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12 Unique Aspects of Equine Nutrition NURSING AND ORPHAN FOALS liquid milk replacer consumed less than 1 kg grain mix per 100 kg BW daily up to 1 month of age, then increased intake During the first week of life, healthy foals will nurse up to 1.5–2 kg grain mix per 100 kg BW daily at 7 weeks of age to seven times per hour, with each nursing bout lasting 1 to (Cymbaluk et al., 1993). Foal body weight averaged about 2 minutes (Carson and Wood-Gush, 1983). Subsequently, 60 kg at birth, and average daily gains ranged from a low of there is a decrease in the frequency and duration of nursing 0.18 kg/d during the first 2 weeks of age to 1.43 kg/d be- bouts; at 4 weeks of age, foals nurse about three times per tween 16 and 24 weeks of age. In another trial, creep feed hour (Carson and Wood-Gush, 1983). During the first 24 intakes of extensively managed nursing foals averaged hours of life, foals consume approximately 15 percent of 0.56–0.84 kg/d (Coleman et al., 1999). Initially, foals were body weight (BW) as milk, increasing to 22–23 percent on approximately 2 months of age with a mean BW of 140 kg. day 2, and approximately 25 percent of BW (15 liters for a At 4 months of age (the end of the study), mean BW was ap- 50 kg-foal) by 7 days postpartum (Ousey et al., 1996). One proximately 200 kg. study of Thoroughbred foals indicated that the digestibility Management of the orphan foal is dependent upon the age of mare’s milk is 98 percent (Ousey et al., 1997). Although at which the foal loses its dam. Young foals should be fed a the dam’s milk will normally supply all the nutritional needs milk-based diet to ensure satisfactory growth and develop- of foals for the first 6–8 weeks of life, foals will begin to ment, whereas older foals may develop adequately with a consume small portions of solid feed within days of birth. diet of high-quality forage and creep feed. For foals orphaned Foals will increasingly seek solid feed sources to supply on the 1st day of life, an important consideration is provision their nutrient needs by consumption of the dam’s feed, or if of high-quality colostrum that contains immunoglobulins available, feed supplied by creep feeding. Creep feeding has vital to competency of the foal’s naïve immune system. Fol- advantages of supplying a nutrient-dense source of feed to lowing ingestion by the foal, immunoglobulins in colostrum foals that is protected from ingestion by mares. are absorbed by specialized cells throughout the epithelium Average daily gain of foals fed creep feed before wean- of the small intestine. The absorption process is most effi- ing can be higher when compared to foals not receiving cient following the first few feedings postpartum, with a creep rations (Coleman et al., 1999). Additionally, foals that rapid decline in uptake efficiency over the first 12 hours of become accustomed to consuming dry feed prior to weaning life. By 24 hours, the small intestine is no longer permeable have reduced weaning stress (McCall et al., 1985; Hoffman to colostral immunoglobulins (Jeffcott, 1972). Colostrum et al., 1995). Creep rations are typically formulated to con- should be fed before any milk replacers to ensure maximal tain 16–20 percent high-quality crude protein (CP), 0.8–1 absorption of immunoglobulins (Stoneham, 2005). The im- percent calcium, and 0.6–0.8 percent phosphorus, although munoglobulin content of colostrum declines rapidly after the there has been limited research to assess the optimal nutri- onset of nursing in postpartum mares. Milk samples col- ent composition of creep feeds for foals. lected 4–8 hours after birth have 15 percent of the im- The decision to provide creep feed will depend on de- munoglobulin concentration of samples collected in the first sired growth rates and post-weaning nutritional programs. 3 hours postpartum (Naylor, 1979). Therefore, the colostrum Voluntary intake of creep feed is expected to vary between fed to orphan foals should have been harvested from mares foals and to be influenced by factors such as herd behavior, soon after parturition. placement of creep feeders, and presence of other foals con- Fostering to a nurse mare is the preferred option for man- suming feeds. One trial reported that orphan foals receiving agement of orphan foals less than 6 to 8 weeks of age. Foals 235

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236 NUTRIENT REQUIREMENTS OF HORSES that cannot be fostered should be fed a mare’s milk substi- lates to physiological function and uses the decline in phys- tute. Fortified cow’s milk, goat milk, and commercially iological function as a threshold for old age. Demographic available milk replacer products specifically designed for age reflects survivorship of an age-group subpopulation rel- foals have been used (Naylor and Bell, 1985; Pugh and ative to the whole. No exact chronological threshold for old Williams, 1992). Milk substitutes should be designed to age in horses has been identified. Several investigators have mimic the nutrient concentrations in mare’s milk (Naylor used 20 years of age to define the threshold of old age (Ral- and Bell, 1985). Research findings on composition of ston et al., 1988, 1989; Malinowski et al., 1997; Brosnahan mare’s milk are provided in Table 16-8. On a dry matter and Paradis, 2003). The 1998 National Animal Health Mon- (DM) basis, foal milk replacers should contain approxi- itoring Systems study (USDA, 1998) estimated that 7.5 per- mately 15 percent fat and 22 percent CP, with a fiber content cent of the horse population (total horse population esti- of less than 0.5 percent (Naylor and Bell, 1985). Milk re- mated at approximately 7 million) was 20 years of age or placers should be fed as a 10–15 percent solution. Milk from older. farm animal species (i.e., goat, pig, sheep, and cow) is dis- Although 20 years of age may serve as an estimate of the similar to mare’s milk. Therefore, commercially available threshold for old age, the degree of variation of this estimate milk replacers are the preferred milk substitute for foals. is not known. Therefore, the combination of chronological However, cow’s milk is suitable if some of the fat is re- age and physical signs of aging may be the most effective moved and sugar added. One recommendation is to feed 2- means of establishing the “old-age” threshold for individual percent-fat skimmed milk to which dextrose has been added horses. Physical signs of aging may include chronically low at the rate of 20 g/L (40 ml of 50 percent dextrose per liter body condition score, loss of muscle mass over the top line of milk) (Naylor and Bell, 1985). yielding a sway-backed appearance, hollowing out of the It is advisable to gradually increase the volume of milk grooves above the eyes, graying of the coat, and dental dis- fed over a 7- to 10-d period. One recommendation is to start ease (Ralston et al., 1988, 1989; Ralston and Breuer, 1996; at 5–10 percent BW at day 1, increasing to 20–25 percent Paradis, 2002). BW by day 10 (Naylor and Bell, 1985). Initially, many small meals should be provided to somewhat mimic natural feed- Energy Requirements of Aged Horses ing patterns (e.g., every hour). As the foal begins to eat solid feed, the frequency of feedings can be gradually reduced. Energy requirements are a function of energy expendi- Alternatively, the foal can be given free choice access to ture and the efficiency with which gross energy (GE) pres- milk in a pail or by use of an automated feeding device de- ent in feeds is converted to net energy (NE). Both of these signed for calves. Fresh water should be available at all factors have the potential to be affected by age. times (Cymbaluk et al., 1993). Orphan foals can be weaned Maintenance energy requirements typically constitute the from milk at 10–12 weeks of age (Naylor and Bell, 1985). largest proportion of total energy expenditure. Maintenance energy requirements in aged humans and dogs have been re- ported to be 15–20 percent lower when compared to OLD AGE younger populations (Harper, 1998a; Bosy-Westphal et al., Several challenges exist in defining nutrient requirements 2003). The decline in maintenance energy expenditure is of old horses. The first challenge is to establish criteria that thought to be a function of declining fat-free mass associ- define the threshold for old age in horses. Secondly, nutrient ated with aging (Bosy-Westphal et al., 2003). Decreased requirements are a function of metabolic requirements and physical activity is thought to be a primary factor in the age- the efficiency of nutrient digestion, absorption, and metabo- related decline in fat-free mass (Harper, 1998a; Roubenoff, lism. Therefore, knowledge of age-related changes in diges- 1999). Whether an age-related decline in maintenance en- tion, absorption, and metabolism is necessary to define the ergy requirement occurs in horses is unknown. Although ap- nutrient requirements of old horses. Finally, the effect of parent loss of muscle mass has been observed in old horses aging-associated disease on nutrient requirements must be (Ralston et al., 1989), age-related change in fat-free mass determined and should be viewed separately from the effects and subsequent changes in maintenance energy requirement of aging itself. Old age is not synonymous with the term have not been quantified in horses. Nor is the effect of age “geriatric,” which refers to diseases of the aged. on physical activity in horses well characterized. Therefore, the extent to which aging alters the maintenance energy re- quirements of horses is not known. Some disease conditions Defining Old Age may increase energy requirements. Mean resting energy ex- Paradis (2002) described three types of age that may be penditure was increased by approximately 41 percent in useful in establishing old age threshold for horses: chrono- horses with recurrent airway obstruction as compared to logic, physiologic, and demographic. Chronologic age is the controls (Mazan et al., 2004). actual number of years of life from birth. Physiologic age re- Fecal energy typically accounts for the largest proportion of gross energy lost from feedstuffs. Therefore, factors that

