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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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Suggested Citation:"1 Energy." National Research Council. 2007. Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. doi: 10.17226/11653.
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1 Energy ENERGY SYSTEMS effect on the gross energy of a feed because the gross energy of a nonstructural carbohydrate such as starch is similar to Conceptual energy systems have been developed to par- the gross energy of a structural carbohydrate such as cellu- tition and quantify the energy utilized by animals and the en- lose. Feeds that are composed mostly of minerals are very ergy contained in feeds. The chemical energy in feeds may low in gross energy. The total amount of gross energy con- be partitioned into the portion that is recovered in product or sumed by an animal is termed the intake energy (IE), as tissue and the portion that is lost (Figure 1-1). Energy re- shown in Figure 1-1 (NRC, 1981). quirements of animals and the energy content of animal foods are expressed in calories in the United States. The en- ergy requirements of horses are often expressed in terms of Digestible Energy kilocalories (kcal) or megacalories (Mcal). Energy require- The apparent digestible energy (DE) content of a ration is ments may also be expressed in joules. One megacalorie is calculated by subtracting the gross energy in the feces from equivalent to 4.184 megajoules (MJ). the gross energy (intake energy) consumed by an animal. The term “apparent” is used because some of the material Gross Energy excreted in the feces does not originate from the feed but from cells sloughed from the gastrointestinal tract and di- The gross energy of a feed represents the amount of heat gestive secretions. The true DE of a feed may be calculated produced from the total combustion of that feed, as mea- if fecal endogenous losses are known. Endogenous fecal en- sured in a bomb calorimeter (NRC, 1998). The chemical ergy losses are not routinely determined in studies with composition of a feed will affect its gross energy. Lipids are horses and thus most DE values represent apparent DE, not higher in gross energy per unit weight than proteins or car- true DE. bohydrates. The type of carbohydrate in a feed has minimal Two factors that impact the amount of DE in a feed are the gross energy content of the feed and the digestibility of the energy-containing components. A feeding trial is the Intake Energy as feed (IE) Digestible Energy (DE) Fecal Energy (FE) most accurate method of estimating the DE content of feeds. undigested food (FiE) Digestible energy values for some common horse feeds have metabolic (FmE) been determined using feeding trials, but the number of Gaseous Energy equine studies is limited compared to studies with other Metabolizable Waste Urinary Energy (UE) species. Prior to 1989, it was common to estimate the DE Heat Energy (HE) basal metabolism (HeE) Energy (ME) value of horse feeds from data compiled in other species digestion and absorption (HdE) (NRC, 1978). Because digestive processes vary among heat of fermentation (HfE) Recovered Energy (RE) species of animals, the DE value of a feed also varies among product formation (HrE) (useful product) species. For example, when fed to dairy cattle, alfalfa meal voluntary activity (HjE) Tissue Energy (TE) has a DE value of 2.6 Mcal/kg (NRC, 2001), but when fed thermal regulation (HcE) Lactation Energy (LE) to swine, the value is 1.83 Mcal/kg (NRC, 1998). waste formation and excretion Conceptus Energy (YE) (HwE) The NRC (1989) adopted the following equations for es- Hair Energy (VE) timating DE content of horse feeds from the chemical com- FIGURE 1-1 Energy flow diagram (NRC, 1981). position of the feeds. 3

4 NUTRIENT REQUIREMENTS OF HORSES Dry Forages and Roughages, Pasture, Range Plants, When the equation was tested using an additional set of ob- and Forages Fed Fresh: servations not included in the original data set, there was no DE × (Mcal/kg) = 4.22 – 0.11 × (%ADF) + 0.0332 × systematic deviation between the actual and predicted val- (%CP) + 0.00112 × (%ADF2) ues for feeds containing less than 35 percent crude fiber and less than 8 percent AEE (DM basis). The relationship be- Energy Feeds and Protein Supplements: tween predicted and actual values was given as r = 0.626 DE × (Mcal/kg) = 4.07 – 0.055 × (%ADF) with a standard error (SE) of the regression equation of 1.012 (Zeyner and Kienzle, 2002). The authors stated that where ADF = acid detergent fiber and CP = crude protein. the DE content of feeds high in fat or highly fermentable These equations were based on work by Fonnesbeck (1981) fiber was somewhat underestimated, but specific values that summarized chemical and biological data from 108 di- were not given. gestion trials conducted with horses. Fonnesbeck (1981) did For most cereal grains, cereal grain byproducts, and some not report any of the characteristics of the specific feeds other byproduct feeds (such as pulps), the equations devel- used in the digestion trials. oped by Pagan (1998) and by Zeyner and Kienzle (2002) to Using data from 30 different diets (120 observations), predict DE content do not appear to have any advantages Pagan (1998) reported that DE could be estimated from the over the equation previously used by the NRC (1989). How- following equation: ever, the equation developed by Zeyner and Kienzle (2002) may predict DE content more accurately than the equations DE × (kcal/kg DM) = 2,118 + 12.18 × (%CP) – 9.37 × in the previous NRC (1989) edition for feeds high in fat. (%ADF) – 3.83 × (%hemicellulose) + 47.18 × (%fat) + 20.35 However, the equation derived by Zeyner and Kienzle × (%non-structural carbohydrate) – 26.3 (%ash); R2 = 0.88 (2002) utilizes chemical components of the diet that may not where hemicellulose = ADF – neutral detergent fiber (NDF) be available in current feed databases, such as acid ether ex- and nonstructural carbohydrate = (100 – %NDF – %Fat – tract, crude fiber, and nitrogen free extract. %Ash – %CP). The DE content of a ration or particular feed may not be constant across all situations. Diet digestibility may be af- DE content predicted by the equations from NRC (1989) fected by individual variation, exercise, and diet form (pro- and Pagan (1998) were reported to be similar for many feeds cessing) (Hintz et al., 1985; Pagan et al., 1998). There is also (Pagan, 1998). However, Pagan (1998) suggested that neither potential for one feed component to affect the digestibility equation accurately predicted the DE content of some high- of another feed component. Kienzle et al. (2002) reported fiber feeds, and feeds that were high in fat. The DE of rice that the addition of concentrate to a diet consisting of poor bran was measured to be 3.17 Mcal/kg (as fed basis), whereas quality roughage (straw) could increase the digestibility of the values estimated by the NRC (1989) and Pagan (1998) the roughage. It was suggested that this associative effect equations were 2.62 and 2.71 Mcal/kg (as fed basis), respec- was due to a positive effect of the concentrate on the ability tively (Pagan, 1998). Pagan (1998) suggested that DE values of the microbial population to digest fiber. However, Martin- for feeds containing more than 5 percent fat could be adjusted Rosset (2000) suggested that the ratio of forage to concen- by increasing the DE (as fed basis) by 0.044 Mcal/kg for each trate does not affect organic matter digestibility in horses. 1 percent fat above 5 percent. The measured DE value for beet Adding fat to equine diets has been reported to reduce fiber pulp was approximately 20 percent higher than the value es- digestibility in some experiments (Jansen et al., 2000, timated from chemical composition (Pagan, 1998). The meas- 2002), but not in others (Rich et al., 1981; Bush et al., 2001). ured value for soybean hulls was approximately 42 percent A complete discussion of the effects of dietary fat can be higher than the value predicted by the Pagan (1998) equation found in Chapter 3. Energy digestibility of a specific feed and 52 percent higher than predicted by the NRC (1989) may also be affected by differences among horses. Pagan equation (Pagan, 1998). The Pagan (1998) and NRC (1989) and Hintz (1986a) reported that energy digestibility of a pel- equations overestimated the DE content of oat hulls by 55 and leted alfalfa-oat diet ranged from 58.8 percent to 65.8 per- 27 percent, respectively. Zeyner and Kienzle (2002) have also developed an equation to estimate the DE content of horse cent among individual horses. feeds, using data from 170 digestion trials. The diets used to develop the equation ranged in composition from 5.7–28.7 Metabolizable Energy percent crude protein, 4.2–34.7 percent crude fiber (CF), 33.8–69.8 percent nitrogen-free extract (NFE), and 1.6–7.9 Metabolizable energy (ME) is calculated by subtracting percent acid ether extract (AEE). The equation derived by urinary energy losses and gaseous losses from DE. Urinary Zeyner and Kienzle (2002) to predict DE of horse feeds was: and gaseous losses in the horse are smaller than fecal losses. When horses were fed a pelleted diet, approximately 87 per- DE (MJ/kg DM) = –3.6 + 0.211 × (%CP) + 0.421 × cent of the DE was converted to ME (Pagan and Hintz, (%AEE) + 0.015 × (%CF) + 0.189 × (%NFE) 1986a). The efficiency of DE conversion to ME will be in- fluenced by the composition of the diet consumed (Vermorel

ENERGY 5 et al., 1997a). Gaseous losses will be higher when feeds are with its ultimate use in the body (maintenance, exercise, lac- digested in the large intestine. Vermorel et al. (1991) re- tation). For example, in the NE system currently in use for ported that the efficiency of DE conversion to ME was 90 dairy cattle, an individual feed may be assigned four differ- percent for a mixed diet and 87 percent for a hay diet. When ent NE values (NRC, 2001). Information relating to the NE ponies were fed diets consisting primarily of oats, the effi- value of each feed for each specific function is considered ciency of DE use for ME was greater than 90 percent (Kane one of the requirements of a useful NE system (NRC, 1981). et al., 1979). In the United States, NE systems have been developed for cattle (NRC, 2000, 2001), but the DE system has been used for horses (NRC, 1989). Work on an NE system for Net Energy horses was initiated in France in the early 1980s. Kronfeld Metabolizable energy is the starting point for net energy (1996) also proposed an approach to modeling NE that fo- (NE) systems (Ferrell, 1988). Metabolizable energy may be cuses primarily on the exercising horse. Although the two transformed to recovered energy (RE) or to heat (Figure systems share some common characteristics, Harris (1997) 1-1). Recovered energy includes energy stored in tissues pointed out that they make different assumptions about the (during pregnancy or weight gain) or secreted in a product efficiency of ME use of various fuels during exercise. In ad- (such as milk). Recovered energy may be designated as NEr dition, the system proposed by Kronfeld (1996) did not en- and may be further partitioned according to its specific type compass all physiological classes of horses or define NE (lactation, growth, conceptus). Total heat production (HE) is values for horse feeds. At this time, the French system is the the amount of energy lost to the environment. Total heat pro- most fully developed NE system for horses. French horse- duction may be partitioned into several components (NRC, feeding standards are based on the Unite Fourragere Cheval 1981). The components of HE include: (UFC; or horse feed unit) (Martin-Rosset et al., 1994; Vermorel and Martin-Rosset, 1997; Martin-Rosset, 2000; HeE = the heat associated with basal metabolism; Martin-Rosset and Vermorel, 2004). The UFC system re- HjE = the heat associated with voluntary activity; lates the NE requirements and the NE value of feeds to a HcE = the heat of thermal regulation; standard unit derived from the NE value of 1 kg of barley HrE = the heat associated with product formation; (1 UFC = NE of 1 kg standard barley). The UFC system uti- HdE = the heat of digestion and absorption; lizes information about the gross energy and digestibility of HwE = the heat associated with waste formation and horse feeds, the efficiency of DE conversion to ME (deter- excretion; mined in horses), the expected proportions of energy sup- HfE = the heat of fermentation. plied by absorbed nutrients, and estimates of the efficiency of ME utilization for those nutrients (Martin-Rosset et al., The term “heat increment” (HiE) encompasses HrE, HdE, 1994; Vermorel and Martin-Rosset, 1997). In the French HwE, and HfE (NRC, 1981). In thermoneutral environments, system the efficiency of converting ME to NE is estimated when HcE is zero, the metabolizable energy for maintenance at 85 percent for glucose, 80 percent for long-chain fatty (MEm) includes HjE, HeE, and HiE. acids, 70 percent for amino acids, and 63–68 percent for Because specific losses can be partitioned, NE systems VFAs. The French system also accounts for the energy cost have the potential to predict more accurately the ability of of eating (Martin-Rosset, 2000) in assessing the efficiency specific diets to meet the true energy needs of an animal. of ME use for NE for various feeds. The French system has However, NE systems require more information and are NE values for many common horse feeds, but it does not more complicated than DE systems. The losses described currently assign different values to feeds based on their effi- above do not constitute a consistent proportion of the ME in ciency of use for different physiological functions. feeds. For example, the energy costs associated with obtain- The advantages and disadvantages of applying NE or DE ing and chewing food can vary. The energy cost of eating systems in equine nutrition have been discussed elsewhere various feeds has been reported to range from 1–28 percent (Hintz and Cymbaluk, 1994; Harris, 1997; Martin-Rosset, of ME (Vermorel et al., 1997b). These losses might be ac- 2000; Cuddeford, 2004). Net energy systems provide a more counted for as either HjE or HdE. The chemical composition complete theoretical basis for matching the energy content of the energy-yielding components of ME will also influ- of feeds to the energy requirements of specific animals. Har- ence the magnitude of the various losses from individual ris (1997) concluded that NE systems better explain the val- feeds. HfE would be expected to be larger for feeds that are ues of various feeds for exercising horses, as a DE system digested by microbial fermentation than for feeds assimi- may overestimate the value of forage compared to grains or lated through enzymatic digestion. Therefore, the NE value fats. However, a system based on NE would be more com- of a feed containing starch could vary depending upon plex than a system based on DE as it accounts for more whether the starch was digested and absorbed in the small losses, many of which are interrelated. Cuddeford (2004) intestine, or if it was fermented and absorbed as volatile has suggested that because of its complexity, a NE system fatty acids (VFAs). The NE value of a feed may also vary may not offer a major advantage over the DE system used in

