed handling may be eased, storage time parent digestibility of nitrogen of crimped sorghum was lengthened, and consumer acceptance increased. Some pro- lower than either of the two sources of oats. Prececal di- cessing methods may have a small effect on total tract di- gestibility of micronized oats appeared to be lower than gestibility for some feeds. In addition, some processing crimped oats or either source of sorghum, although other methods may influence the site of digestion for specific feed sources of variation apparently were so large that there were components, such as starch. Therefore, improvements in the no significant differences due to grain type or processing utilization of energetic nutrients may be more related to site method. of digestion rather than observable differences in total tract The effect of processing on amino acid absorption in the digestibility. As energetic efficiency is higher with small in- small intestine requires further investigation, especially in testinal starch digestion, shifting the percentage of starch di- horses fed large amounts of protein in meal feeding. By cal- gested prececally should also increase the energy value of culating expected absorption of amino acids from results of grains. Benefits appear to be related to differences in the several different trials, Gibbs and Potter (2002) emphasized structure of starch molecules of different grains, and with the potential for deficiencies of amino acids in diets fed to processing methods that combine sufficient heat and steam growing horses, even when rations met protein requirements to affect the chemical structure of starch. Research is lim- based on total tract digestion. ited, and results have varied between trials. However, pro- Underprocessing may leave a deleterious level of anti-nu- cessing has had the most dramatic effect on the small intes- tritional factors, such as trypsin in soybean meal, which has tinal starch digestibility of corn. Effects have also been been shown to affect performance and growth in other observed from the micronizing of barley and oats. Estimates species. Excessive heating may reduce the availability of es- from the previous version of the NRC (1989) of 2–5 percent sential amino acids (Dale, 1996). Research on the utilization increase in the digestibility of barley, with more benefits to of feedstuffs by other species shows that some processing processing hard-seed coat grains such as corn and milo, are methods can decrease the availability of amino acids be- supported with recently reported research on the site of cause of the Maillard reaction, in which protein combines starch digestion. Improvements with energy utilization are with other nutrients to form large compounds with differing expected to be largest with processes that significantly dis- rupt the structure of the starch molecule, such as microniz- ing, steam flaking, and popping. TABLE 8-15 Comparison of Small Intestinal Nitrogen Quantifying expected benefits of processing for all feed- Digestibility of Diets Containing Micronized and Crimped ing plans and horses is difficult. The relative benefit of pro- Oats and Sorghuma,b cessing will depend on many influencing factors including Crimped Micronized Crimped Micronized grain type, horse differences, processing method, and level Oats Oats Sorghum Sorghum Mean of starch and dry matter intake. Processing has a significant potential for reducing digestive upset of horses that are fed Nitrogen intake (mg/kg meals containing high amounts of starch that is resistant to BW/meal) 130 132 138 130 132 small intestinal digestion. Horses with limited ability to Prececal chew rations because of poor dentition should respond more apparent favorably to processed feeds than those horses able to break digestibility (%) 45.4 35.9 51.5 52.0 46.2 up feed prior to swallowing. Total tract Behavioral and intake differences observed with differ- apparent ently processed feeds could be large enough to alter other digestibility 68.6d,e 70.9e 62.2c 65.1c,d 66.7 management routines (see Chapter 11). Slower intakes may aAdapted from Klendshoj (1979) as reported in Potter et al. (1992b). be desirable to reduce boredom with confined horses. How- bDigestibility reflective of total ration of grain fed with a soybean meal ever, the relative behavioral and physiological importance of based supplement in a 50:50 ratio with Bermudagrass hay. processing appears to be specific to intake, feedstuffs, pro- c,d ,eValues with different superscripts reported different at P < 0.05. cessing method, and horses.