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UNIQUE ASPECTS OF EQUINE NUTRITION 237 influence DM digestibility have a large impact on digestible Micronutrient Requirements energy yield. Mean crude fiber apparent digestibility tended Requirements for micronutrients in old horses remain (P = 0.10) to be lower (~ 5 percent) in a group (n = 7) of old relatively uninvestigated. Only two reports related to the ef- horses (26 ± 5 years of age) when compared to a small group fect of aging on micronutrient nutrition were identified as of (n = 5) of younger horses (2.3 ± 0.5 years of age) (Ralston this writing. Ralston et al. (1989) found decreased phospho- et al., 1989). Because the magnitude of this change in crude rus apparent digestibility (–4 ± 19 vs. 11 ± 6 percent) in old fiber apparent digestibility was similar to that of the young (26 ± 5 years of age) vs. younger (2.3 ± 0.5 years of age) horses used in the study following resection of left and right horses. Additionally, initial reports suggested vitamin C sta- colons, the authors suggested that aged horses have a re- tus may be different between young and old horses (Ralston duced absorptive and/or digestive function in the large in- et al., 1988), but more recent evidence does not support this testine (Ralston et al., 1989). This is in contrast to findings idea (Deaton et al., 2004). in humans and dogs that suggest gastrointestinal function, at least with regard to macronutrients, is well preserved with aging (Harper, 1998b; Russell, 2000). It is also likely in Feed Form some instances that age-related changes to teeth may impair Dental abnormalities, which are common in older horses a horse’s ability to masticate feed, subsequently decreasing (Paradis, 2002; Graham, 2002), can limit the ability to pre- digestibility in the remainder of the digestive tract. hend and chew feed, decrease the digestibility of nutrients, Another area of potential interest in old horses is the ef- and lead to substantial loss of body weight. Alterations in fect of caloric restriction on aging and age-related disease. the physical form of the ration can be beneficial for old Caloric restriction has been demonstrated to extend the life- horses with dental disease. A common approach is to feed span in a variety of species (Heilbronn and Ravussin, 2003). older horses processed, complete feeds. In one study, the Larson et al. (2003) reported that lifetime dietary restriction feeding of a complete feed containing extruded ingredients improved glucose tolerance and had a favorable effect on was more effective at maintaining the body weight of old disease and survival in dogs. horses with low body condition (condition score less than 3) Equine Cushing’s disease is a chronic progressive disease when compared to a more traditional ration consisting of a of the intermediate pituitary gland of older horses. This dis- grain mix and timothy-alfalfa hay (Ralston and Breuer, ease is discussed in detail elsewhere (McCue, 2002). Hyper- 1996). Other options include the feeding of ensiled forage glycemia and hyperinsulinemia are common findings in (haylage) or chopped hay (e.g., early cut alfalfa) or forage horses with equine Cushing’s disease (Garcia and Beech, cubes that have been soaked in water. Oil can be added to in- 1986). Therefore, providing calories from sources that do crease the energy density of the ration. not contribute substantial quantities of glucose to the blood stream (e.g., fiber) appears prudent. Conclusion The true effect of aging and age-related disease on nutri- Protein and Amino Acid Requirements ent requirements remains to be determined in horses. There The effect of aging and age-related disease on protein re- appears to be a large degree of variation in the way old quirements of horses is unknown. Ralston et al. (1989) re- horses respond to similar diets (Ralston and Breuer, 1996), ported lower crude protein apparent digestibility (67 ± 3 vs. suggesting chronological age alone is not sufficient to cate- 73 ± 3 percent) in aged horses (26 ± 5 years of age) when gorize horses relative to age-related changes in nutrient compared to younger horses (2.3 ± 0.5 years of age). requirements. Whether this finding reflects the old horse population in general and significantly impacts protein requirements of FEEDING MANAGEMENT OF HORSES IN COLD OR old horses remains to be determined. Supplemental lysine HOT WEATHER and threonine (0.25 percent on a DM basis or approximately 20.0 g/d and 0.2 percent of DM or 15 g/d, respectively) have Chapter 1 includes a discussion of the effects of climate been suggested to maintain muscle mass in old (22.4 ± 0.87 on heat production in horses and also describes factors that years) as well as younger horses (9.1 ± 0.29 years) under- affect upper critical temperature (UCT) and lower critical going light exercise (Graham-Thiers and Kronfeld, 2005). temperature (LCT) for horses. The UCT is the upper range Healthy humans appear to have a decreased protein require- of the thermoneutral zone and is the temperature above ment with aging. Millward et al. (1997) reported a 33 per- which evaporative heat loss must be increased to control cent reduction in protein metabolic demand and no signifi- body temperature. The LCT is the lowest temperature in the cant impairment in efficiency of protein utilization in thermoneutral zone and is the temperature below which healthy, mobile, elderly persons, suggesting a decline in pro- metabolic heat production must be increased to maintain tein requirements with aging. body temperature. For horses kept in environments outside of the thermoneutral zone, adjustments in nutrient require-

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238 NUTRIENT REQUIREMENTS OF HORSES ments will occur and changes in feeding management may TABLE 12-1 Guidelines for Feeding Horses during Cold be necessary. Weathera 1. The lower critical temperature (LCT) for young horses can range from –11 to 0°C and for adult horses can be as low as –15°C in northern Feeding in Cold Weather continental climates. In more temperate climates, LCT was reported to Two studies with growing horses reared in cold, outdoor be 5°C. Specific horses may have higher or lower LCT. If cold weather persists at temperatures below LCT, then an increased provision of di- weather gave differing results concerning voluntary feed in- etary energy may become necessary. take (Cymbaluk and Christison, 1989a; Cymbaluk, 1990). 2. Growing horses may require an additional 1.3 percent digestible en- As temperatures fell below LCT, yearling horses in the first ergy (DE) for each degree below LCT plus the DE required for weight study were found to eat less, not more, feed and, therefore, gain (Cymbaluk, 1990). Adult horses should be given an additional 2.5 consumed less digestive energy (DE). Although decreases in percent DE for maintenance per degree below LCT (McBride et al., 1985). DE intakes were only 5.7 percent and 8.8 percent less at tem- 3. Hays should be provided free-choice to allow horses to eat to their en- peratures below –10° and below –20°C compared to above ergy demands. Use of digestible hays facilitates higher total intake –10°, weight gains were lower than expected (Cymbaluk and (Dulphy et al., 1997). Concentrate may need to be added to ensure ad- Christison, 1989a). In the second study, growing horses ate equate energy intake especially for growing, thin, worked, or aged 0.2 percent more feed per Celsius degree decrease in ambi- horses. If a concentrate is to be fed, the horse must be fully adapted to this feed. ent temperature below LCT (Cymbaluk, 1990). Based on 4. Additional mineral and vitamin supplementation does not appear to be this study, DE intakes for growing horses must be increased necessary during cold weather beyond the requirements needed for the by 1.3 percent per Celsius degree below LCT. By compari- specific production level of the horse. son, maintenance DE for adult horses must be increased 5. Water should be provided ad libitum and can be heated up to 20°C to 2.5 percent per Celsius degree below their LCT of –15°C maximize intake (Kristula and McDonnell, 1994). (McBride et al., 1985). The disparity between feeding rec- 6. Well-bedded and wind-protected shelter should be provided to mini- mize energy loss. ommendations for adults and growing horses is that the LCT aSOURCE: Modified from Cymbaluk and Christison (1990). for adults was based on maintenance only, and those for young horses was based on energy intakes for maintenance and gain. Young, thin, or aged mature horses are less cold- tolerant than mature horses. When cold weather occurs, diet 1995, 1999; McCutcheon and Geor, 1996). The primary changes for susceptible horses must be made much sooner electrolytes lost in sweat are sodium, potassium, and chlo- than for mature horses in good body condition. ride. The amount and type of electrolyte supplementation The overall conclusion from the preceding data is that depends on the extent of heat stress and the amount and du- cold weather creates an increased demand mainly for en- ration of physical stress that is imposed. At a minimum, ergy. Modified guidelines to those previously published for free-choice access to salt should be available to horses dur- feeding horses in cold weather are given in Table 12-1 ing hot weather (McCutcheon and Geor, 1996). General (Cymbaluk and Christison, 1990). Feeding good-quality hay guidelines for feeding horses during hot weather are given in free-choice may be the simplest way to supply additional Table 12-2. DE for most idle, adult horses during cold weather. The in- In hot weather conditions, the feeding program for idle fluence of diet composition and nutrient content on energy horses should be designed to minimize heat load. Although metabolism during cold weather is described in Chapter 1. high-fat diets may prove potentially useful in reducing heat In addition to diet, the effects of cold weather on non- load in hot weather (Kronfeld, 1996; Kronfeld et al., 1998), acclimated horses can be ameliorated through