6 NUTRIENT REQUIREMENTS OF HORSES the NRC (1989) system. In France, the NE values of com- system is more familiar to equine nutritionists, veterinari- mon feeds were related to the NE value of a standard, well- ans, feed manufacturers, and horse owners. accepted feed (barley), creating a system where users can compare feeds on a substitution basis rather than on an ab- ENERGY SOURCES solute energy basis (Vermorel and Martin-Rosset, 1997). This substitution system may be more easily understood and The following section provides a brief overview of en- applied in practical situations. Barley was chosen as the ref- ergy sources used by the horse. More detailed discussions of erence feed in the French system, but it might not be as carbohydrate, fat, and protein metabolism in horses can widely accepted as the standard horse feed elsewhere in the be found in Chapters 2, 3, and 4. A discussion of energy- world. In addition, the French system does not currently ac- yielding feedstuffs and information on the effect of feed pro- count for differences among feeds in their efficiency of use cessing on energy availability can be found in Chapter 8. for different purposes. This concern may not be significant Adenosine triphosphate (ATP) is the major source of for situations in which maintenance requirements represent readily available chemical energy in cells. Cells generate the majority of the daily energy needs. However, failure to ATP from the catabolism of carbohydrates, fats, and pro- account for differences in the efficiency of energy use for teins. Carbohydrates and fats are the predominant sources of various physiological functions may induce errors into the ATP under normal circumstances. The balance between car- estimates for certain types of horses, such as horses in heavy bohydrate and fat utilization may be influenced by the phys- work or rapidly growing horses. Few studies have compared iological status of the horse, feeding state, physical condi- the two systems in practice, although several authors made tioning, and type of diet being consumed. comparisons based on theoretical diets. Hintz and Cym- Glucose is the primary form of carbohydrate used for baluk (1994) found that the estimated amount of feed re- ATP production. Cells may obtain glucose from the circula- quired by broodmares as calculated using the UFC system tion or from intracellular stores of glycogen. Glucose in the was similar to the amount calculated using the DE system in circulation may originate from hepatic gluconeogenesis, the 1989 NRC system. Martin-Rosset and Vermorel (2004) from hepatic glycogenolysis, or from food consumed and di- calculated that a diet of hay, barley, and soybean meal that gested by the horse. Red blood cells and brain cells depend met the DE requirements (NRC, 1989) of growing horses almost entirely on glucose as an energy source under normal exceeded the NE requirement estimated by the UFC system circumstances. Other tissues may use fat as an energy source by about 19 percent. Many feeding trials with different and, in some cases, amino acids. Amino acid catabolism classes of horses and different types of feeds will be neces- may be accelerated during starvation, but in most other sary to completely compare the effectiveness of the two cases, amino acids are a relatively minor component of the systems. energy used by the body. Even though NE systems are more complex, they offer an Fat is the most abundant energy source in the body. The opportunity to partition energy more completely in individ- long-chain fatty acids utilized by cells may originate from ual situations. NE systems have been well accepted and ap- recently consumed food, but most of the long-chain fatty plied with success in other sectors of the livestock industry. acids that are oxidized for energy probably come from either The most fully developed NE system for horses has been intracellular stores or adipose tissue. The triglycerides in created in France. However, the system still lacks informa- adipose tissue are broken down to long-chain fatty acids and tion about the NE values of all feeds of all classes of horses. glycerol that are released into the blood. In addition, more information is needed to verify that diets Short-chain (volatile) fatty acids can also be used for en- based on UFC units meet the requirements for horses for ergy production. Most short-chain fatty acids originate from different activities and physiological states. The DE system the large intestinal fermentation of carbohydrates. VFA pro- is retained in this document as the method for expressing the duction in the cecum may be sufficient to meet up to 30 per- energy requirements of horses. The DE system is retained, cent of a horse’s energy needs at maintenance (Glinsky et al., in part, because the French UFC system is considered to be 1976). Additional VFAs are produced in the colon. It has somewhat incomplete and because the use of barley as a ref- been estimated that horses consuming a diet composed pri- erence unit is potentially confusing to individuals who do marily of hay will meet more than 80 percent of their energy not use it as a common horse feed. This should not be taken needs from VFAs (Vermorel et al., 1997a). VFAs may be as a criticism of the French UFC system, as it has many pos- available as energy sources to cells or they may be metabo- itive attributes and has advanced the understanding of en- lized to long-chain fatty acids or glucose. After measuring ergy use by horses. However, the DE system has a few prac- blood acetate concentrations and estimating blood flow, tical advantages over the French UFC system. More is Pethick et al. (1993) suggested that acetate oxidation might known about estimating the DE content of horse feeds from contribute about 30 percent of the energy utilized by the hind chemical composition. Most feeding experiments with limb at rest. Acetate that is not oxidized for energy could be horses in the United States (and perhaps the world) have ex- converted to long-chain fatty acids and stored in adipose tis- pressed the dietary energy content on a DE basis. The DE sue, or converted to long-chain fatty acids for secretion into

ENERGY 7 milk. Acetate is the predominant VFA produced in the large to prevent a change in the total energy contained in the intestine, but significant amounts of propionate are also gen- body of these animals can be considered the maintenance erated. Lieb (1971) demonstrated that a large portion of pro- requirement. pionate infused into the cecum does not pass the liver. In ad- Maintenance energy requirements have been estimated in dition, Argenzio and Hintz (1970) demonstrated that mature horses using a variety of methods, including indirect propionate infusion can elevate blood glucose levels in fasted calorimetry, metabolic balance trials, and simple feeding tri- ponies. More recent work has suggested that up to 50–60 als. In addition, maintenance energy requirements have been percent of circulating glucose in forage-fed ponies originates expressed on a body weight basis and on a metabolically from absorbed propionate (Simmons and Ford, 1991). scaled basis, usually BW0.75 or BW0.67. The convention of Excess energy consumed by horses can be stored as expressing energy requirements on a metabolically scaled glycogen or as triglyceride. It is most efficient to immedi- basis was established to account for differences in the rela- ately utilize absorbed energy sources because there is an en- tionship of surface area to body weight in animals of widely ergetic cost of storing, and then mobilizing, endogenous en- differing sizes. However, because many other nutrient re- ergy sources. These energy costs are included in HdE—the quirements are expressed on a body weight basis, it is con- heat production associated with digestion and absorption. It venient to express energy requirements on a body weight has been estimated that the energy cost of incorporating glu- basis as well. In addition, Pagan and Hintz (1986a) con- cose into glycogen for later use is about 5 percent (Blaxter, cluded that maintenance energy requirements varied linearly 1989). However, if glucose is metabolized for the synthesis with body weight for equids weighing 125 to 856 kg. The of long-chain fatty acids and then the long-chain fatty acids previous edition of this document used the results of Pagan are oxidized, the energy cost is greater (Blaxter, 1989). and Hintz (1986a) as a reason to express daily energy re- McMiken (1983) estimated that a 500-kg horse with 5 per- quirements on a body weight basis. It is recognized that their cent body fat would have almost 10 times more calories study utilized a small number of horses. However, in the ab- stored in fat than in glycogen. However, several studies have sence of a study illustrating the benefits of using a metabol- reported the body fat of horses to be greater than 10 percent ically scaled body weight, energy requirements calculated in of body weight (Robb et al., 1972; Schryver et al., 1974; this document will be calculated on a body weight basis. Westervelt et al., 1976; Lawrence et al., 1986; Kane et al., A number of studies have estimated the amount of ME 1987). Therefore the amount of energy stored as fat can vary required by adult horses or ponies at maintenance by mea- among individuals. Several indirect methods have been used suring daily heat production. The components of daily heat to estimate body fat percentage in horses. Westervelt et al. production are shown in Figure 1-1. Studies that have mea- (1976) reported that rump fat thickness as determined by ul- sured heat production in horses have utilized a variety of trasound was related to percent extractable fat in the carcass. methods. These studies have used all forage as well as Kane et al. (1987) also reported a relationship between ultra- mixed diets and results have been reported on either a meta- sonically determined rump fat thickness and percent body fat bolically scaled basis or on a body weight basis. Data from in horses. These studies used small numbers of animals, and several studies are summarized in Table 1-1 (Wooden et al., effects due to breed, gender, and age were not investigated. 1970; Pagan and Hintz, 1986a; Martin-Rosset and Vermorel, However, in the absence of more comprehensive studies val- 1991; Vermorel et al., 1991; Vermorel et al., 1997a). When idating indirect estimates of body fat in horses, many subse- studies reported results on a metabolic body weight basis, quent studies have utilized the methods described by Wester- these values were converted to a body weight basis using the velt et al. (1976) or Kane et al. (1987) to estimate body fat in body weights reported in the study. When data were re- sedentary and athletic horses. These indirect estimates indi- ported for individual horses, the values were averaged to cate body fat in mature, sedentary horses in moderate or provide a mean value for a particular diet or feeding level. fleshy body condition may exceed 15 percent (Kubiak et al., The average heat production ranged from 22.2 to 30.6 1987; Kearns et al., 2002a), while the body fat of competitive kcal/kg BW across treatments and studies. The digestible endurance horses was estimated at less than 11 percent energy for maintenance (DEm) was calculated using the ef- (Lawrence et al., 1992b). Similarly, Kearns et al. (2002b) es- ficiency of DE use for ME reported for each diet within each timated that the body fat of elite harness racing horses was study. The DEm estimated across studies ranged from 25.7 to approximately 8 percent, and Webb et al. (1987) reported an 35.1 kcal/kg BW. estimated body fat in cutting horses of 12 percent. It is not surprising that variation exists in the estimated values of DEm that are reported by different studies. Coenen (2000) suggested that the DE requirements for maintenance ENERGY REQUIREMENTS range from 0.48 to 0.62 MJ/kg BW0.75/day (d) (approxi- mately 24.3 to 31.4 kcal/kg BW/d for a 500-kg horse). Some Maintenance of the factors that can affect maintenance requirements are Animals that are not pregnant, lactating, growing, or per- discussed below. Within a study, heat production is often forming work are often considered to be in a physiological higher when the diet consists of hay than when mixed diets state of maintenance. The amount of dietary energy needed are fed (Table 1-1). Higher fiber diets would increase HfE; it