FEEDS AND FEED PROCESSING 173 APPENDIX 8-1 SELECTED TERMINOLOGY RELATED Cracked, cracking. Particle size reduced by a combined TO FEED IDENTIFICATION AND PROCESSING breaking or crushing action. Crimped, crimping. Rolled by use of corrugated rollers, SOURCE: Adapted from AAFCO (2005). which may curtail tempering or conditioning and cooling. Additive. An ingredient or combination of ingredients Crumbled, crumbling. Pellets reduced to a granular form. added to the basic feed mix or parts thereof to fulfill a Customer-formula feed. Consists of a mixture of commer- specific need. Usually used in micro quantities and re- cial feeds and/or feed ingredients each batch of which is quires careful handling and mixing. manufactured according to the specific instructions of the Balanced. A term that may be applied to a diet, ration, or final purchaser. feed having all the known required nutrients in proper Dehulled, dehulling. Having removed the outer covering of amount and proportion based upon recommendations of grains or other seeds. recognized authorities in the field of animal nutrition, Dehydrating, dehydrated. Having been freed of moisture such as the National Research Council, for a given set of by thermal means. physiological animal requirements. Diet. Feed ingredients or mixture of ingredients including Blocks. Agglomerated feed compressed into a solid mass water, which is consumed by animals. cohesive enough to hold its form and weighing over 2 Expanded. Subjected to moisture, pressure, and tempera- pounds, and generally weighing 14 to 22 kg. ture to gelatinize the starch portion. Bricks. Agglomerated feed, other than pellets, compressed Extracted, mechanical. Having removed fat or oil from ma- into a solid mass cohesive enough to hold its form and terials by heat and mechanical pressure. Similar terms: weighing less than 0.91 kg. expeller extracted, hydraulic extracted, “old process.” Byproduct. Secondary products produced in addition to the Extracted, solvent. Having removed fat or oil from materi- principal product. als by organic solvents. Similar term: “new process.” Carriers. An edible material to which ingredients are added Extruded. A process by which feed has been pressed, to facilitate uniform incorporation of the latter into feeds. pushed, or protruded through orifices under pressure. The active particles are absorbed, impregnated, or coated Feed is fed through the extruder from a holding bin, into or onto the edible material in such a way as to phys- through a mixing cylinder and into the extruder barrel. ically carry the active ingredient. The feed is subjected to increased pressure, friction, and Chopped, chopping. Reduced in particle size by cutting attrition as it passes through the barrel. As the feed is re- with knives or other edged instruments. leased from the extruder barrel, it expands violently as Cleaned, cleaning. Removal of material by such methods as steam is released because of the sudden drop in pressure. scalping, aspirating, magnetic separation, or by any other Feed(s). Edible material(s) that are consumed by animals method. and contribute energy and/or nutrients to the animal’s Commercial feed. As defined in the Uniform State Feed diet. (Usually refers to animals rather than humans.) Bill, all materials except unmixed whole seeds or physi- Fines. Any materials that will pass through a screen whose cally altered entire unmixed seeds, when not adulterated, openings are immediately smaller than the specified min- which are distributed for use as feed or for mixing in imum crumble size or pellet diameter. feed. Flakes. An ingredient rolled or cut into flat pieces with or Complete feed. A nutritionally adequate feed for animals without prior steam conditioning. other than humans; by specific formula is compounded to Fodder. The green or cured plant, containing all the ears or be fed as the sole ration and is capable of maintaining life seed heads, if any, grown primarily for forage. and/or promoting production without any additional sub- Formula feed. Two or more ingredients proportioned, stance being consumed except water. Note: Other regula- mixed, and processed according to specifications. tory agencies also include those feeds that provide all of Gelatinized, gelatinizing. Having had the starch granules the nutritional requirements necessary for maintenance completely ruptured by a combination of moisture, heat, of life or for promoting production except water and and pressure, and, in some instances, by mechanical roughage within the definition of complete feeds. shear. Concentrate. A feed used with another to improve the nu- Germ. (Part) The embryo found in seeds and frequently tritive balance of the total and intended to be further di- separated from bran and starch endosperm during the luted and mixed to produce a supplement or complete milling. feed. Gluten. The tough, viscid nitrogenous substance remaining Conditioning. Having achieved predetermined moisture when the flour or wheat or other grain is washed to re- characteristics and/or temperature of ingredients or a move the starch. mixture of ingredients prior to further processing. Grain. Seed from cereal plants. Cooked, cooking. Heated in the presence of moisture to alter Ground, grinding. Reduced in particle size by impact, chemical and/or physical characteristics or to sterilize. shearing, or attrition.

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Nutrient Requirements of Horses: Sixth Revised Edition Get This Book
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Proper formulation of diets for horses depends on adequate knowledge of their nutrient requirements. These requirements depend on the breed and age of the horse and whether it is exercising, pregnant, or lactating.

A great deal of new information has been accumulated since the publication 17 years ago of the last edition of Nutrient Requirements of Horses. This new edition features a detailed review of scientific literature, summarizing all the latest information, and provides a new set of requirements based on revised data. Also included is updated information on the composition of feeds, feed additives, and other compounds routinely fed to horses. The effects of physiological factors, such as exercise, and environmental factors, such as temperature and humidity, are covered, as well. Nutrient Requirements of Horses also contains information on several nutritional and metabolic diseases that horses often have.

Designed primarily as a reference, both practical and technical, Nutrient Requirements of Horses is intended to ensure that the diets of horses and other equids contain adequate amounts of nutrients and that the intakes of certain nutrients are not so excessive that they inhibit performance or impair health. This book is primarily intended for animal nutritionists, veterinarians, and other scientists; however, individual horse owners and managers will also find some of this material useful. Professors who teach graduate courses in animal nutrition will find Nutrient Requirements of Horses beneficial as a textbook.

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