physical mod- few studies have critically examined the metabolic effects of ifications including housing or application of a rug or blan- high-fat diets on thermoregulation of idle horses in hot ket. Shelters, rugs or blankets, and shelters plus rugs or weather conditions. Under normal thermal conditions, blankets reduced heat loss during cold exposure by 9, 18, and horses fed high-fat diets reportedly had lower respiratory 26 percent, respectively (MacCormack and Bruce, 1991). exchange ratios than those fed high-carbohydrate diets (Pagan et al., 2002). Lower blood pH, higher blood glucose, a calculated reduction in urine volume, reduced feed and Feeding in Hot Weather water intake, and weight gain have also been reported Water intake is markedly increased (30–75 percent) by (Zeyner et al., 2002). Mathiason-Kochan et al. (2001) ob- both acute and chronic heat loads (Geor et al., 1996; Marlin served that horses fed a fat-supplemented diet (a concentrate et al., 2001). The high intake of water compensates for water with 10 percent fat of DM, fed in a 65:35 ratio with hay) had lost through rapid respiration and increased sweating rate higher sweat losses after a high-intensity standard exercise when conditions are hot and dry but not hot and humid (Mc- test (SET) but lower packed cell volumes than horses fed a Cutcheon et al., 1995). The increased sweating rate in hot high-hay diet (60 percent hay) or the basal diet with no conditions, especially when superimposed with exercise, re- added fat. The authors suggested this might reflect an in- sults in substantial electrolyte losses (McCutcheon et al., crease in extracellular fluid, but in the absence of plasma

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UNIQUE ASPECTS OF EQUINE NUTRITION 239 TABLE 12-2 Guidelines for Feeding Horses during Hot codes for the sodium channel, a membrane protein involved Weather in the regulation of cellular sodium and potassium content. 1. The upper critical temperature (UCT) for horses is about 38°C for This ion channel facilitates movement of potassium into the foals and 25°C for adults, depending on duration of exposure to hot cell and outward movement of sodium. ambient temperatures. Climatic conditions of an area will determine the absolute value of the UCT of the horses living in those regions. 2. At ambient temperatures that exceed UCT, water should be supplied in Etiology and Genetic Basis a manner that allows voluntary intake by the horse. There appears to be no preference for iced water by horses exposed to warm ambient In normal muscle cells, an electrical gradient is estab- temperatures (McDonnell and Kristula, 1996). lished across the cell membrane as sodium is pumped out- 3. Although the benefit of feeding grain-based and fat-supplemented diets side of the cell and potassium into the cell. That electrical to horses in hot weather is unclear, these types of diets may be theoret- gradient is typically on the order of 85 millivolts (mV) and ically useful for horses. is maintained in part by the ability of the sodium channel to 4. Salt should be available to horses during hot weather (McCutcheon and Geor, 1996). restrict entry of sodium into the cell. As long as that gradi- 5. A shade, preferably that allows unimpeded air movement, may reduce ent is maintained, the cell is at rest and no muscle contrac- heating effects of direct sunlight. Coat clipping may help dissipate heat tion occurs. During excitation, a nerve impulse is received in horses with a long hair coat (Morgan et al., 2002). by the muscle cell that results in a conformational change of the sodium ion channel and the generation of an “action po- tential.” During this event, sodium rushes into the cell, protein data, the effect on body fluid distribution is difficult potassium rushes out, and the electrical gradient approaches to interpret. 0 mV. This action potential, in a coupled reaction, causes the Conflicting data have been reported on the impact that fat release of calcium from storage sites within the sarcoplas- may have on utilization of other nutrients (see Chapter 3; mic reticulum of the muscle cell, with movement of calcium Beynen and Hallebeek, 2002). Although soybean oil added ions to the area of the muscle fibrils actin and myosin. Cal- to provide dietary fat at 5–10.8 percent resulted in reduced cium binds to sites on myosin, and the muscle contracts and dry matter, fiber, and protein digestibility by horses (Worth remains contracted until the calcium is resequestered in the et al., 1987; Jansen et al., 2001), no effect of fat was found storage sites by the action of a calcium pump. on nutrient digestion coefficients derived by various meth- In the muscle cells of horses with the HYPP trait, regula- ods (Meyers et al., 1987; Bush et al., 2001). Others have tion of the movement of sodium and potassium ions via the found an increase in fiber utilization (Hughes et al., 1995; sodium channel is disturbed such that there is a constant Julen et al., 1995). Fat supplementation of diet did improve “back leakage” of sodium ions into the cell. As a conse- utilization of either natural or β-carotene or α-tocopherol by quence, there is repetitive depolarization of some muscle horses (Keinzle et al., 2003). More importantly, the stability cells. Under certain conditions, these repetitive contractions and effects of long-term storage of high-fat horse diets under become more severe and are clinically recognized as muscle high heat conditions have not been fully explored. fasiculations. During clinical episodes of HYPP, fasicula- tions are often first evident over the rib cage and flank areas, but may spread to other muscle groups. There can be pro- NUTRITIONAL MANAGEMENT OF SPECIFIC lapse (eversion) of the third eyelid and the horse may show DISEASE CONDITIONS signs similar to colic. In severe episodes, the contractions become tetanic; the horse may sit like a dog and later Hyperkalemic Periodic Paralysis become recumbent. Death due to respiratory failure is pos- Hyperkalemic periodic paralysis (HYPP) is a co-dominant sible. In horses that experience severe episodes, serum single autosomal gene disorder that appears to have originated potassium concentrations may increase from 3 to 4 mEq/L as a point mutation in the Quarter horse stallion Impressive to as much as 12 mEq/L or higher (Meyer et al., 1999). (Spier et al., 1994). The genetic nature of this condition has Recognition of such a marked increase in serum potassium been well documented and a highly reliable test based on concentration is useful in differentiating HYPP from other polymerase chain reaction (PCR) technology has been devel- muscle diseases. oped to identify horses with this condition (Meyer et al., 1999). The test was developed originally for a condition in Nutritional Management humans that is essentially the same genetic abnormality (Lehmann-Horn et al., 2002). To be affected, the horse must Treatment and management of HYPP aims to limit in- have inherited the defective gene from an affected parent de- creases in serum potassium concentration by one or a com- scended from Impressive. Affected horses are either het- bination of three methods: (1) limiting the dietary intake of erozygous (H/N) or homozygous (H/H) for HYPP. Horses potassium, (2) promoting entry of potassium into cells, or that are homozygous are more severely affected than those (3) eliminating excess extracellular potassium from the that are heterozygous (Carr et al., 1996). The gene in question body via the urine.

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240 NUTRIENT REQUIREMENTS OF HORSES The most important management practice for HYPP- Exertional Rhabdomyolysis Syndromes positive horses is a restriction in potassium intake. Recent Exertional rhabdomyolysis (ER) in horses is a syndrome work has demonstrated that the risk of clinical episodes is of muscle pain and cramping associated with exercise. It can heightened when dietary potassium exceeds 1 percent of the be categorized into sporadic exertional rhabdomyolysis, in total diet (Reynolds, 1997; Reynolds et al., 1998a,b). When which horses have sporadic or infrequent episodes of mus- dietary potassium content is maintained below this 1 cle necrosis and pain associated with exercise, or chronic ex- percent threshold, HYPP episodes can often be controlled ertional rhabdomyolysis, in which affected horses have re- without other preventative measures. peated episodes (Valberg et al., 1999a). Two forms of The largest single source of potassium in a horse’s diet is chronic exertional rhabdomyolysis have been described: (1) forage (hay). Grains contain potassium, but are generally recurrent exertional rhabdomyolysis (RER), which com- less than 0.5 percent potassium. Forages may, on the other monly afflicts Thoroughbreds but also may affect other hand, contain in excess of 3 percent potassium. Forages vary breeds such as Standardbreds and Arabians; and (2) poly- widely in their potassium content by type, region of the saccharide storage myopathy (PSSM), which predominantly country, and even different areas within a single field. Fer- affects Quarter horses but also has been described in other tility level of the field, rainfall or irrigation, and stage of ma- breeds including Paints, European Warmbloods, Ap- turity all have an effect on the potassium content of forages. paloosas, Morgan horses, and draft breeds (Valberg et al. Higher fertility levels, increased moisture, and cutting hay at 1999a). In Thoroughbreds, RER has been identified as a her- an early stage of maturity all tend to increase the potassium itable defect in intracellular calcium regulation that leads to concentration of the forage (Minson, 1990). Given this wide muscle necrosis during exercise (MacLeay et al., 1999). variation in potassium content, even within forage types, Polysaccharide storage myopathy involves increased storage laboratory analysis is required for accurate estimation of of glycogen and abnormal polysaccharide in skeletal mus- potassium content. However, in general, grass forages tend cle. In both RER and PSSM, there is some evidence that a to have less