8 NUTRIENT REQUIREMENTS OF HORSES TABLE 1-1 Summary of Studies that Measured Heat Production in Horses at Maintenancea Number Heat of Production Efficiency Author Animals Type of Diet Level of Feeding BW (kg) kcal/kg BW DE/ME DEm Pagan and Hintz, 1986a 4 Alfalfa and oats Near maintenance 421.8 22.3 86.3 25.8 (125–856) Pagan and Hintz, 1986a 4 Alfalfa and oats Above maintenance 421.8 27.0 87.6 30.8 (125–856) Pagan and Hintz, 1986a 4 Alfalfa and oats Above maintenance 421.8 24.5 86.6 28.3 (125–856) Wooden et al., 1970 2 Alfalfa and concentrate Near maintenance 454.5 27.2 90.0 30.2 (413–496) Wooden et al., 1970 2 Alfalfa and concentrate Above maintenance 463.5 28.7 90.8 31.6 (417–510) Vermorel et al., 1991 4 Meadow hay Maintenance and 490 29.7 87.2 34.1 above maintenance Vermorel et al., 1991 4 Meadow hay and Maintenance and 470 28.6 90.0 31.8 concentrate above maintenance Vermorel et al., 1997a 6 Grass hay and Below maintenance 475 30.6 87.2 35.1 protein supplement Vermorel et al., 1997a 6 Grass hay and barley Maintenance 475 28.3 90.1 31.4 Vermorel et al., 1997a 6 Grass hay Maintenance 475 27.8 87.2 31.9 Vermorel et al., 1997a 6 Grass hay Above maintenance 475 29.4 88.2 33.3 Vermorel et al., 1997a 6 Grass hay and barley Maintenance 475 25.3 89.5 28.3 Vermorel et al., 1997a 6 Grass hay and barley Above maintenance 475 25.4 90.5 28.1 Vermorel et al., 1997a 6 Grass hay Above maintenance 475 27.6 89.1 31.0 Vermorel et al., 1997a 6 Grass hay and corn Above maintenance 475 25.7 90.2 28.5 Vermorel et al., 1997a 6 Grass hay and beet pulp Above maintenance 475 27.6 89.1 31.0 Vermorel et al., 1997a 6 Straw and concentrate Above maintenance 475 26.1 89.9 29.0 Vermorel et al., 1997a 8 Alfalfa hay Above maintenance 475 27.8 87.1 31.9 Vermorel et al., 1997a 8 Grass hay Above maintenance 475 27.0 84.6 31.9 Vermorel et al., 1997a 8 Grass hay and corn Above maintenance 475 26.3 89.7 29.3 Vermorel et al., 1997b 8 Grass hay and corn Above maintenance 475 25.1 87.7 28.6 Vermorel et al., 1997b 6 Grass hay Near maintenance 208 26.3 85.2 30.9 Vermorel et al., 1997b 6 Grass hay Near maintenance 208 27.3 85.2 32.0 Vermorel et al., 1997b 6 Grass hay and corn Near maintenance 208 22.2 86.4 25.7 Vermorel et al., 1997b 6 Grass hay and corn Maintenance 208 23.5 87.7 26.8 aFor several studies, values expressed as kcal/kg BW0.75 were converted to a body weight basis. is also possible that high-forage diets could increase tissue morel et al. (1997b) later reported that MEm requirements of mass in the gastrointestinal tract, which would impact other ponies were lower per unit of metabolic body size (BW0.75) components of HE. McLeod and Baldwin (2000) reported than for horses. Another source of variation could be body that lambs fed a high-forage diet at 2-times maintenance had composition. Oxygen consumption is more closely related heavier gut tissues than lambs fed an isocaloric high- to lean body mass than to total body mass (Blaxter, 1989; concentrate diet. The gastrointestinal tract has a high meta- Kearns et al., 2002a). Therefore, the energy cost of main- bolic rate, and, thus, an increase in gut size could account taining a 500-kg horse with a higher lean body mass may be for elevated maintenance requirements in forage-fed ani- different than the energy cost of maintaining a 500-kg horse mals. Potter et al. (1989) reported that less DE was needed with a lower lean body mass. The effect of body composi- to maintain body weight when horses were fed a fat- tion on maintenance requirements has been demonstrated in supplemented diet than when they were fed a diet with no dairy cattle, where fasting heat production was inversely re- supplemental fat. As noted previously, individual differ- lated to body condition score (Birnie et al., 2000). ences can exist among horses in their ability to digest en- It has been suggested that equations used to estimate ergy; therefore, the horse-to-horse variation for DEm could maintenance requirements should be applied with consider- be relatively high. Differences may exist due to age and ations for factors such horse-to-horse variation, environ- breed as well. Martin-Rosset and Vermorel (1991) found ment, and diet composition (NRC, 1989). Pagan and Hintz that MEm requirements were lower for horses approximately (1986a) noted that their estimates of maintenance require- 11 years old than for horses approximately 4 years old. Ver- ments were based on measurements made while horses were

ENERGY 9 confined to metabolism stalls in a thermoneutral environ- horse/d) was estimated by increasing the minimal require- ment. They suggested that estimates of maintenance re- ment by approximately 20 percent. quirements for horses kept in typical environments should The daily DE intakes of horses as determined using the include an adjustment for normal activity and proposed that estimates for minimum, average, and elevated maintenance the maintenance requirement could be estimated by DE are compared to the recommendations from NRC (1989) in (kcal/d) = 1,375 + (30 × BW). This equation was later mod- Table 1-2. The average maintenance requirement of 33.3 ified by the NRC (1989) to DE (Mcal/d) = 1.4 + (0.03 × kcal/kg BW results in a daily DE requirement that is very BW) for horses weighing 600 kg or less. For horses weigh- similar to the previous requirement for 500-kg horses (NRC, ing more than 600 kg, maintenance DE was estimated from 1989). The average daily requirement of 33.3 kcal/kg BW the equation (NRC, 1989): DE (Mcal/d) = 1.82 + (0.0383 × results in a lower daily requirement for ponies than the pre- BW) – (0.000015 × BW2). vious recommendation (NRC, 1989), which appears to be The equations developed by Pagan and Hintz (1986a) and consistent with work of Vermorel et al. (1997a). However, NRC (1989) provide estimates of the average maintenance the requirement of 33.3 kcal/kg BW/d is higher than the pre- requirements of horses of different body weights. They do vious estimate for draft horses (NRC, 1989) and is not con- not, however, give guidance on how to adjust the require- sistent with Potter et al. (1987), who estimated the mainte- ment for horses with needs above or below the average re- nance DE requirement of draft horses to be 24.6 kcal/kg quirement. BW. This estimate was derived from the relationship of When the studies in Table 1-1 are summarized, the mean weight gain to DE intake (Potter et al., 1987), and the au- estimate for DEm is 30.3 kcal/kg BW/d. Stillions and Nelson thors noted that their methodology could have underesti- (1972) estimated the daily DEm of mature geldings at 33.8 mated DE intake of some horses. Morrison (1961) recom- kcal/kg BW, which was similar to the value determined by mended that idle draft horses (818 kg) receive 11 to 13.4 lb Anderson et al. (1983). Barth et al. (1977) estimated the (5 to 6.1 kg) of total digestible nutrients (TDN) per day. If daily DEm for pony stallions at 39.6 kcal/kg BW. These 1 kg of TDN contains 4.4 Mcal DE (NRC, 1978), then the studies used BW maintenance to estimate the DE require- Morrison (1961) recommendation for 818-kg horses would ment, and the estimates are higher than the mean value (30.3 be equivalent to 22 to 26.8 Mcal DE/d or 26.9 to 32.8 kcal/kg BW) obtained from Table 1-1. kcal/kg BW. It is possible that the minimum maintenance The mean daily DEm derived from Table 1-1 (30.3 kcal estimate (30.3 kcal/kg BW) should be applied to idle draft DE/kg BW) was calculated from pooled data from horses horses. and ponies that were confined during the experiments. The minimum and elevated values given here should not Therefore, this value (30.3 kcal DE/kg BW) may be consid- be considered to be the absolute minimum or maximum that ered to represent the average minimum requirement for would be appropriate for a specific animal. Many studies es- horses at maintenance. This average minimum maintenance timated DEm to be lower than 30.3 kcal/kg BW (Table 1-1). requirement may be appropriate for horses that have a very Several of the values in Table 1-1 were obtained with diets sedentary lifestyle, either due to confinement or due to a fed above maintenance. However, other values were ob- docile, nonreactive temperament. Horses in this group might tained with diets containing more concentrate than would include older animals that live in stables or small pens with commonly be fed to maintenance horses. Therefore, it is limited turnout, or horses that engage in limited voluntary suggested that the minimum, average, and elevated values activity even when kept in larger paddocks or pastures. be used as guides in formulating diets for maintenance An estimate for DEm for horses with average voluntary horses and that adjustments be made to meet individual activity was obtained by increasing the minimum value by situations. 10 percent to 33.3 kcal/kg BW. The average daily mainte- nance requirement of 33.3 kcal DE/kg (16.7 Mcal/500 kg BW horse/d) represents the needs of adult horses with alert temperaments and moderate voluntary activity. Horses in this group would probably be turned out for several hours TABLE 1-2 Three Proposed Levels of Digestible Energy per day but could include stabled horses that are active in Intake for Maintenance (Mcal/d) in Adult Horses as their stalls. Examples of horses in this group might be open Compared to the Previous Recommendation (NRC, 1989) broodmares and some performance horses that are being Body Minimum Average Elevated rested. weight (30.3 kcal/kg (33.3 kcal/kg (36.3 kcal/kg NRC A third estimate of DEm was derived for adult horses with (kg) BW) BW) BW) (1989) nervous temperaments or high levels of voluntary activity. 200 6.1 6.7 7.3 7.4 Members of this group might include stallions or young 400 12.1 13.3 14.5 13.3 adult horses that are noticeably active in their stalls or dur- 500 15.2 16.7 18.2 16.4 600 18.2 20.0 21.8 19.4 ing periods of turnout. An elevated daily maintenance re- 800 24.2 26.6 29.0 22.9 quirement of 36.3 Mcal DE/kg BW (18.2 Mcal DE/500 kg

