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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

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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-

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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-

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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.

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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),

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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

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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

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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

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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

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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.

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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)

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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.

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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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174 NUTRIENT REQUIREMENTS OF HORSES Hay. The aerial portion of grass or herbage especially cut REFERENCES and cured for animal feeding. AAFCO (Association of American Feed Control Officials, Inc.). 2005. Of- Hulls. Outer covering of grain or other seed. ficial Publication. Oxford, IN: Association of American Feed Control Ingredient, feed ingredient. A component part or con- Officials. stituent of any combination or mixture making up a com- Adams, L. G., J. W. Dollahite, W. M. Romane, T. L. Bullard, and C. H. Bridges. 1969. Cystitis and ataxia associated with sorghum ingestion in mercial feed. horses. J. Am. Vet. Med. Assoc. 155:518–524. Kibbled, kibbling. Cracked or crushed baked dough, or ex- Aiken, G. E., G. D. Potter, B. E. Conrad, and J. W. Evans. 1989. Voluntary truded feed that has been cooked before the extrusion intake and digestion of coastal Bermuda grass hay by yearling and ma- process. ture horses. J. Equine Vet. Sci. 9:262–264. Meal. 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