potassium than legume forages. reduction in dietary starch and sugar and/or an increase in In view of the potassium concentrations in feeds, the use dietary fat are beneficial in the management of affected of grass hays or pastures such as Bermudagrass, prairie hay, horses (see McKenzie et al., 2003). or timothy instead of legume hays or pastures such as alfalfa seems prudent. If alfalfa must be included in the diet, then other preventative measures may be necessary. Also, the use Polysaccharide Storage Myopathy of cereal grains as a major portion of the diet will reduce the overall potassium content of the ration. In addition, grain in- Polysaccharide storage myopathy is characterized by take will stimulate insulin release that may facilitate the up- high concentrations of glycogen and glucose-6-phosphate in take of potassium by muscle cells. However, the inclusion of muscle and the accumulation of amylase-resistant, periodic commercially prepared concentrates that contain large acid Schiff (PAS)-positive inclusions in up to 30 percent of amounts of molasses, soybean meal, or dehydrated alfalfa type II muscle fibers (Valberg et al., 1992). Clinical signs in- should be monitored, as these feed ingredients may be rela- clude ER, exercise intolerance, muscle stiffness, back pain, tively high in potassium (> 2 percent). shifting lameness, gait changes, muscle atrophy, a camped- In cases where control of potassium intake is insufficient out stance, and colic-like signs. Detection of amylase- to manage episodic events, additional interventions may be resistant polysaccharide in muscle of horses with a history required. The uptake of potassium by the cells is enhanced of recurrent ER is considered diagnostic for PSSM. How- by a number of factors, including mild exercise, insulin re- ever, the accumulation of polysaccharide may be a gradual lease, and administration of other cations into the extracel- process. In a small group of Quarter horse foals with clini- lular fluid. Insulin concentrations can be increased by oral or cal and laboratory evidence of chronic, intermittent ER, intravenous (IV) glucose administration, or the feeding of polysaccharide accumulation in skeletal muscle was not ap- cereal grains high in starch. Cations such as sodium and parent until 2 years of age (De La Corte et al., 2002). calcium-administered IV promote the intracellular move- A glycogen storage disorder also has been described in ment of potassium in order to maintain electrical neutrality Warmblood horses (Hunt et al., 2005), draft horses and re- of the extracellular fluid (ECF). As a further preventive lated breeds, and Welsh ponies (Valentine et al., 1997, 2000, measure, a diuretic such as acetazolimide may be adminis- 2001a,b). In draft horses, this disorder has been termed tered under the supervision of a veterinarian. This drug in- equine polysaccharide storage myopathy (EPSM) (Valentine creases the excretion of potassium in urine. et al., 2001a). Clinical signs of polysaccharide myopathy in draft horses and related breeds include muscle soreness of the hindquar- ters and back, stiffness, muscle atrophy, and occasionally overt ER. In Belgian draft horses, PSSM may occur concur-

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UNIQUE ASPECTS OF EQUINE NUTRITION 241 rently with “shivers,” a disorder characterized by muscle exercise and/or turnout, in addition to diet change, showed tremors and hindlimb hyperflexion that may progress to significant clinical improvement (Hunt et al., 2005). Quarter muscle atrophy, weakness, and recumbency (Firshman et horses with PSSM also were more likely to show improve- al., 2005). The results of one epidemiologic study indicated ment in the severity and frequency of ER when changes in that PSSM and shivers are common but unrelated disorders both diet and physical activity were instituted vs. a change of Belgian draft horses (Firshman et al., 2005). in diet only (Firshman et al., 2003). Nutritional Factors Pathogenesis There is evidence that diet modifies the clinical expres- The mechanisms underlying enhanced glycogen storage sion of PSSM (and EPSM). In clinical reports, the frequency in Quarter horses with PSSM have been partially elucidated. and severity of ER episodes were higher when PSSM- Unlike skeletal muscle glycogenoses in humans and other affected horses received little exercise and were fed energy species (DiMauro and Lamperti, 2001), excessive glycogen concentrates containing moderate amounts of starch and storage is not due to reduced capacity for glycogen utiliza- sugar such as straight grains or sweet feed mixes (Valentine tion. During controlled exercise protocols, net glycogen et al., 2001b; McKenzie et al., 2003). Conversely, controlled breakdown and accumulation of lactate in skeletal muscle laboratory experiments in Quarter horses with PSSM (middle gluteal m.) were similar in affected Quarter horses (Ribeiro et al., 2004) and uncontrolled clinical trials in and controls (Valberg et al., 1999a,b). Similarly, the activi- Quarter horses (Firshman et al., 2003), Warmblood horses ties of key glycolytic enzymes, measured in homogenates of (Hunt et al., 2005), and draft horses (Valentine et al., 2001a) muscle biopsies, did not differ between affected and control with evidence of muscle polysaccharide accumulation have horses (Valberg et al., 1998). Instead, excessive muscle shown that the feeding of a ration with restricted starch and glycogen storage may be related to enhanced insulin sensi- sugar content (on a total ration basis, < 8 percent DE from tivity and uptake of glucose into skeletal muscle. Glucose starch and sugar) and added fat (> 10 percent of total DE clearance following bolus intravenous administration of glu- from fat) resulted in clinical improvement of affected cose (0.5 g/kg BW) was 1.5 times faster in affected Quarter horses. horses when compared to healthy control horses, while glu- In one study, Quarter horse mares (n = 4) were fed cose concentrations after oral glucose administration were isocaloric diets ranging in DE from 21.2 percent (diet A), significantly lower (De La Corte et al., 1999a). Affected 14.8 percent (B), 8.4 percent (C), to 3.9 percent (D) for horses had lower resting insulin concentrations and lower starch, and 7.2 percent DE (diet A), 9.9 percent (B), to 12.7 insulin concentrations than controls after intravenous or oral percent DE (diet C and D) for fat (Ribeiro et al., 2004). The administration of glucose. Furthermore, intravenous insulin diets were fed for 6-week periods in a 4 × 4 Latin square de- resulted in a more profound hypoglycemia when compared sign. During the last 4 weeks of each period, the horses un- to controls (De La Corte et al., 1999a). Blood glucose and derwent 15–30 minutes of treadmill exercise (trotting). insulin concentrations were also lower in affected horses Blood samples for measurement of serum creatine kinase than in healthy controls after consumption of a meal of (CK) activity were taken 4 hours after each exercise session. sweet feed (De La Corte et al., 1999b), findings consistent The log of serum CK activity was significantly (P < 0.05) with enhanced glucose clearance and insulin sensitivity. The higher when horses were fed diets A, B, and C when com- strongest evidence of enhanced insulin sensitivity in PSSM pared to diet D. Postprandial glucose and insulin responses was provided by a more recent study that demonstrated a were lower in diet D when compared with diet A, while 2-fold higher rate of glucose clearance in affected vs. con- serum free fatty acid (FFA) concentrations (measured 4 trol horses during a euglycemic-hyperinsulinemic clamp hours after exercise) were consistently higher in diet A when (Annandale et al., 2004). compared to the other diets. However, muscle glycogen and glucose-6-phosphate concentrations and the percentage of Nutritional Management muscle fibers with abnormal polysaccharide accumulation did not differ among the diets (Ribeiro et al., 2004). Dietary recommendations (Firshman et al., 2003; Whereas there are reports that draft horses with PSSM McKenzie et al., 2003) for management of horses with can show clinical improvement with diet change alone PSSM include: (1) feeding a minimum of 1.5 percent of BW (Valentine et al., 2001b), studies in Quarter horses (Firsh- as forage per day, ideally a grass or oat hay as these forages man et al., 2003) and Warmbloods (Hunt et al., 2005) have have lower nonfiber carbohydrate content when compared demonstrated that both a change in diet and institution of to legumes; (2) removal of all concentrates containing grain daily exercise (including turnout) are necessary for a favor- and molasses from the ration; and (3) use of alternative en- able response. In a study of 65 Warmblood or Warmblood- ergy sources such as vegetable oil, rice bran, and/or non- cross horses with PSSM, only horses that received regular molassed beet pulp when DE requirements are higher than