10 NUTRIENT REQUIREMENTS OF HORSES Effect of Environment on Maintenance Requirements The ambient temperature to which the horse is accus- tomed determines its TNZ. The TNZ for Standardbred Horses are adaptable to wide temperature ranges and horses adapted to moderate (10°C) outdoor temperatures thrive in many diverse climatic conditions. The five climatic ranged from 5 to 25°C (Morgan et al., 1997; Morgan, 1998), variables that affect horses are ambient temperature, wind whereas the TNZ for maintenance-fed adult horses acclima- velocity, global solar radiation, precipitation, and relative tized to cold outdoor winter temperatures was –15 to 10°C humidity. The main climatic factor affecting thermoregula- (McBride et al., 1985). Thus the LCT for adults can be 5°C tion is ambient temperature, which affects insensible (evap- in mild climates or as low as –15°C in mature horses orative) and sensible heat (convection, conduction, and radi- adapted to very cold outdoor temperatures. ation) exchange. However, the impact of climate on horse Young horses may be cold-stressed at milder tempera- productivity is not a one-factor model but is the combined tures than adult horses. The TNZ for cold-adapted yearling effect of all climatic factors, often called the effective ambi- Quarter horses fed highly digestible diets was estimated at ent temperature. A model has been proposed that integrates –10 to 10°C (Cymbaluk and Christison, 1989). However, in the effect of the climatic factors on heat balance of exercised a subsequent study, an LCT of 0°C was estimated for Stan- horses (Mostert et al., 1996), but to date no useful five-factor dardbred yearlings (Cymbaluk, 1990). An LCT of about index has been developed that will predict nutritional needs 20°C was determined for 2- to 9-day-old foals, but individ- created by climatic influences. The two-factor index of am- ual variability was wide. Below the LCT, foals had an in- bient temperature and wind velocity called windchill was creased metabolic rate, shivering, and piloerection (Ousey highly correlated to weight gain of young horses (Cymbaluk et al., 1992). However, neonates kept at temperatures well and Christison, 1989). below 20°C gained weight normally and remained healthy Horses are homeotherms and must maintain a nearly con- when shelter, bedding, and feed were plentiful (Cymbaluk stant body core temperature. Cold or hot weather can cause et al., 1993). The UCT for newborn foals was determined body core temperature to decrease or increase, respectively. to be about 36–40°C (Ousey et al., 1992). The UCT for The horse responds to cold or heat through acute or chronic adult horses is harder to establish because various indices physiologic, metabolic, and behavioral responses. During can be used to define this criterion but each index has a dif- sudden cold spells, horses respond by eating more to in- ferent temperature threshold (Morgan, 1998). In Table 1-3, crease metabolic heat production and by postural changes to the UCT is based on an increase in evaporative heat loss. reduce heat loss (Booth, 1998). Chronic metabolic re- Unlike other species, idle horses exposed to tempera- sponses to cold involve longer feeding periods, increased tures above UCT thermoregulate principally through evap- hair coat, decreased rectal temperature, and decreased respi- orative heat loss, i.e., by sweating and/or by breathing more ratory rate (Cymbaluk, 1990; Cymbaluk and Christison, rapidly. 1993; Booth, 1998). Weanling horses housed at tempera- Full acclimation by horses to either hot or cold ambient tures of about 1°C had 27 percent lower respiratory fre- temperatures appears to take about 21 days, although con- quency, slightly lower heart rates, 20 percent more dense siderable acclimation has occurred by 10–14 days after heat haircoats, and peripheral skin temperatures that were or cold exposure. Acclimation of exercised horses to hot 8–15°C degrees lower than cohorts housed at 17°C (Cym- weather occurred largely within 14–15 days with minor im- baluk and Christison, 1993). Metabolic responses by horses provement in response over the next week (Geor et al., 1996; to acute heat exposure include an increased sweating rate, Marlin et al., 1999; Geor et al., 2000). However, the reten- increased respiratory rate, decreased feed intake, and in- tion time of heat (or cold) acclimation is uncertain. Based on creased water intake (Geor et al., 1996; Morgan, 1997; Mor- data collected from a small number of grazing horses, accli- gan et al., 1997; Marlin et al., 2001). mation to short-term thermal stress (cold or warm) was Cold and heat stress occur at temperatures below and cyclic occurring in a time series manner over 10–11 days above (respectively) the thermoneutral zone (TNZ). The (Senft and Rittenhouse, 1985). Thus, the temperature values TNZ is the temperature range when metabolic heat produc- for the LCT and UCT are dynamic. tion does not need to be increased to maintain thermostabil- Temperature stress or exposure to ambient temperatures ity (NRC, 1981). The TNZ itself consists of three divisions: at which the horse is unacclimated will alter the resting cool, optimal, and warm. The lowest temperature of the metabolic rate. Adult horses with an LCT of –15°C that TNZ is termed the lower critical temperature (LCT) and is were acclimated to winter temperatures had an average the temperature below which metabolic heat production is hourly metabolic rate of about 102.9 kcal/100 kg BW at increased to maintain body core temperature. The upper crit- temperatures in their TNZ (McBride et al., 1985). As ambi- ical temperature (UCT) is the high temperature end of the ent temperatures decreased below –10°C, hourly metabolic TNZ and is the temperature above which evaporative heat rates increased linearly to 117.9 kcal/100 kg BW at –20°C, loss must be increased to control body temperature. The to 146.9 kcal/100 kg BW at –30°C, and to 181.6 kcal/kg BW TNZ, LCT, and UCT for horses vary with age, body condi- at –40°C. It was concluded that DE intakes of adult horses tion, breed, season, adaptation, and climate (Table 1-3). at maintenance should be raised 2.5 percent for each degree

ENERGY 11 TABLE 1-3 Lower and Upper Critical Temperatures in Horsesa Lower Critical Temperature (°C) Upper Critical Feed Exposure Life Stage Average Range Temperature (°C) Intake Level Type Reference 2–4 days 22 16–26 36 Suckle; limited Acute cold Ousey et al., 1992 7–9 days 19 13–23 ~40 Suckle Acute cold Ousey et al., 1992 Yearling 0 Unknown Not determined 2 × maintenance Acclimated Cymbaluk, 1990 Yearling –11 Unknown Not determined 2–2.5 × maintenance Acclimated Cymbaluk and Christison, 1993 Mature –15 –20 to –9.4 Not determined Maintenance Acute or acclimated McBride et al., 1985 Mature 5 Unknown 25 Maintenance Acute cold or heat Morgan et al., 1997; Morgan, 1998 aSOURCE: Adapted from Cymbaluk (1994). Celsius below the LCT (McBride et al., 1985). Cymbaluk ditions. Less DE may be needed to maintain body weight and Christison (1990) suggested that the maintenance re- when horses are fed a fat-supplemented diet than when they quirement of growing horses was increased by about 33 per- are fed a diet with no supplemental fat (Potter et al., 1989). cent in cold housing conditions (–15°C) and by more than By lowering DE intake, a decrease in heat production, and 50 percent in severely cold conditions (–25°C). Therefore, it thus heat load might be expected. may be appropriate to increase the total DE intake of grow- ing horses by 1.3 percent for each degree Celsius below the GROWTH LCT (Cymbaluk, 1990). As noted previously, the entire cli- matic effect on horses encompasses more than just tempera- Energy Requirements for Maintenance and Gain ture and includes other conditions such as rain or wind. Horses exposed to cold and wet conditions have been re- The energy requirement of a growing horse is the sum of ported to have DE requirements elevated as much as 50 per- the energy needed for maintenance and the energy needed cent above maintenance (Kubiak et al., 1987). Maintenance for gain. The daily DE requirement was previously esti- energy needs are expected to increase when horses are ex- mated by the following equation (NRC, 1989): posed to conditions above the UCT. However, the magnitude of this increase has not been quantified. DE (Mcal/d) = (1.4 + 0.03 × BW) + Because heat production can be affected by feed intake (4.81 + 1.17X – 0.023X2) × ADG and feed composition, the type and amount of feed may as- sist the horse in coping with cold or heat stress. Pagan and where X = age in months; BW = body weight in kilograms; Hintz (1986a) reported that heat production increased with and ADG = average daily gain in kilograms. There is good increased feed intake. However, in that study, increased DE agreement between actual energy intakes that have been re- intake was coupled with increased feed intake, so it is not ported in several studies and the energy intakes predicted by clear whether heat production increased as a result of in- this equation (Figure 1-2). The studies that were reviewed creased dry matter intake or increased DE intake. In another for Figure 1-2 were Ott et al. (1979); Knight and Tyznik study, feeding at 1.3 times maintenance increased heat out- (1985); Ott and Asquith (1986); Gibbs et al. (1987); put by 0.39-fold (Vernet et al., 1995). Ponies fed a 70:30 Schryver et al. (1987); Cymbaluk and Christison (1989); hay-grain diet at maintenance feeding levels had a lower Cymbaluk et al. (1989); Davison et al. (1989); Scott et al. daily maintenance heat production compared to an all-hay diet (Vermorel et al., 1997b). However, weanling horses fed 30 Predicted DE Intakes (Mcal/d) Y = 0.823X + 3.3792 high-hay or high-grain diets and kept at either warm (17°C) 25 R2 = 0.8482 or cold (0°C) temperatures showed no difference in energy 20 utilization or gain (Cymbaluk and Christison, 1993). Simi- 15 larly, heat production did not differ when horses were fed ei- 10 ther hay or hay-grain diets at maintenance levels (Vernet et 5 al., 1995). In hot weather conditions, the feeding program for idle 0 0 5 10 15 20 25 30 horses should be designed to minimize heat load. Although Actual DE Intakes (Mcal/d) high-fat diets may prove potentially useful in reducing heat load in hot weather (Kronfeld, 1996; Ott, 2005), few studies FIGURE 1-2 Comparison of digestible energy (DE) intakes of have critically examined the metabolic effects of high-fat growing horses as predicted by NRC (1989) and actual intakes re- diets on thermoregulation of idle horses in hot weather con- ported in the literature.

12 NUTRIENT REQUIREMENTS OF HORSES (1989); Buffington et al. (1992); Saastamoinen et al. (1993); –20.6 to 26.6°C. In that study, the maintenance component Ott and Asquith (1995); Coleman et al. (1997); Ousey et al. of the daily energy requirement may have been elevated due (1997); Cooper et al., (1999); Ott and Kivipelto, (1999); to environmental effects, but other factors such as increased Bell et al. (2001); and Ott et al. (2005). voluntary activity and increased ratio of surface area to body In the equation developed in 1989 (NRC, 1989), the en- weight could also result in an increased maintenance com- ergy required for maintenance in growing horses was esti- ponent in young horses. Figure 1-3 shows the effect of age mated using the same method as the energy required for on DEm when the values from Ousey et al. (1997) and Cym- maintenance in adult horses. This equation predicts that baluk et al. (1989) are combined with estimates for 24- and maintenance requirements on a body weight basis will be 36-month-old horses of 36.3 and 33.3 kcal/kg BW, respec- somewhat higher for a 200-kg weanling (37 kcal/kg BW) tively. These values for horses at 24 and 36 months are the than for a 500-kg horse (32.8 kcal/kg BW). Coenen (2000) same as the elevated and average maintenance requirements suggested the maintenance requirement of growing horses derived for mature horses. decreases from 210 kcal/kg BW0.75 at 3 to 6 months of age Based on the values shown in Figure 1-3, DEm for grow- to 151 kcal/kg BW0.75 for horses between 13 and 18 months ing horses may be estimated using the following equation: of age. Horses older than 18 months were estimated to have maintenance requirements similar to mature horses (Co- DE (kcal/kg BW/d) = 56.5X–0.145 (R2 = 0.99) (1-1) enen, 2000). Using the estimates of Coenen (2000), the maintenance component for a 200-kg weanling would be where X = age in months. This equation appears to fit the about 56 kcal/kg BW. In addition, Arieli et al. (1995) deter- available data well, but it is based on very few data points, mined that heat production in young calves ranged from 105 and therefore may under- or overestimate DEm in growing to 130 kcal/kg BW0.75, which is much higher than estimated horses. However, the estimates derived by Equation 1-1 for fasting heat production in mature cows (NRC, 2001). Ousey growing horses between the ages of 6 and 18 months are et al. (1997) estimated the daily net energy for maintenance only slightly higher than estimates that would be derived (NEm) of neonatal foals at about 67 kcal/kg BW. They also from scaling the maintenance requirements of growing estimated that the conversion of DE to NE was greater than horses to metabolic body size. Therefore, the estimates of 95 percent in milk-fed foals. In limit-fed and ad libitum–fed maintenance derived by Equation 1-1 are used in this docu- growing horses (6–24 months of age), Cymbaluk et al. ment for growing horses. Additional research is needed to (1989) reported maintenance requirements to be 37.8 and provide accurate estimates of the maintenance component 35.6 kcal DE/kg BW, respectively. The horses in that study for horses of different ages. were kept in a barn with 6 hours of daily turnout exercise. To obtain estimates of the amount of DE required for Mean daily temperature during the study was above freez- each kilogram of gain, data from several studies were sum- ing, but minimum and maximum temperatures ranged from marized (Ott et al., 1979; Knight and Tyznik, 1985; Ott and 80 70 –0.1452 Y = 56.539X 60 R2 = 0.9985 DEm (kcal/kg) 50 40 30 20 10 0 0 5 10 15 20 25 30 35 40 Age in Months FIGURE 1-3 Effect of age on digestible energy for maintenance (DEm) (kcal/kg BW) in growing horses.

ENERGY 13 Asquith, 1986; Gibbs et al., 1987; Schryver et al., 1987; might have occurred in the various studies. There was insuffi- Cymbaluk and Christison, 1989; Cymbaluk et al., 1989; cient information to determine effects due to gender and there Davison et al., 1989; Scott et al., 1989; Buffington et al., was only one value for horses older than 18 months. This 1992; Saastamoinen et al., 1993; Ott and Asquith, 1995; equation may underestimate the DE required for gain in ani- Coleman et al., 1997; Ousey et al., 1997; Cooper et al., mals more than 18 months of age. Using the data in Figure 1- 1999; Ott and Kivipelto, 1999; Bell et al., 2001; Ott et al., 4, Equation 1-2 was obtained. Because Equation 1-2 was de- 2005). For each study the maintenance requirement (deter- veloped from mean values reported in the literature and not mined using Equation 1-1) was subtracted from DE intake from observations on individual horses, it is likely that the R2 to obtain the DE available for gain. The amount of DE avail- is overestimated: able for gain was then divided by ADG to arrive at the en- ergy cost (DE) for 1 kg of gain. Figure 1-4 shows the rela- DE (Mcal) for gain = (1.99 + 1.21X – (0.021X2) × (1-2) tionship of age to the amount of DE required per kilogram ADG (R2 = 0.65) of gain. It is apparent in Figure 1-4 that there is considerable variation in the amount of DE needed for a kilogram of gain where X = age in months and ADG = average daily gain in at any age. There are several potential explanations for the kilograms. observed variation in DE required per kilogram of gain. The Using Equation 1-2, the amount of energy required for DE required for each unit of body weight gain will be influ- gain by growing horses of different ages may be estimated. enced by the composition of the gain (amount of fat and pro- The predicted amount of DE necessary for 1 kg of gain in- tein). Not all horses at a given age were growing at the same creases from 6 months to 24 months of age (Table rate, and it is possible that rate of gain influenced composi- 1-4). This equation is not appropriate for predicting the DE tion of gain. In addition, gender may influence composition necessary for gain in mature horses. For young growing of gain. Most studies did not separate their results by gen- horses, the values for DE/kg gain in Table 1-4 are somewhat der. In addition, the composition of the diet may contribute lower than the values suggested previously (NRC, 1989). to differences in the amount of DE required per kilogram of gain. The efficiency of DE use in high-forage diets may be Growth Rate lower than the efficiency of DE use when high-concentrate diets are fed. In the studies that were reviewed, the compo- The daily energy requirements for growing horses are sition of the diets varied from milk replacer to predomi- greatly influenced by the rate of growth of the animal. nantly concentrate to predominantly forage. Some diets con- Growth rate often varies among breeds or types in cattle and tained added fat. Finally, the amount of DE utilized for gain swine, and nutrient recommendations have been developed for horses in each study was determined as the difference to account for some of these differences (NRC, 1998, 2001). between the maintenance component and the DE intake. The Extensive, well-documented growth data are available for maintenance component was related to age only and did not Thoroughbreds (Green, 1969; Hintz et al., 1979; Ruff et al., account for differences in activity or thermal challenges that 1993; Thompson, 1995; Jelan et al., 1996; Pagan et al., 25 20 DE (Mcal/kg gain) 15 10 5 Y = 1.99 + 1.21X – 0.021X2; R2 = 0.65 0 0 5 10 15 20 25 Age in Months FIGURE 1-4 Relationship between age (in months) and the amount of digestible energy (DE) (Mcal) required above maintenance per kilo- gram of gain in growing horses.

14 NUTRIENT REQUIREMENTS OF HORSES TABLE 1-4 Effect of Age on the Amount body weight for Morgan horses was obtained from the same of Digestible Energy (DE) Required per farm that provided the growth measurements of the Morgan Kilogram of Gain (above maintenance) for horses (Green, personal communication). Mature body Growing Horses weights for Swedish warmbloods (Johnston et al., 2002) DE for Gain were used to estimate the mature weight of Hanovarians. Age of Horse (Mcal/kg gain) The following mature body weights (Kg) were used: Thor- oughbred, 580; Hanovarian, 580; Swedish Standardbred, 6 months 8.5 12 months 13.5 500; Quarter horse, 555; Belgian, 863; Morgan, 454; Ara- 18 months 17.0 bian, 455; pony, 195. 24 months 18.9 Once mean growth data for each breed were converted to a percentage of mature body weight, the data were fitted to a curve. The NLIN procedure of the Statistical Analysis 1996; Kavazis and Ott, 2003), but the available data for System (SAS) (SAS/STAT, 1999) was used to calculate other breeds are less comprehensive or entirely absent. weighted least squares estimates of parameters a and c of the Growth data were found for Hanovarians (Vervuert et al., following non-linear model: 2003), Morgan horses (Lawrence et al., 2003), Swedish Stan- dardbreds (Sandgren et al., 1993), Arabians (Reed and Dunn, Percent mature body weight = a + ((100 – a) (1 – e–ct)) 1977), and ponies (Jordan, 1977). No recent data sets con- taining sequential body weight measurements were found for where a = percent mature body weight at birth; c = rate con- Quarter horses, but numerous studies have reported body stant; t = age in months; e = 2.7183. The resulting equation weights at different ages (Russell et al., 1986; Davison et al., was derived: 1989; Lawrence et al., 1987, 1991; Coleman et al., 1997). Some of the studies cited above separated growth informa- Y = 9.7 + (100 – 9.7) × (1– (e(–0.0772 × X)) (R2 = 0.99) (1-3) tion by gender, but others did not. Because of the limited data available for most breeds, it where Y = percent mature body weight and X = months of was not possible to develop breed-specific growth curves at age. Equation 1-3 was derived from mean data that were this time. However, it has been proposed that the body pooled within a breed, and not from observations on indi- weight of a growing horse at any given age can be predicted vidual horses. Therefore the R2 given for Equation 1-3 is from mature body weight (Coenen, 2000; Austbo, 2004), probably overestimated. which would allow prediction of growth rate for horses of The percent of mature body weight predicted by Equa- different breeds and mature body weights. This method tion 1-3 is compared to the estimates of Austbo (2004) and would also allow prediction of differences between males Coenen (2000) in Table 1-5. and females based on expected differences in mature body Equation 1-3 may be used to calculate expected body weight. weight during growth if an estimate of mature body weight Body weight data for growing horses were summarized is available. Table 1-6 shows body weights predicted by by breed using previously cited growth studies and a few ad- Equation 1-3 for growing horses expected to mature at 200, ditional unpublished data sets on Quarter horses (Cooper, 400, 500, 600, and 900 kg. Table 1-6 also shows the body personal communication; Cymbaluk, personal communica- weights previously suggested for horses of various ages tion) and Belgian foals (Cymbaluk, personal communica- (NRC, 1989). Equation 1-3 predicts slightly lower body tion). Mature weights were estimated from the body weights weights at most ages for growing horses expected to mature of sires and dams cited in individual studies (Jordan, 1977) at 500 kg or less than NRC (1989). However, Equation 1-3 or from the mature weight given in the study (Reed and tends to predict larger body weights during growth than Dunn, 1977). When mature weights were not available for a NRC (1989) for horses 600 kg and greater. Body weights in specific study, the literature was searched for estimates of NRC (1989) appear to have been scaled to assume a longer mature body weights in each breed. The mature body weight for Thoroughbreds was developed from body weights of broodmares and stallions (Siciliano et al., 1993; Pagan et al., TABLE 1-5 Summary of Estimates of the Relationship 1996; Ordakowski et al., 2001; Williams et al., 2001). Ma- Between Age and Percentage of Mature Body Weight in ture body weight for Quarter horses was developed from Growing Horses body weights of mares (Webb et al., 1991; Lawrence et al., 1992a; Coleman et al., 1997; Cymbaluk, personal commu- Birth 6 mo 9 mo 12 mo 18 mo nication). Mature body weight for Swedish Standardbreds Equation 1-3 9.7 43.2 54.9 64.2 77.5 was estimated from data of Malinowski et al. (2002) and Coenen, 2000a 9.95 46.2 56.4 64.2 75.9 Palmgren Karlsson et al. (2002). No mature weights were Austbo, 2004 10.0 47.0 58.0 67.0 82.0 found for Morgan or Hanovarian horses. Therefore, mature a500-kg horse.