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242 NUTRIENT REQUIREMENTS OF HORSES that provided by forage alone. Clinical improvement of degree of ALD. It was predicted that 40 percent of these horses with PSSM may be dependent upon the addition of foals would need corrective hoof trimming, 8 percent would fat to the ration. It has been reported that signs of muscle need surgical correction, 3–5 percent would develop con- dysfunction can persist when affected horses are fed an all- tracted tendons, 6 percent would become wobblers, 5 per- forage ration with low starch and sugar content (< 10 per- cent would develop OC, and 10 percent would be unsold as cent DE) content, whereas clinical signs of muscle dysfunc- yearlings because of the bone abnormalities. Leibsle et al. tion abate when even a small amount of vegetable-source fat (2005) found that only 3 percent of newborn (average age 2 is added to the ration (McKenzie et al., 2003). There are days) Thoroughbred foals had “straight” carpal conforma- conflicting views on the amount of dietary fat required for tions compared to 55 percent with correct fetlocks. At 1 to clinical improvement of horses with PSSM. Valentine et al. 1.5 years of age, only 7 percent of the same foals were con- (2001b) reported that horses with ESSM showed greatest sidered straight at the carpus (knee), while 79 percent were improvement when fed a ration that provided, on a total diet straight at the fetlock. Since a considerable number of young basis, at least 20–25 percent of DE from fat. Other re- horses are born with less than perfectly aligned joints, sub- searchers have reported clinical improvement when affected sequent mismanagement of the dietary program and hus- horses were fed rations with only 10–15 percent of DE pro- bandry of the foal could potentially exacerbate any develop- vided by fat (Firshman et al., 2003; Ribeiro et al., 2004). As ing osteochondral problems. some horses with PSSM are overweight, the feeding of a Bone growth disorders in young horses have multifactor- high-oil diet is problematic without instigation of an exer- ial causes. Pool (1993) divided the causes of OC into those cise program. On balance, it appears that a reduction in di- that are idiopathic or acquired. Idiopathic causes are consti- etary starch and simple sugar is the most important dietary tutional and hereditary; acquired causes are associated with recommendation for horses with PSSM. When forage alone biomechanically induced trauma or nutritional, toxic, iatro- does not meet daily DE needs, a source of fat such as veg- genic, and other determinable causes. Thus, the collective etable oil (as much as 600 ml/d for a 500-kg horse), rice risks for bone growth disorders in a foal originate with breed bran (0.5–2 kg/d), or other sources of fat should be added to risk, the conformation of the foal inherited from its dam and the diet. For horses in heavy training, other feedstuffs such sire, its prenatal and postnatal diet, and the husbandry meth- as nonmolassed beet pulp may be needed to meet DE re- ods and housing practices used during the critical growth pe- quirements and ensure palatability of the diet. riod from birth to at least 2 years of age. Future studies to As mentioned, the implementation of a daily exercise elucidate the etiology and pathogenesis of DOD will con- regimen is also important for successful management of tinue to provide conflicting and nonedifying conclusions Quarter horses and Warmbloods with PSSM. A combination until all of these factors are controlled and all criteria used of a low-starch, high-fat diet and regular exercise may result to confirm the clinical and pathological diagnoses of DOD in clinical improvement via a decrease in muscle glycogen are standardized. The extent to which these criteria were storage (Firshman et al., 2003) and/or an increase in lipid controlled and standardized should be considered in the in- metabolism in muscle (Ribeiro et al., 2004). terpretations of conclusions in the studies described below. Developmental Orthopedic Disease Pathogenesis Developmental orthopedic disease (DOD) is a complex Developmental orthopedic diseases originate from abnor- of musculoskeletal abnormalities that can afflict growing malities of endochondral ossification in one or both of the horses. These conditions include angular limb deformities growing areas of bone, articular-epiphyseal cartilage com- (ALD, or “crooked-legged foals”), physitis, subchondral plex and the growth plates (physis) (Pool 1993; McIlwraith, bone cysts, osteochondrosis (OC), flexural limb deformities 2004). Osteochondrosis is the pathologic description of (“contracted tendons”), and cervical vertebral malformation bone diseases of young horses in which there is failure of or (“wobbler syndrome”) (Pool, 1993; McIlwraith, 2004). abnormal cartilage maturation. The abnormalities of joint Congenital contracted tendons are not considered part of the cartilage and subchondral bone in osteochondrosis dissecans DOD complex (Kidd and Barr, 2002) and will not be dis- (OCD) have progressed to cracks and fissures in the carti- cussed here, nor will cervical vertebral malformation. lage. If osteochondrotic cartilage loosens, bone chips or The incidence of DOD disorders (physitis and OC) sub- joint mice may occur within the joint or synovial space mitted to a North American veterinary hospital and in young (McIlwraith, 2004). Although OC and OCD are often used Irish Thoroughbreds was 68–81 percent (Gabel, 1986; interchangeably, the pathology associated with OCD is more O’Donohue et al., 1992). Of these foals, 11.3 percent re- advanced than that seen in OC. Clinical symptoms and quired treatment (O’Donohue et al., 1992). Conformational pathologies are different depending on the affected site and leg abnormalities in the neonate are high (Aldred, 1998; affected bone. A detailed review of the cellular pathogenesis Leibsle et al., 2005). Aldred (1998) reported that 80 percent of equine osteochondrosis can be found in Jeffcott and Hen- of Thoroughbred foals born in Australia annually have some son (1998) and some of the site specific modulators of the

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UNIQUE ASPECTS OF EQUINE NUTRITION 243 physeal growth zones including dietary nutrients were re- (Mase, 1987; Firth and Hodge, 1997; Whitton, 1998) lead- viewed by Orth (1999). ing to carpal physitis or other forms of DOD. Osteochondrosis has been described as a dynamic bone The impact of genotype on osteochondrosis has been re- disorder that can develop or regress in specific joints de- ported in several studies. Based on a survey of 753 Swedish pending on the joint’s “window of susceptibility” (Pool, Standardbred trotters aged 6–21 months, 14.3 percent had 1987; Dik et al., 1999; Barneveld and van Weeren, 1999). In tibiotarsal OC while 11.8 percent had fetlock joint involve- 1987, Pool suggested that the pathologic insult to a suscep- ment (Grondahl and Dolvik, 1993). The incidence of fet- tible bone likely was of short duration and occurred ran- lock, hock, and stifle OC ranged from 17, 6, and 3 percent domly during the period of joint vulnerability. This hypoth- and 24, 6, and 11 percent, respectively, in nonlame and clin- esis appears to be supported by subsequent studies. Mild to ically lame Swedish Warmblood horses (Beneus, 2005). Ra- moderate radiographic abnormalities were observed in the diographic evidence of OC was found in 16.6 percent of 350 intermediate ridge of the distal tibia in 67.4 percent of 1- Maremmano Warmblood horses with a heritability index of month-old Dutch Warmblood foals (Dik, 1999; Dik et al., 0.14, which increased with inbreeding (Pieramati et al., 1999; van Weeren et al., 2003), but tended to regress over 2003). These authors predicted that through genetic selec- time so that at 11 months of age, only 18.4 percent of hocks tion, OC in their studied population could be reduced from were still abnormal. Stifle abnormalities occurred between 16 percent to 2 percent within five generations. 3–4 months of age but reverted to normal by 8 months of In North America, the breeds with the highest incidence age. Similarly, Sondergaard (2003) observed an age-related of DOD were Quarter horses, Thoroughbreds, Arabians, and decrease in radiographic OCD from weaning through 2 Paints (Wagner, 1986), while Standardbred followed by years. Although most lesions were temporary, the “age of no Thoroughbred horses were at highest risk of OC (Mo- return” when regression was less likely to occur in the hock hammed, 1990). Ponies and draft horses have been sug- was after 5 months of age and for the stifle was after 8–12 gested to have a low incidence of osteochondrotic bone dis- months of age (Dik, 1999; Dik et al., 1999). The important orders (Stromberg, 1979). In a veterinary hospital, only 5 nutritional conclusion from these observations is that dietary percent of the admitted draft horse population had OCD mismanagement preceding and during these critical ages and/or subchondral bone cysts (Riley et al., 1998). The may tip the balance from regression to progression of sub- heavy breeds most often presented in the latter study were clinically abnormal cartilage and bone to clinical lesions. Clydesdale and Percheron horses. Hence, a balanced and appropriately managed diet is neces- Foals at risk for osteochondrosis were described as those sary prior to and at the time of weaning. that are the most rapidly growing foals in the group (Turner and Fretz, 1977; Stromberg, 1979; Thompson et al., 1988a,b), which is consistent to observations in other species (Olsson, Genetics and Growth Rate 1978; Stromberg and Rejno, 1978; Stromberg, 1979). Some The genetic growth potential of the foal, including its ambiguity between the association of weight gain with DOD conformational traits and growth, are governed by the geno- has been raised (McIlwraith, 2004), and this uncertainty is type of the dam and sire (Saastamoinen, 1990; Preisinger et supported by the conflicting data presented in studies exam- al., 1991; Árnason and Bjarnason, 1994; Koenen et al., ining these relationships. Foals affected with hock OC 1995; Molina et al., 1999; Zechner et al., 2001; Leibsle et tended to have heavier birth weights, had higher average al., 2005). Skeletal dimensions and conformation were mod- daily gain (ADG), and were heavier at 1.5 years of age, but erately to highly heritable traits in Andalusian and Lipiz- those with palmar/plantar osteochondral fragments tended zaner horses (Molina et al., 1999; Zechner et al., 2001) and to weigh less than nonaffected foals (Sandgren, 1993). Foals Trakehner foals (Preisinger et al., 1991). Associations be- with stifle OC were significantly taller at the withers and tween parent and progeny were found for carpal but not fet- had 7–20 percent higher weight gains only at 3 and 5 months lock conformation (Leibsle et al., 2005). These authors also of age; there was no relationship between rates of weight reported a significant relationship between heavier birth gain and OC in the hock joint (van Weeren et al., 1999). The weights and an offset carpal conformation (P < 0.01) and average incidence of DOD in Thoroughbred foals evaluated fetlock inward deviation at 46 days (P < 0.005), effects that between 4 through 18 months of age was 16.1 percent persisted beyond 1 year of age (Leibsle et al., 2005). Foals (range 12.9–28.8 percent) and the highest incidence was with hock OC and palmar/plantar osteochondral fragments recorded at 4 months of age (Jelan et al., 1996). Body tended to have outwardly rotated limb axes and periodically weights were not statistically different between affected and had a more upright pastern (Sandberg, 1993). The high inci- nonaffected foals, although the authors noted that affected dence of conformational imperfections in foals (Leibsle et foals were generally heavier. Based on farm data, OCD- al., 2005), coupled with the relative immaturity of the radio- affected and normal Thoroughbred or Hanoverian foals did carpal physis at birth (Mase, 1987), and/or a high body mass not differ in body weight (Pagan, 2003; Vervuert et al., could exacerbate abnormal biomechanical pressure, asym- 2003). The total incidence of hock and fetlock osteochon- metrical growth, or direct trauma occurring at the physis drotic lesions in the latter study was 31.5 percent. Foals in