ENERGY 15 TABLE 1-6 Body Weight Predicted by Equation 1-3 Equation 1-3 is best suited to estimating weight of horses and Expected Mature Body Weight and Body Weight with an expected mature weight of 450 to 650 kg. Estimated in the Previous NRC (1989) for Growing Horses The ability of Equation 1-3 to predict body weight in (values in parentheses represent % of mature body weight) growing horses was tested using three independent data sets Mature Wt/Age NRC (1989) Equation 1-3 that contained 550 measurements on horses between birth and 16 months of age. Expected mature weight was calcu- 200 kg 4 mo 75 (37.5%) 67.4 (33.7%) lated from dam weight and sire weight, from dam weight 6 mo 95 86.4 12 mo 140 128.5 and the average weight of sire’s breed, or from the breed av- 18 mo 170 155 erage, depending upon the information contained in each 24 mo 185 (92.5%) 172 (86%) data set. Horses included in the data sets were of Thorough- 36 mo No value 189 bred or stock-type (Quarter horse, Appaloosa, or Paint) 400 kg 4 mo 145 (36.3%) 134.8 (33.6%) breeding. For these data sets, the body weights predicted 6 mo 180 173 12 mo 265 257 from expected mature body weight and Equation 1-3 and the 18 mo 330 310 actual body weights were not different for horses less than 24 mo 365 (91.2%) 343.4 (85.9%) 9 months of age, or older than 12 months of age (P > 0.05). 36 mo No value 379 However, between 9 and 12 months of age, the average pre- 500 kg 4 mo 175 (35%) 168.5 (33.7%) dicted body weight was higher (P < 0.05) than the actual 6 mo 215 215.9 12 mo 325 321.2 body weight. Equation 1-3 predicts that ADG will decrease 18 mo 400 387.5 with age. However, data collected on horses in commercial 24 mo 450 (90%) 429.2 (86%) environments do not always reflect this trend. Pagan et al. 36 mo No value 472 (1996) reported that ADG of Thoroughbred yearlings in 600 kg 4 mo 200 (33.3%) 202.1 (33.7%) central Kentucky was lower in January and February than in 6 mo 245 259 12 mo 375 385.5 April and May, possibly due to milder temperatures and in- 18 mo 475 465 creased pasture availability in April and May. Rates of gain 24 mo 540 (90%) 515 (86%) for Thoroughbreds in Japan have been reported to be 0.40 to 36 mo No value 566.4 0.50 kg/d at 11 months and 0.4 to 0.7 kg/d at 13 to 15 900 kg 4 mo 275 (30.6%) 303.2 (33.7%) months, with the slower growth observed in winter and the 6 mo 335 388.6 12 mo 500 578.2 faster growth observed in summer (Asai, 2000). Staniar et al 18 mo 665 697.5 (2004) also reported a lag in growth during the winter 24 mo 760 (84.4%) 773 (85.9%) months followed by accelerated growth in the spring for 36 mo No value 850 pastured Thoroughbred horses. Therefore, Equation 1-3 overestimates percent of mature body weight and ADG dur- ing the winter for pastured horses kept in northern climates and underestimates ADG in the subsequent spring. How- ever, for horses with an expected mature body weight of 580 maturation curve for heavy-weight horses. For example, kg, the body weight and ADG predicted at 6 months and 15 NRC (1989) estimated that a foal expected to mature at 200 months of age (250 kg and 415 kg, respectively) are similar kg would have reached 37.5 percent of mature weight at 4 to body weights reported by Pagan et al. (1996) for Thor- months of age, whereas a foal expected to mature at 900 kg oughbreds. In addition, the predicted weight at 16.5 months would have reached 30.6 percent of mature body weight at of age for horses expected to mature at 580 kg is similar to the same age. The data sets used to develop Equation 1-3 the actual weight reported by Staniar et al. (2004) for Thor- were insufficient to determine whether large breeds mature oughbreds. Independent data for horses older than 16 more rapidly than small breeds using statistical analyses. months of age were not available to test Equation 1-3. How- However, there were a few data points for ponies, Thor- ever the body weight predicted by Equation 1-3 for an 18- oughbreds, and draft horses at similar ages. The percentage month-old horse with an expected mature body weight of of mature weight at 4 months of age was 33.3 percent, 33.7 580 kg was similar to the body weight reported for 18- percent, and 32.2 percent, for ponies, Thoroughbreds, and month-old Thoroughbreds at a commercial sale (Pagan et Belgians, respectively. At 6 months of age, percentage of al., 2005b). mature weight was 42 percent for ponies and 42.4 percent Segmental models have also been used to predict growth for Thoroughbreds (data not available for draft horses). At in horses (Kavazis and Ott, 2003; Delobel et al., 2005). Seg- 12 months of age, percentage of mature weight was 75 per- mental models may provide a better estimate of growth in cent for ponies and 69 percent for Thoroughbreds (data not horses affected by seasonal factors; however, it was the con- available for draft horses). The similarity of the available sensus of the Committee on Nutrient Requirements of values, particularly in early growth, indicates that one curve Horses that it was more desirable to use a continuous model may be used for a variety of breeds. However, it is likely that to predict growth. Therefore, Equation 1-3 is used in this

16 NUTRIENT REQUIREMENTS OF HORSES document to estimate body weight and ADG of growing DE (Mcal/d) = (56.5X–0.145) × BW + (1-4) horses. (1.99 + 1.21X – 0.021X2) × ADG Ideally, ADG and body weight will be known for indi- vidual horses. However, when it is not possible to weigh where X = the age in months; ADG = average daily gain in horses, body weight and ADG may be estimated using the kilogram; and BW = body weight in kilograms. equations above. Although this prediction equation may be The relationship between predicted daily DE intakes and used to predict body weight and average daily gain of horses reported daily DE intakes from published studies is shown beyond 18 months of age, users should recognize that the in Figure 1-5. Although Equation 1-4 appears to predict imposition of training can markedly affect the accrual of daily DE intakes of growing horses more closely than the body weight. In addition, horses used for intense competi- equation used previously (NRC, 1989), this may be because tive purposes, such as racing, will typically weigh less than the studies used to generate Figure 1-5 included those that a similarly aged horse used for breeding or for recreation. were used to generate Equation 1-2, which is a component Because many of the values used to generate these predic- of Equation 1-4 (Ott et al., 1979; Knight and Tyznik, 1985; tion equations were obtained from commercial horse enter- Ott and Asquith, 1986; Gibbs et al., 1987; Schryver at al., prises, this prediction equation should represent the average 1987; Cymbaluk and Christison, 1989; Cymbaluk et al., of modern-day growth rates. However, it is recognized that 1989; Davison et al., 1989; Scott et al., 1989; Buffington et it may be desirable to accelerate growth rate to produce al., 1992; Saastamoinen et al., 1993; Ott and Asquith, 1995; horses suitable for specific purposes such as competition or Coleman et al., 1997; Ousey et al., 1997; Cooper et al., sale (NRC, 1989; Martin-Rosset, 2000). In addition slower 1999; Ott and Kivipelto, 1999; Bell et al., 2001; Ott et al., growth rates may be suitable for horses that will not be uti- 2005). In addition, the values shown in Figure 1-5 represent lized or marketed until they reach a later stage of maturity. means for groups of horses, rather than observations on in- Therefore, users should modify dietary nutrient intakes to dividual horses. Therefore it is likely that the variation asso- attain the desired growth rate necessary to meet manage- ciated with differences in predicted and actual DE intakes of ment goals. individual horses will be much greater than is illustrated by Figure 1-5. Unfortunately, independent data sets were not Daily DE Intakes for Growing Horses available to test this equation for individual horses. Daily DE requirements for growing horses are deter- mined as the sum of the requirement for maintenance and REPRODUCTION the requirement for gain. As noted previously, the mainte- nance requirement will vary with age of horse and body Breeding Stallions weight (Equation 1-1). The requirement for gain will vary Stallions are generally considered to have higher mainte- with age of horse and rate of gain (Equation 1-2). The daily nance requirements than mares or geldings (Cuddeford, DE requirement of growing horses can be calculated by 2004; Martin-Rosset and Vermorel, 2004). The amount of combining Equation 1-1 and Equation 1-2 to form Equa- dietary energy required in the breeding season will likely be tion 1-4: affected by breeding frequency. Horses that make a few 30 25 Predicted DE Intakes (Mcal/d) Y = 0.96X + 0.77; R2 = 0.88 20 15 10 5 0 0 5 10 15 20 25 30 Actual DE Intakes (Mcal/d) FIGURE 1-5 Digestible energy (DE) intakes of growing horses as predicted by Equation 1-4 and ac- tual intakes reported in the literature.

ENERGY 17 mounts each week in live cover or artificial insemination placental development, then there would be an increase in programs would be expected to have lower energy require- uterine tissue of 4 kg during gestation. There are insufficient ments than horses that make more than a dozen mounts each data to determine whether the rate of nonfetal tissue accre- week in intensively managed live cover programs. Limited tion during gestation follows a linear function in mares. data on DE intakes of breeding stallions are available. One However, Bell et al. (1995) reported that tissue accumula- field study estimated that Thoroughbred stallions (590 kg) tion in the gravid uterus of cows was a linear process. There- covering 70 to 90 mares/month were receiving approxi- fore the rate of total nonfetal tissue accretion (uterus and mately 25 Mcal DE/d (42.4 kcal/kg BW) (Siciliano et al., placenta) in a 500 kg-horse is estimated at about 45 g/d 1993). This estimate is slightly higher than the recommen- (0.09 g/kg maternal BW/day) from day 150 through day 330 dation of the previous edition of this publication (NRC, of gestation. Although the amount of nonfetal tissue accre- 1989), where the requirement for breeding stallions was tion is not great, this tissue appears to be very active meta- considered to be equivalent to the requirement for light bolically (Fowden et al., 2000a,b), and thus has a higher work. The DE requirement for breeding stallions in heavy maintenance requirement per unit of weight. use is estimated to be 20 percent higher than maintenance, The existing data on fetal development have been drawn and breeding stallions are considered to have an elevated from studies on foals that were aborted or stillborn between maintenance requirement during the nonbreeding season day 150 and term of gestation (Meyer and Ahlswede, 1978; (36.3 kcal/kg BW). Platt, 1978; Giussani et al., 2005). Using the data reported in these papers, the percent of birth weight accumulated during days 150 to 330 of gestation were calculated and are pre- Pregnancy sented in Figure 1-6. Birth weight was estimated at 9.7 per- During pregnancy, energy will be used for maintenance cent of dam weight. of the dam, deposition of fetal and placental tissue, hyper- Several equations were developed from the data in Figure trophy of the uterus, mammary development, and mainte- 1-6. The resulting equations (where X = days of gestation) nance of the fetus, placenta, mammary and additional uter- were: ine tissue. In studies with other livestock species, sacrifice of pregnant females has provided information about the Fetal weight as a percent of birth weight = (1-5a) amount of fetal, placental, and uterine tissue accumulated at 0.5321X – 87.996 (R2 = 0.855) various stages of gestation. In the pig, which also has a dif- fuse placenta, more than 50 percent of the final placental Fetal weight as a percent of birth weight = (1-5b) and uterine weight had accumulated by midgestation (Ji et 2.456 + 0.0015X2 – 0.2198X (R2 = 0.874) al., 2005). At parturition, placental weight (without fluids) is about 4 kg for a 500-kg mare (Allen et al., 2002; Whitehead Fetal weight as a percent of birth weight = (1-5c) et al., 2004). The few measurements of uterine and placen- 0.8875e0.0144X (R2 = 0.903) tal tissue accretion in midgestation mares indicate that uter- ine and placental tissue accretion occurs during the second Fetal weight as a percent of birth weight = (1-5d) trimester (Ginther, 1992). If uterine development parallels 1 × 10–7X3.5512 (R2 = 0.929) 140 Fetal Weight as a percentage of Birth Weight 120 100 80 60 40 20 0 0 100 200 300 400 Days of Gestation FIGURE 1-6 Fetal weight during gestation as a percentage of birth weight.

18 NUTRIENT REQUIREMENTS OF HORSES Although the coefficients of determination were rela- ergy cost of tissue deposition was determined using the as- tively high for all equations, not all equations predicted the sumptions that the efficiency of DE use for tissue deposition same gestation length. For example, using Equation 1-5a, during pregnancy is 60 percent (NRC, 1989) and that each the fetus would reach birth weight at 353 days, whereas with unit of gain is 20 percent protein and 3 percent lipid. Equation 1-5c the fetus would reach birth weight at about The resulting estimates of DE intakes for mares with 328 days of gestation. Equations 1-5b and 1-5d predict that body weights between 400 and 600 kg are slightly higher the fetus would reach birth weight at 339 and 342 days, re- than previous estimates (NRC, 1989) that were calculated at spectively. Equation 1-5d appears to provide a more biolog- 11 percent, 13 percent, and 20 percent increments above ical approximation of the data than Equation 1-5b (Figure maintenance in the 9th, 10th and 11th months of gestation, 1-7), and thus Equation 1-5d is used to predict the fetal respectively. Mares in late gestation with DE intakes 10 per- weight as a percent of birth weight. cent above the previous recommendation have been reported Fetal weight during gestation may be calculated from to maintain fat mass (Filho et al., 2005). In another study, birth weight, which is estimated as 9.7 percent of the ex- mares with estimated DE intakes slightly above the previous pected mature weight (Equation 1-3). Because the data used recommendation did not gain weight during the last 60 days to develop Equation 1-5d were taken from non-normal foals, of gestation but had apparently normal foals (Kowalski et it is possible that the resulting estimates may underestimate al., 1990). Because fetal growth is greatest during the last 60 development of normal foals. However, data on fetal devel- days of gestation, an increase in mare body weight would be opment of normal foals at various stages of gestation were expected if the energy requirements for maintenance and tis- not found during a search of the literature. sue deposition were met. The failure of mares to gain weight Using information discussed above and Equation 1-5d, suggests that the amount of DE consumed was not adequate the total accretion of fetal and nonfetal tissue during gesta- to meet the needs for maintenance and tissue deposition, and tion was calculated and the additional maintenance require- that mares were mobilizing body stores to meet the needs of ment of these tissues as well as the energy retained in these fetal development. The current recommendations for 500-kg tissues estimated. The accumulated fetal and nonfetal tissues pregnant mares are approximately 5 to 8 percent above the associated with pregnancy are more metabolically active 1989 recommendations; in addition, the current recommen- than other maternal tissue and a higher maintenance cost dations suggest increasing DE intakes above maintenance (66.6 kcal/kg BW) was estimated. This estimate was based earlier in gestation. on studies by Fowden et al. (2000a) using horses and by The current estimates for DE intakes by draft-type mares Reynolds et al. (1986) using cattle. This rate was applied are much higher than most previous estimates (NRC, 1989; only to the accumulating tissues of conception. Therefore, Coenen, 2000; Martin-Rosset, 2000). However, one study the additional maintenance requirement was calculated as utilizing pregnant draft-type mares (Doreau et al., 1991) fed the product of weight of tissue accumulated and a mainte- diets providing DE intakes similar to those recommended nance DE cost for those tissues of 66.6 kcal/kg BW. The en- here. The higher estimates of DE intakes by draft-type mares 140 120 Fetal Weight as a Percentage of Equation 1-5b 100 Equation 1-5d 80 Birth Weight 60 40 20 0 0 100 200 300 400 -20 Days of Gestation FIGURE 1-7 Comparison of two equations (1-5b and 1-5d) that predict fetal weight as a percentage of birth weight.