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244 NUTRIENT REQUIREMENTS OF HORSES controlled studies fed excess energy or starch did not differ Thoroughbreds monitored from birth through 2 years oc- in final body weights or average daily gain, although curred post-weaning in early winter and gradually resolved dyschondroplasia was prevalent in one study (Savage et al., over the next year (O’Donohue et al., 1992). Seasonal fluc- 1993a) but not the other (Ott et al., 2005). tuations in DOD were also evident in yearlings fed high That genotype affects the rate of weight gain is evident starch (SS) or high fat-fiber (FF) diets (Hoffman et al., from the wide range of weight gains of various breeds of 1999). The most severe clinical physitis in suckling foals oc- horses (see Chapter 1), but certain lines of horses within curred over 2 months in late summer and autumn, and were breeds are also recognized to have different body dimen- not influenced by the foal’s growth rate or sex (Finkler- sions (Zechner et al., 2001). A highly positive correlation Schade et al., 1999; Gee et al., 2005b). However, in other (0.97) was found between body weight and growth rate at 12 studies the seasonal clinical observations were coincident months in Finnhorses indicating that selection for rapid with changes in serum bone markers, including osteocalcin, growth rates is possible in horses (Saastamoinen, 1990). The carboxypropeptide of type I collagen and type III collagen heritability for body weight from birth to 2 years of age was propeptide, and with average daily gain (Price et al., 1997, 0.22–0.88 (SE = 0.46 to 0.87); the overall heritability from 2001). The production of sex hormones may also stimulate birth to 48 months was 0.86 ± 0.32 (Saastamoinen, 1990). growth spurts. Weight gains increased abruptly in 14-month- The variance in growth rates exceeded variance in absolute old Thoroughbred fillies concurrent with a pubertal spike in body measurements. Variation in absolute body weight progesterone secretion (Nogueira et al., 1997). ranged from 7.3–14.9 percent and from 2.4–4.6 percent for Thus, sustained modest growth rates may be preferred for withers height. Variance in ADG in the age categories from young horses to minimize some DOD-like conditions, espe- birth–6 months, 6–12, 12–24, 24–36, 36–48, and 0–48 cially prior to the earliest window of susceptibility for spe- months was 10.9, 26.8, 24.1, 64, 45.3, and 7.9 percent, re- cific joints. Supplying creep feed to suckling foals at 2–3 spectively. The calculated variances in biweekly ADG for months of age has been suggested as a method to maintain weanlings fed a high-forage diet (70 percent alfalfa) in lim- growth during the suckling period and to minimize the im- ited amounts or free-choice, or a high-concentrate diet (34.7 mediate post-weaning weight loss seen in foals (Coleman et percent alfalfa) fed free-choice were 7.8, 14.2, and 17 per- al., 1999; Peterson et al., 2003). Although creep feeding did cent, respectively, indicating a more fluctuating weight gain not entirely prevent weight loss post-weaning, creep-fed with ad libitum feeding (Cymbaluk, 1989). Thus, rate and foals were perceived to be less stressed than noncreep-fed amplitude of weight gain are not only affected by diet but by foals just after weaning (Coleman et al., 1999). method of feeding, which is further confounded by heri- tability of gain. Dietary Nutrients and Bone Growth Disorders Fluctuating growth rate may be a precursor to DOD (Barneveld and van Weeren, 1999), yet fluctuating biweekly The potential effect of dietary nutrient intake on bone weight gains are evident even in normal Thoroughbred foals growth in foals has been reviewed by Jeffcott and Savage between birth and 1 year of age (Thompson, 1995). Al- (1996) and Harris et al. (2005). Documented evidence has though erratic weight gains were more prevalent in free- confirmed that horses kept under pratical management situ- choice than in limit-fed foals (Cymbaluk, 1989), it was un- ations are often fed diets that may not be adequately de- clear whether this alone would contribute a sufficient signed to meet the predicted nutrient requirements (Knight biomechanical insult during the critical window of suscepti- et al., 1985; Hacklander et al., 1996; Finkler-Schade et al., bility to alter endochondral ossification. Other management 1999; Gibbs and Cohen, 2001; Paragon et al., 2003). The conditions that have been reported to result in nonuniform most common nutrient imbalances identified included ex- growth rates are dietary stress (Hintz et al., 1976), environ- cess energy intake, and excesses or deficiencies in protein, mental stress (Rooney, 1984; Cymbaluk and Christison, macromineral, and trace mineral content, as well as calcium/ 1989a; Cymbaluk, 1990), season (Jelan et al., 1996; Hoff- phosphorus imbalances. Feeding programs on Thorough- man et al., 1999), hormonal maturation (Noguiera et al., bred and Quarter horse breeding farms were well managed 1997), and unidentified factors (Jelan et al., 1996). Con- on about half of the farms, but the remaining farms were felt tracted tendons and/or physitis in young horses followed a likely to be using unbalanced diets (Gibbs and Cohen, suspected compensatory gain following a switch from nutri- 2001). Although the role of diet in foal growth cannot be tionally marginal to abundant diets (Hintz et al., 1976), fol- disputed, the complexity of the relationships among nutri- lowing environmental stress either as extreme cold (Cym- ents and their interactions in abnormal bone growth are not baluk and Christison, 1989a) or prolonged inclement yet fully elucidated. It is, however, important to recognize weather (Rooney, 1984). that when formulating practical diets, nutrient concentra- Seasonal growth spurts were associated with an increased tions can deviate from NRC recommendations without caus- occurrence of bone abnormalities in young horses (O’Dono- ing abnormal growth in young horses (Ott and Kivipelto, hue et al., 1992; Hoffman et al., 1999). The peak incidence 2002). However, all variances in dietary nutrient concentra- (4.1 percent) of clinical musculoskeletal conditions in young tions in horse diets should be evaluated to ensure that the

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UNIQUE ASPECTS OF EQUINE NUTRITION 245 nutrient concentration falls into an accepted normal range fed a fat-fiber (FF) concentrate. However, Ropp et al. (2003) for the class of horse for which the diet is intended. compared the insulin-glucose responses of weanling Quar- ter horses fed a more typical concentrate containing 33.9 percent nonfiber carbohydrates vs. a concentrate containing Energy, Protein, and Fat Intake 10 percent fat and 24 percent nonfiber carbohydrate for 75 Excessive energy intake has received considerable inter- days and found no treatment by time interactions on day 0 est in the etiology of equine DOD in part based on the hy- or 60 for glucose and no interaction effect for insulin on day pothesis that excessive amounts of nonfiber carbohydrates 0 or 30. There was no change in growth hormone secretion may contribute to hormonal abnormalities in foals, specifi- or plasma insulin-like growth factor I (IGF-I) concentra- cally through modified insulin responses (Kronfeld et al., tions, and only a minor glucose sparing effect. Ott et al. 1990). High-energy diets were felt to cause OCD irrespec- (2005) fed weanling horses either a medium- (2.9 starch/kg tive of feed composition (Stromberg, 1979). The data of BW) or high- (6.5 starch/kg BW) starch diet without induc- Savage et al. (1993a), who fed weanling foals diet contain- ing changes in bone mineral content or creating new osteo- ing 29 percent more energy than recommended by NRC chondrotic bone lesions. During the study, the incidence of (1989), supported this hypothesis. Histological dyschon- preexisting radiographic OCD bone lesions decreased about droplastic changes in the stifle, hock, and fetlock joints were 29 percent irrespective of whether the foals were fed hay observed in all foals fed high-energy diets (29 percent above plus a concentrate containing either 17 or 37.5 percent control) created by adding 0.25 kg of corn oil to a basal diet starch. These data confirm that foals fed some types of of 13 percent oaten chaff and 77 percent rice-based pellets grain-based concentrates can have altered insulin responses. (Savage et al., 1993a). The source of the additional dietary Unlike growing horses, ponies were significantly more hy- energy in the high-energy diet in this study was derived from perglycemic and hyperinsulinemic when fed a high-fat (11.1 corn oil, but all diets were based on rice concentrate, a feed percent) diet than when fed a high-sugar diet (Schmidt et al., that is high in starch, and the foals were partially confined. 