ENERGY 19 arise from the maintenance component associated with the affects the well-being of the equine fetus is unknown. In dam, which is higher than in the previous edition of this pub- studies cited above, the amount of energy fed was less than lication (NRC, 1989). For mares with body weights above amounts recommended for pregnant mares, but was close to 700 kg, a maintenance requirement of 30.3 kcal DE/kg BW or slightly above the maintenance requirements of the (minimal maintenance) may be more appropriate. mares. The effects of long-term or more drastic energy re- The current recommendations do not allow for accretion striction on gestating mares and the development of their of maternal fat stores during gestation, and thus apply to foals has not been studied. Silver and Fowden (1982) re- mares that enter midgestation in at least moderate body con- ported that complete feed withdrawal for 12 to 30 hours dur- dition (CS > 5). Mares that have inadequate body condition ing late gestation resulted in increased uterine prostaglandin (CS < 5) in early or midgestation should be fed additional production. These authors suggested that extended interrup- energy to reach a body condition score of at least 5 by the tions in feed intake should be avoided during late gestation. 9th month of gestation. Additional DE would also be neces- sary to meet the maintenance requirements of mares kept in Lactation environmentally stressful conditions during gestation. Foal birth weight is estimated at 9.7 percent of nonpreg- Studies with lactating mares have produced variable esti- nant mare weight. Additional weight accrues from the other mates of daily milk output. Estimates in early lactation have products of conception, such as the placenta and associated ranged from 2.3 kg milk/100 kg BW (Gibbs et al., 1982) to fluids. Therefore, mares that conceive in a moderate body 3.8 kg milk/100 kg BW (Doreau et al., 1992; Martin et al., condition would be expected to increase in body weight by 1992). The differences in measurements may arise from 12 to 15 percent during gestation. In the computer program variation in breeds, diets, or method of measurement. Using that accompanies this document, the majority of the increase draft-type mares, Doreau et al. (1986) reported that daily in mare weight coincides with the majority of the increase in milk output increased from 2.6 kg/100 kg BW in week 1 to weight of the fetus that occurs primarily in the last 3 months 3.1 kg/100 kg BW in weeks 4 and 8. In a later study, milk of gestation. However, it appears possible that an increase in output of draft mares was 2.7 kg/100 kg BW in week 1 and mare body weight in the last 3 months of gestation is not 3.8 kg/100 kg BW in week 8 (Doreau et al., 1992). Using necessary if mares gain weight earlier and are in adequate Thoroughbred and Standardbred mares, Oftedal et al. (1983) body condition. Kowalski et al. (1990) found that mares reported that daily milk output was 3.1, 2.9, and 3.4 kg/100 with an average condition score of 6 did not gain weight kg BW at 11, 25, and 39 days postpartum, respectively. The during the last 90 days of gestation but produced apparently daily amounts of milk (kg/d) estimated by Oftedal et al. normal foals. Even though the mares did not gain weight (1983) were similar to amounts measured by Glade (1991) during the last 90 days, the difference in pre- and post- using Thoroughbred mares. In light horse mares (Anglo- foaling weight was 14 percent of the post-foaling weight. In Arab and Selle Francais), milk production at 4 weeks of lac- another study, mare body weight increased by 14 percent tation was 3.4 kg/100 kg BW. The lowest daily milk outputs during gestation but the majority of weight gain occurred in were reported by Gibbs et al. (1982) in Quarter horse mares, the second trimester, rather than the last trimester (Lawrence which may suggest a breed effect on milk production. How- et al., 1992a). Banach and Evans (1981) restricted the en- ever, Martin et al. (1992) reported milk outputs above 3.8 ergy intake of pregnant mares during the last 90 days, caus- kg/100 kg BW in Australian Stockhorses. Most of the previ- ing the mares to lose weight. They reported that mares de- ously mentioned studies used isotope dilution methods to livered normal weight foals (49 kg; 9.6 percent of estimate daily milk output, although Gibbs et al. (1982) and postpartum dam weight). The mean gestation length for the Glade (1991) utilized weigh-suckle-weigh techniques. In mares was 350 days. Hines et al. (1987) also restricted en- the study by Glade, foals were allowed to nurse every 2 ergy intake by mares in late gestation, resulting in loss of hours, whereas in the study by Gibbs et al. foals were al- weight and body condition. Mares fed the energy-restricted lowed to nurse every 3 hours. It is possible that less frequent diet had longer gestation lengths than mares fed energy- nursing resulted in the somewhat lower milk output esti- adequate diets. These authors did not report foal weight. mates of Gibbs et al. (1982). Henneke et al. (1984) also restricted energy of mares in late A few studies have examined the effects of dietary energy gestation, causing a reduction in body condition. Gestation and protein intake on milk output by mares. Doreau et al. length of restricted mares did not differ from the gestation (1992) found that mares fed a high-concentrate, high- length of mares fed to increase body condition during late energy diet produced about 10 percent more milk than mares gestation. Birth weight of foals from restricted mares was fed a high-forage diet that was adequate in energy. However, similar to the birth weight of foals from the mares fed to the milk produced by mares receiving the high-energy, high- gain condition during late gestation. The results of these concentrate diet was slightly lower in fat and protein than studies indicate that mares in moderate body condition can milk produced by mares receiving the high-forage diet. As a utilize body stores to support fetal development if energy is result, even though the amount of milk produced was differ- partially restricted in late gestation. Whether this situation ent between diets, the total milk energy output was similar

20 NUTRIENT REQUIREMENTS OF HORSES and foal growth was not affected by the diet of the mares. high-energy, high-concentrate diet (Doreau et al., 1992). Mares receiving the high-energy, high-concentrate diet Pagan and Hintz (1986c) reported that increased energy in- gained more weight than mares receiving the high-forage take decreased the concentration of fat and energy in milk diet. Pagan et al. (1984) studied the effect of energy restric- from pony mares. Davison et al. (1991) found that adding fat tion on lactating pony mares. Although milk production was to the diets of lactating mares increased the concentration of not measured, average daily gain of the foals was not af- fat in the milk. However, Hoffman et al. (1998) did not re- fected by either mild energy restriction or mild energy ex- port an increase in milk fat when pastured mares received a cess. Mares receiving restricted dietary energy lost weight, supplement containing 10 percent fat. It appears that the en- whereas mares receiving excess energy gained weight. Di- ergy concentration may be influenced by several factors, in- etary protein has been reported to have a small effect on cluding energy source and amount as well as stage of milk yield (Gibbs et al., 1982), but Martin et al. (1991) did lactation. not find any effect of a protein supplement on milk yield in The DE requirement for lactation was calculated as the pastured mares. sum of the DE utilized for milk production and the DE uti- A review of studies on milk production suggested that lized for maintenance. The amount of energy secreted in mares will produce approximately 3 kg milk/100 kg BW in milk each day was calculated as the product of daily milk early lactation and 2 kg milk/100 kg BW in late lactation output and the gross energy content of milk (estimated at (NRC, 1989). More recently published studies have reported 500 kcal/kg milk). To estimate daily milk production for higher levels of milk production in early lactation, and San- each month of lactation, the total milk produced per month tos et al. (2005) reported that milk production in Lusitano was calculated and a daily average was obtained. The mares was greater than 2 percent of body weight at day 120 amount of DE needed for milk production is calculated of lactation. Therefore, it is possible that milk production is using an efficiency of DE use for lactation of 60 percent somewhat higher that previously estimated (NRC, 1989). (NRC, 1978). Because the transition from early lactation to late lactation The maintenance requirement of lactating mares has pre- is probably gradual, data from available studies were sum- viously been assumed to be the same as for other adult marized to estimate milk production by day of lactation. The horses (NRC, 1989). However, several factors may con- equation to estimate milk production is: tribute to an elevated maintenance requirement in lactating mares such as increased activity associated with maternal Y = a × (d0.0953) × e(–0.0043d) (1-6) behavior and increased feed intake; therefore, the elevated maintenance estimate that was discussed in this chapter’s where Y = daily milk yield in kilograms; a = 0.0274287 × maintenance section of 36.3 kcal/kg BW was used for lac- mature weight in kilograms; and d = day of lactation. tating mares. The gross energy content of mare milk has previously The recommendations for DE intakes by lactating mares been summarized as 580 kcal/kg, 530 kcal/kg, and 500 assume that the mare enters lactation with a body condition kcal/kg, in weeks 1 to 4, 5 to 8, and 9 to 21, respectively score of at least 5 (moderate), and that she will not gain or (NRC, 1989). Martin et al. (1992) reported the mean gross lose weight during lactation. If a change in body condition energy of milk to be 480, 470, and 450 kcal/kg, at 11, 41, of the mare is desirable, then either more or less energy and 71 days of lactation. Doreau et al. (1992) found that should be fed. Small to moderate increases or decreases in gross energy declined from 560 kcal/kg milk in week 1 of energy intake of lactating mares in moderate body condition lactation to 450 kcal/kg milk in week 8. These data suggest do not appear to alter milk production. Pony mares with DE that the concentration of gross energy in milk decreases dur- intakes approximately 15 percent above or 15 percent below ing the lactation period. However, Hoffman et al. (1998) re- the estimated requirement gained or lost body weight, but ported that the concentrations of fat and lactose were higher their foals grew similarly to foals of mares consuming DE at at 4 months of lactation than at 1 month of lactation in Thor- the estimated requirement (Pagan et al., 1984). However, oughbred mares. Hoffman et al. (1998) did not report gross when energy restriction is more severe or extended, lactation energy concentration in their samples. If gross energy is es- may be affected (Banach and Evans, 1985). timated from the concentration of fat, protein, and lactose, For 200-kg mares, the current estimates for DE intakes then gross energy content was 580 kcal/kg milk at 1 and 2 are somewhat lower than previous estimates (NRC, 1989). months of lactation and 600 kcal/kg milk at 6 months of lac- Milk production for pony mares in early lactation was pre- tation. When all data were summarized, a significant rela- viously estimated at 4 percent of body weight (NRC, 1989). tionship between milk energy concentration and stage of Pagan et al. (1984) estimated the DE requirements of 200- lactation was not obtained. Dietary energy amount or source kg lactating mares at 12.9 Mcal/d, which is also lower than appears to influence the energy composition of milk. Mares the previous estimate (NRC, 1989). For mares with mature receiving an energy-adequate, high-forage diet had higher weights of 400–600 kg, the current DE recommendations concentrations of fat and energy in milk than mares fed a for early lactation are somewhat higher than previous rec-