2001). The role of insulin resistance in developmental bone The foals in this study had similar net gains in skeletal and disorders is unclear and conflicts with the observation that weight growth to control foals (Savage et al., 1993c); con- ponies are more insulin resistant than horses (Rijnen and van sequently, the dyschondroplastic bone lesions may have der Kolk, 2003), yet have an apparently low incidence of been induced through a hormonal alteration, although this DOD or osteochondrosis (Stromberg, 1979). was not verified. The suspicion that insulin resistance may contribute to Dietary composition, in addition to absolute nutrient con- developmental bone disorders (Kronfeld et al., 1990) has tent, of foal diets has also been scrutinized as a possible in- stimulated interest in finding alternative energy sources to fluence on the incidence of DOD. The assumption that rapid nonstructural carbohydrates. Fat, in limited amounts (< 11 growth obtained through use of grain-based, high nonfiber percent), was added to highly fibrous constituents (oat straw, carbohydrates has contributed to a higher incidence of DOD soybean hulls–FF) (Hoffman et al., 1999; Staniar, 2002) or has led to examination of dietary effects on the insulin- to concentrate-based supplements (Ropp et al., 2003) to glucose axis and the role of insulin sensitivity in foals. The lower the nonfiber carbohydrate while maintaining energy plasma insulin-glucose response to feeding a diet of 50:50 content of the diet. Carpal physitis scores and joint effusion textured grain:alfalfa-grass hay was compared in OCD- did not differ between FF and SS foals, but hind fetlock affected to normal young horses (Ralston, 1996). Although physitis was higher (P < 0.5) for the SS group at 8 and 12 glucose:insulin ratios did not differ, plasma insulin concen- months (Hoffman et al., 1999). Foals in the FF group had tration was 29–79 percent higher in OCD-affected horses higher (P < 0.05) scores for angular limb deformities at than controls. Pagan (2003) correlated the post-meal feeding 1 month, 8 months, and 10 months of age. (2 h) plasma insulin concentrations of 218 10-month-old The conflicting data obtained on the effects of energy in- foals to surgical OCD lesions present at the time of blood take on the incidence of DOD have in part arisen through sampling or which developed over the next 6–10 months and different methods of feeding. Regulated feeding of concen- observed that the 27 OCD-affected foals had higher plasma trate mixes has not resulted in increased bone abnormalities glucose and insulin concentrations but no difference in the (Ott and Asquith, 1989; Reynolds et al., 1992; Ott et al., glucose:insulin ratio compared to normal cohorts. Yearling 2005). Quarter horse weanlings fed a diet of 25 percent Thoroughbreds fed a 72 percent hydrolysable carbohydrate Bermudagrass hay plus 75 percent concentrate (given twice diet had a significantly higher glycemic (P < 0.043) and in- daily) from 6 to 12 months of age gained weight at 0.65 sulinemic (P < 0.031) response than foals fed a high fat-fiber kg/d, and all subchondral and cystic lesions, except for the concentrate (Staniar, 2002). That high nonfiber carbohy- cystic carpal lesions, identified radiographically at 6 months drate concentrates modify the insulin responsiveness was had regressed by 1 year of age (Reynolds et al., 1992). Un- shown by Treiber et al. (2005), who observed a 37 percent regulated feeding of any diet, whether high in fiber or not, reduction in insulin sensitivity in weanlings fed a 49 percent has led to transient physitis and flexural limb deformities nonfiber carbohydrate (SS) concentrate compared to foals (Cymbaluk and Christison, 1989b). In addition to the effect

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UNIQUE ASPECTS OF EQUINE NUTRITION 257 Feeding grain (e.g., 0.5 kg/100 kg BW) and about 1 cup spores, small fungal spores, and dust mites. The role of dust (220 ml) of apple cider vinegar twice daily may be benefi- mites in exacerbating the allergenicity of molds is unclear. cial in promoting mild acidification of colonic contents. Fi- Storage mites infest many feedstuffs and were linked to the nally, daily pasture turnout may reduce risk of enterolith spread and growth of molds within those infected feeds. In formation. addition to these usual allergens, endotoxins (lipopolysac- charides) can amplify the symptoms of RAO (Pirie et al., 2003). The small size of respirable particles allows escape Recurrent Airway Obstruction from the turbinate trap in the upper respiratory tract (Art et Recurrent airway obstruction (RAO) is the accepted ter- al., 2002). If not captured by the bronchial mucociliary minology for the disease entity in mature horses formerly mechanism, these particles can traverse to the alveoli. The known as chronic obstructive pulmonary disease (COPD) or threshold limiting value (TLV) is the level of exposure of an heaves (Robinson, 2001). Approximately 9.2 percent of agent above which disease or organic dysfunction can occur. clinical cases referred to North American veterinary teach- The TLV of spores for horse stables has been calculated at 33 ing hospitals have RAO (Ward and Couetil, 2005). Epi- particles/cm3 (Webster et al., 1987). demiologic studies indicate that affected horses were typi- Thermotolerant and thermophilic fungal species (As- cally 4 years or older and symptoms occurred more often in pergillus, Rhizomucor, Penicillium, and Stachybotrys spp.) winter and spring and occurred more often in the southern produce the majority of small respirable, fungal spores in than northern hemisphere (Couetil and Ward, 2003; Ward hays, straw, and shavings (Clarke and Madelin, 1987). The and Couetil, 2005). The monthly prevalence of RAO was high temperature conditions (> 38°C) preferred by these significantly correlated to total pollen counts measured 3 fungi most often occur in poorly cured hay, which heats dur- months before clinical symptoms and to total mold counts ing storage. Fungal invasion was shown to occur in the occurring 1 month before or during the same month of clinic stand-ing crop, during curing, and during storage, but admission (Ward and Couetil, 2005). A genetic predisposi- growth during storage was enhanced in square alfalfa hay tion has been reported with exacerbation by environmental bales when moisture content of the hay was high (29 percent contaminants in stables and age of horse (Marti, 2001; Marti ) at baling (Wittenberg et al., 1989). For horses with heaves, and Ohnesorge, 2002). The incidence of RAO in German hay should contain few mold and fungi. Hay has been Warmblood horses was 17, 48, and 69 percent when neither, graded at levels I through III based on numbers of mold and one, or both parents, respectively, were affected by RAO fungi type. Grade I hay had less than 999 particles/mg symptoms. Using a relative risk analysis, Marti and Ohne- source material, grade II ranged from 1,000–4,999 parti- sorge (2002) reported that the relative risk of RAO is low in cles/mg source material, and grade III contained more than offspring (13 percent) if neither dam nor sire is affected, but 5,000 particles/ mg source material (Clarke and Madelin, increased 3.2-fold (P < 0.05) if either parent was affected 1987). Hays and bedding with a grade III score are undesir- and 4.6-fold (P < 0.05) if both parents were affected. able for horses because millions of spores can be inhaled Environmental pollutants, generically labeled as “stable from these materials by a horse. dust,” include airborne fungi, thermophilic actinomycetes, Bedding has also been shown to contribute to horse sta- dust mites, endotoxins, and inorganic compounds (Clarke ble dust. Harvesting conditions often determine the quality and Madelin, 1987; McGorum et al., 1998; Art et al., 2002) of the straw for bedding. Based on their mold content, and typically enter the microenvironment (stall) and straws used to bed horse stalls were generally classified as macroenvironment (stable) through feed and bedding. RAO good or poor quality (Clarke and Madelin, 1987; Vandenput symptoms are not exclusive to indoor-stabled horses. A et al., 1997). Straws and shavings had a similar complement condition known as “summer-pasture-associated obstructive of molds as contaminated hays, whereas peat harbored few pulmonary disease” (SPAOPD; also known as “summer- potential allergenic molds (Clarke and Madelin, 1987). By pasture associated allergy”) has been recorded in horses on contrast, good-quality wheat and flax straw had fewer res- pasture (Seahorn and Beadle, 1993; Costa et al., 2000). pirable particles and allergenic spores than wood shavings (Vandenput et al., 1997). All-natural bedding materials were inferior to cardboard bedding, which had low levels of res- Nutritional Factors pirable dust and spores (Kirschvink et al., 2002c). Pelleted Stable dusts have been classified as nonrespirable (> 10 newspaper may be an alternative to straw and shavings as mm) or respirable (large or small) particles (Clarke and bedding, although particle mass less than 10 µm was ini- Madelin, 1987). Large respirable particles are 5–10 mm and tially higher in the newspaper product than either natural include particles such as plant structures, large pollen grains, bedding material when spread in the stall (Ward et al., and “fair weather” spores (Alternaria, Cladosporium spp). 2001). This study also confirmed that microbial numbers in These particles typically are not inhaled because of rapid the breathing zone and on the legs of horses were greater in sedimentation to the stable floor. Small respirable particles autumn than in summer. When bedding down the stall, are 0.1–5 mm and are typically comprised of actinomycetes horses should be removed from the barn. In one study, the

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258 NUTRIENT REQUIREMENTS OF HORSES high exposure to respirable dust particles during “bedding idized and reduced glutathione, glutathione redox ratio, and down” increased airborne particles in the breathing zone of ascorbic acid have been characterized in normal horses un- the horse 3- to 6-fold (Webster et al., 1987). Good stable dergoing RAO episodes and those in remission (Art et al., ventilation is indispensable in the control of respirable par- 1999; Kirschvink et al., 2002a; Deaton et al., 2004). Horses ticles in the breathing zone of the horse, yet many stables with an induced RAO or clinical RAO crisis have increased were inadequately ventilated to accomplish this objective production of elastase, decreased ascorbic acid concentra- (Webster et al., 1987). tion in bronchoalveolar lavage fluid, and epithelial lining fluid (Deaton et al., 2004, 2005a,b) and increased total glu- tathione and oxidized glutathione (Art et al., 1999). Al- Pathogenesis and Clinical Symptoms though these data would indicate an increased production of Exposure of a sensitized horse to excessive amounts of free radicals or reactive oxygen species and that ascorbate respirable dust particles or organic dust in hay and moldy might be protective, the acute neutrophilic episode did not bedding has been shown to initiate neutrophilic inflamma- result in a marked pulmonary oxidative stress (Deaton et al., tion of the small airway (bronchioles) and/or the entire tra- 2005b; Deaton, 2006). When healthy horses were given an- cheobronchial tree (Robinson et al., 1996). Altered re- tioxidant supplements including ascorbic acid, there was no sponses by the mucociliary system result in increased mucus effect on ascorbic acid concentrations in pulmonary epithe- and/or mucopus production in the bronchial and bronchiolar lial lining fluid (Deaton et al., 2002), yet an antioxidant sup- tree (Gerber et al., 2004), mucosal edema, and broncho- plement containing vitamins C and E plus selenium given to spasm, eventually followed by varying degrees of fibrosis heaves-affected horses in remission resulted in improved ex- and fibroplasia of the submucosa. These pathologies reduce ercise tolerance and lower airway inflammation (Kirschvink oxygen exchange and a progressively reduced exercise tol- et al., 2002b). Further data will be required to determine if erance of the horse. Other symptoms, which can be mild to dietary ascorbic acid supplementation ameliorates or pro- severe, include coughing, respiratory distress, mucus or mu- tects the equine airway from pathology associated with the copurulent nasal discharge, abnormal lung sounds, in- inflammatory responses seen in RAO. creased work in breathing, and an abdominal “heave” line after chronic and sustained respiratory impairment (Robin- Nutritional Management and Prevention son et al., 2003). Importantly, the symptoms of RAO can be induced by a moldy hay/straw challenge in affected horses The best environment for RAO-prone horses has been to (McGorum et al., 1993). house them outdoors on pasture (Art et al., 2002). Horses The hypothesized causes for the airway hyperresponsive- with symptoms of heaves become asymptomatic within 4–6 ness seen in RAO-affected, RAO-prone horses, and SPAOD- days of being turned outdoors (Vandenput et al., 1998). Im- affected horses are numerous. Airway inflammation seen in provement in respiratory function was observed within 3 RAO has been suggested to originate at a cellular level, in days after pasture turnout in horses who were induced into a part from increased activity of transcription factors such as RAO episode by environmental modification (Jackson et al., nuclear factor-KB and activator protein-1 (Bureau et al., 2000). However, every precaution must be taken to mini- 2000). The profound neutrophilia seen in RAO may promote mize respiratory allergens even when the heaves-prone the continued production of mucus by the mucociliary ap- horses are kept outdoors. Supplementary hays and grains, if paratus (Gerber et al., 2004), perhaps through upregulation fed, should have minimal organic dust content. Despite the of equine MUC5AC mucin mRNA (Gerber et al., 2003). apparent benefit of housing RAO horses outside, the outdoor Other pathologic indicators in RAO horses include an in- environment may not be acceptable for horses prone to creased expression of IL-4 mRNA and IL-13 mRNA and a SPAOPD. decreased expression of IFN-γ mRNA indicative of an IgE- Unfortunately, horse athletes, including RAO-affected mediated response (Robinson, 2001; Bowles et al., 2002), horses, must often be stabled. If bedded and fed with feeds increased IL-1 β mRNA (Matera et al., 2005), higher sys- with low allergenicity, even RAO-affected horses appear to temic levels of endothelin-1 (Benamou et al., 1998), and re- be able to stay asymptomatic indoors. Thompson and duced densities of the β-adrenergic receptors of the lung and McPherson (1984) showed that horses with symptoms of bronchi particularly of the β-1 subtype, which may increase RAO became asymptomatic within 4–24 days after mea- airway smooth muscle sensitivity (Abraham et al., 2006). sures were taken to control dust in the stable by bedding The upregulation of the inflammatory genes has been with shredded paper and feeding a complete pelleted diet. linked to the redox conditions initiated by the increase in re- Similar observations were observed in heaves-affected active oxidant species (ROS) or free radicals during inflam- horses bedded with cardboard or good-quality straw and fed mation (Kirschvink and Lekeux, 2005). The overabundance a completed pelleted diet (Vandenput et al., 1998; of ROS in airway inflammation relative to the level of neu- Kirschvink et al., 2002c). tralizing oxidants has been speculated to exacerbate airway Good-quality hay for RAO-affected horses has low num- pathology. The concentrations of antioxidant indicators, ox- bers of thermophilic and thermotolerant molds and fungi

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UNIQUE ASPECTS OF EQUINE NUTRITION 259 and dust mite allergens, but, contrary to popular belief, sta- Annandale, E. J., S. J. Valberg, J. R. Mickelsen, and E. R. Seaquist. 2004. ble operators were less able to detect mold through smell or Insulin sensitivity and skeletal muscle glucose transport in horses with polysaccharide storage myopathy. Neuromusc. Dis. 14:666– 674. appearance than expected (Clarke and Madelin, 1987). Árnason, T., and Bjarnason, T. 1994. Growth, development and size of Ice- Moreover, feed quality, based on allergenicity, cannot be landic toelter horses. Búvísindi 8:73–83. guaranteed because weather conditions at the time of har- Art, T., N. Kirschvink, N. Smith, and P. Lekeux. 1999. Indices of oxidative vest, methods of harvest, and storage all profoundly influ- stress in blood and pulmonary epithelium lining fluid in horses suffer- ence mold growth in hays, straws, silage, and grains. Al- ing from recurrent airway obstruction. Equine Vet. J. 31:397–401. Art, T., B. C. McGorum, and P. Lekeux. 2002. Environmental control of though pelleted or cubed alfalfa and grass silage can have respiratory diseases. In Equine Respiratory Diseases, P. Lekeux, ed. low levels of aeroallergens and dust and have been effec- Document No. B0334.0302. Ithaca, NY: International Veterinary Infor- tively used in diets of stabled, heaves-affected horses (Ray- mation Services. mond et al., 1994; Vandenput et al., 1997), these observa- Bailey, S. R., C. M. Marr, and J. Elliott. 2004. Current research and theo- tions may not apply universally to similar feeds produced in ries on the pathogenesis of acute laminitis in the horse. Vet. J. 167:129–142. other geographic areas. Poor-quality meals and uncoated Barneveld, A., and P. R. van Weeren, 1999. Conclusions regarding the in- pellets can expose animals to a large number of respirable fluence of exercise on the development of the equine musculoskeletal particles (Li et al., 1993), so quality of these products should system with special reference to osteochondrosis. Equine Vet. J. Suppl. also be evaluated. 31:112–119. Management to prevent and control symptoms of RAO Barneveld A., R. van Weeren, J. Knaap. 1999. Influence of early exercise on the locomotion system. 50th Annual Meeting of the European Asso- involves medical treatment, as well as eliminating or mini- ciation for Animal Production, Zurich. mizing the respiratory tract allergens (stable dust, especially Bell, R. A., B. D. Neilsen, K. Waite, D. Rosenstein, and M. Orth. 2001. respirable particles) in the horse’s macroenvironment (barn) Daily access to pasture turnout prevents loss of mineral in the third and microenvironment (stall). There are several ways to re- metacarpus of Arabian weanlings. J. Anim. Sci. 79:1142–1150. duce exposure of the horse to respirable dust: keep the horse Benamou, A. E., T. Art, D. J. Marlin, C. A. Roberts, and P. Lekeux. 1998. Variations in systemic and pulmonary endothelin-1 in horses with re- outdoors, use only dust and mold-free feeds, moisten feeds, current airway obstruction (heaves). Pulm. Pharmacol. Therap. use nondusty bedding, wash stalls regularly to remove dust, 11:231–235. and provide effective and sufficient ventilation to remove Benders, N. A., K. Junker, T. H. Wensing, S. G. A. M. van den Ingh, and J. dust particles in barn and stall air (Thompson and McPher- H. van der Kolk. 2001. Diagnosis of secondary hyperparathyroidism in son, 1984; Webster et al., 1987; Raymond et al., 1994; Van- a pony using intact parathyroid hormone radioimmunoassay. Vet. Rec. 149:185–187. denput et al., 1998). Soaking or steaming can reduce res- Beneus, L. 2005. Lokalisation av osteochondrosforandrengar hos Svenska pirable particles in hay. The major impact of soaking hay to havhodshastar. Ph.D. Thesis. Uppsala University, Sweden. reduce respirable particles was realized within 30 minutes; Bertone, J. J. 1992. Nutritional secondary hyperparathyroidism. Pp. no further benefit was observed in soaking hay for 12 hours 119–122 in Current Therapy in Equine Medicine, 3rd ed., N. E. Robin- (Moore-Colyer, 1996). Although soaking hay up to 30 min- son, ed. Philadelphia: W. B. Saunders. Beynen, A. C., and J. M. Hallebeek 2002. High-fat diets for horses. Pp 1–13 utes or steaming for 80 minutes reduced 93 percent of res- in Proc. 1st Europ. Equine Nutr. Health Cong., Antwerp Zoo, Belgium. pirable particles, steaming had no effect on nutrient loss Available at http://www.equine-congress.com. Accessed April 15, 2006. whereas soaking reduced phosphorus, potassium, magne- Blackman, M., and M. J. S. Moore-Colyer. 1998. 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