ENERGY 21 ommendations (NRC, 1989). Lactating Thoroughbred Reproductive Efficiency mares weighing 460–475 kg maintained body weight when fed diets containing 28–31 Mcal DE/d (Glade, 1991). Simi- The energy status of broodmares can affect several com- larly, Quarter horse mares (approximately 520 kg) main- ponents of reproductive efficiency, including conception tained body weight when fed diets estimated to contain rate, length of the anovulatory period, and number of cycles 29–31 Mcal DE/d (Gibbs et al., 1982). The current estimates to conception. Henneke et al. (1984) found that mares en- for draft-type mares are markedly higher than the 1989 esti- tering the breeding season in a moderate body condition re- mates due to the higher maintenance requirement used in the quired fewer cycles for conception and had higher concep- current recommendations. In a study by Doreau et al. tion rates than mares entering the breeding season in thin (1992), draft-type mares (730 kg) consumed diets contain- condition. These researchers also reported that mares that ing approximately 71 or 47 Mcal DE/d. Mares consuming foaled in a thin body condition and remained thin during 71 Mcal/day gained 1.18 kg/d, whereas mares consuming 47 lactation had a longer interval from parturition to the second Mcal DE gained 0.18 kg/d. These data suggest that the draft postpartum estrus. The condition scoring system used by mares used approximately 24 Mcal of DE/kg of gain and these researchers is shown in Table 1-7. Other condition that the mares fed the lower energy diet would have been in scoring systems have been developed for horses, but this energy balance at a daily DE intake of 43 Mcal/day. Using system has been the most widely applied in nutrition studies the current set of assumptions (with a maintenance compo- with horses. Where condition scores are noted in this chap- nent of 36.3 kcal/kg BW) to determine DE, a 730-kg lactat- ter, they were determined using the system in Table 1-7. ing mare would require approximately 46 Mcal of DE in Kubiak et al. (1987) reported that mares entering the early lactation. If a maintenance component of 33.3 kcal/kg breeding season with a mean condition score of 5.3 ovulated BW is used, the daily DE intake is estimated at about 44 sooner than mares that entered the season at condition Mcal. Morrison (1961) suggested that lactating draft mares scores below 5. Mares with a condition score above 5 in that (730 kg) performing light work should be fed 10 to 12.5 kg study also tended to have a shorter initial estrus period. TDN or 44 to 55 Mcal DE. The conversion of TDN to DE Mares that entered the breeding season in a moderately fat assumes 1 kg TDN = 4.4 Mcal DE (NRC, 1978). When daily or fat body condition did not have impaired reproductive ef- DE intake recommendation is corrected for the additional ficiency compared to mares that entered the breeding season work, the resulting DE intake is approximately 38–47 in moderate or thin condition (Henneke et al., 1984). Gentry Mcal/d for a lactating 730-kg mare. Therefore, for lactating et al. (2002) found that open mares maintained in moder- mares weighing more than 700 kg, a daily maintenance ately fat to fat body condition (condition score 6.5 to 8) dur- component of 33.3 kcal/kg BW is suggested. ing the fall and winter often continued to cycle throughout TABLE 1-7 A Condition Scoring System for Horsesa Score Description 1 Poor Animal extremely emaciated; spinous processes, ribs, tailhead, tuber coxae, and ischii projecting prominently; bone structure of withers, shoulders, and neck easily noticeable, no fatty tissue can be felt 2 Very thin Animal emaciated; slight fat covering over base of spinous processes; transverse processes of lumbar vertebrae feel rounded; spinous processes, ribs, tailhead, tuber coxae, and ischii prominent; withers, shoulders, and neck structure faintly discernible 3 Thin Fat buildup about halfway on spinous processes; transverse processes cannot be felt; slight fat cover over ribs; spinous processes and ribs easily discernible; tailhead prominent, but individual vertebrae cannot be identified visually; tuber coxae appear rounded but easily discernible; tuber ischii not distinguishable; withers, shoulders, and neck accentuated 4 Moderately thin Slight ridge along back; faint outline of ribs discernible; tailhead prominence depends on conformation, fat can be felt around it; tuber coxae not discernible; withers, shoulders, and neck not obviously thin 5 Moderate Back is flat (no crease or ridge); ribs not visually distinguishable but easily felt; fat around tailhead beginning to feel spongy; withers appear rounded over spinous processes; shoulders and neck blend smoothly into body 6 Moderately fleshy May have slight crease down back; fat over ribs spongy; fat around tailhead soft; fat beginning to be deposited along the side of withers, behind shoulders, and along the sides of neck 7 Fleshy May have crease down back; individual ribs can be felt, but noticeable filling between ribs with fat; fat around tailhead soft; fat deposited along withers, behind shoulders, and along neck 8 Fat Crease down back; difficult to feel ribs; fat around tailhead very soft; area along withers filled with fat; area behind shoulder filled with fat; noticeable thickening of neck; fat deposited along inner thighs 9 Extremely fat Obvious crease down back; patchy fat appearing over ribs; bulging fat around tailhead, along withers, behind shoulders, and along neck; fat along inner thighs may rub together; flank filled with fat aSOURCE: Adapted from Henneke et al. (1983).

22 NUTRIENT REQUIREMENTS OF HORSES the winter. Mares in fat body condition at parturition had fected by feed restriction. However, restricted mares had a foaling characteristics similar to mares in moderate body reduced LH response to gonadotropin releasing hormone condition at parturition (Kubiak et al., 1988). In addition, (GnRH) and a reduced prolactin response to thyroid releas- mares in fat and moderate body condition had similar inter- ing hormone (TRH). Leptin concentrations decreased in vals from parturition to foal heat ovulation, and from partu- both groups with time, but were lower in mares receiving the rition to the second postpartum ovulation (Cavinder et al., restricted diet. Although it is possible that changes in leptin 2005). are involved in the mechanism that allows diet to affect re- Sudden or chronic energy restriction, as well as energy productive efficiency, a clear relationship has not been supplementation of broodmares, may also affect reproduc- demonstrated at this time. tive efficiency. When mares that entered the fall in a moder- A number of studies have demonstrated that energy re- ately fat or fat body condition were fed energy-restricted striction or low body condition can negatively impact repro- diets, all mares became anovulatory within 12 weeks of the ductive efficiency in mares. Therefore, it is recommended onset of restriction and remained anovulatory for an ex- that mares used for breeding be maintained at a condition tended period (Gentry et al., 2002). Similarly, Kubiak et al. score of at least 5. Maintaining broodmares at a high condi- (1987) found that mares that entered the breeding season in tion score (condition scores of 7 or 8) does not impair or im- a thin body condition and remained thin had an extended prove reproductive efficiency (Cavinder et al., 2005). Mares anovulatory period. In some instances, the reproductive effi- maintained in situations that elevate energy requirements ciency of mares that began the breeding season in a thin (such as cold) should be fed sufficient energy to meet the in- condition was improved when sufficient energy was sup- creased needs for maintenance, or they should have suffi- plied to increase body weight and condition (Henneke et al., cient body stores to meet energy needs without decreasing 1984; Kubiak et al., 1987). However, in a field study con- condition score below 5. Similarly, mares that have diffi- ducted by Henneke et al. (1984), mares that entered the culty maintaining body condition during late gestation or breeding season in thin condition and gained weight during lactation should be fed to accrete sufficient body stores to the breeding season remained less reproductively efficient prevent a decline in body condition below 5 when energy than mares that entered the breeding season at condition needs increase. scores above 5. Many studies have been conducted to investigate the EXERCISE mechanism that governs the interaction between energy sta- tus and reproductive efficiency in mares. Initial attention Oxygen Utilization During Exercise focused on the effects of energy restriction on luteinizing hormone (LH) responses in mares (Hines et al., 1987). Sub- The amount of energy utilized during exercise must be sequent studies have examined the relationships among known in order to estimate the DE requirements of exercis- short-term feed deprivation or long-term feed restriction on ing horses. The amount of energy utilized during exercise growth hormone (GH), IGF-I (insulin-like growth factor I), depends upon the duration and the intensity of the exercise. thyroid hormones, prolactin, insulin, and leptin. Energy re- The duration of an exercise bout is relatively easy to mea- striction appears to result in a rapid decrease in IGF-I con- sure, but the intensity of exercise is much more difficult to centrations in nonpregnant mares, but growth hormone and characterize. Factors that influence intensity include the thyroid hormone responses were less affected (Sticker et al., speed of travel, ground resistance (moving on sand versus a 1995). McManus and Fitzgerald (2000) reported that leptin flat surface), and the incline of the terrain. Other factors ger- concentrations decreased in mares subjected to 24 hours of mane to equine events include number and height of jump- feed deprivation, while serum concentrations of follicle ing efforts, performance of extended and collected gaits, and stimulating hormone (FSH), prolactin, and LH were not af- the amount of weight carried or pulled. fected. Similarly, Buff et al. (2005) reported that 48 hours of Oxygen consumption during exercise is often measured feed deprivation resulted in decreased leptin concentrations. as a means of estimating energy expenditure during work. Gentry et al. (2002) compared the hormonal responses of Several studies have investigated the relationship between mares receiving a nutritionally adequate diet and a restricted oxygen consumption and speed of travel of horses. Hiraga et diet during the fall and winter. Mares in the adequately fed al. (1995) reported that oxygen utilization was linearly re- group maintained condition scores of 6.5 to 8, whereas lated to speed for horses running on a level treadmill. Eaton mares in the restricted group began the study with condition (1994) has also reported a linear relationship between speed scores of 6.5 to 8 but eventually reached condition scores of and oxygen. Subsequently, Eaton et al. (1995b) reported a 3 to 3.5. Most of the mares in the adequately fed group con- nonlinear relationship between speed and oxygen utilization tinued to cycle during the fall and winter, but all mares in the for horses exercising on a level treadmill. Similarly Pagan restricted group entered an anovulatory state. Basal concen- and Hintz (1986b) found a nonlinear relationship between trations of LH, FSH, thyroid stimulating hormone (TSH), energy expended (calculated from oxygen consumption) and GH, or insulin that were measured at each week were not af- speed when horses exercised on a level track. The equations

ENERGY 23 of Eaton et al. (1995b) and Pagan and Hintz (1986b) yield galloping on a 10 percent incline has been observed to be similar values for horses exercising at 300 meters/minute approximately 100–150 percent higher than when horses (m/min). However, at speeds above 400 m/min, the equa- gallop on a flat surface (Eaton et al., 1995b; Hiraga et al., tions produce very different estimates of energy use. As the 1995). When horses walked or trotted on a dry treadmill or measurements made by Eaton et al. included a wider range on a treadmill in water at the depth of the horse’s fetlock or of speeds, it is likely that their equation provides a better es- elbow, heart rate was increased (Voss et al., 2002). Although timate of oxygen use than the equation of Pagan and Hintz oxygen utilization was not measured, the increase in heart (1986b) at speeds above 400 m/min. However, the equation rate suggests that oxygen utilization is increased by other from Eaton et al. (1995b) probably overestimates oxygen factors that increase resistance to movement. Changing the utilization at slow speeds (walking and slow trotting). amount of weight carried will change the amount of oxygen Although it seems reasonable that an equation relating utilized (Thornton et al., 1987). Therefore, if estimates of speed of travel to oxygen use could be used to estimate the oxygen utilization are used to estimate energy requirements energy requirements of exercising horses, there are several in horses they should take into account the amount of weight complicating factors. The equation of Eaton et al. (1995b) carried (i.e., the rider). was derived from horses running on a treadmill, rather than Measurements of oxygen utilization made during exer- on a typical track or arena surface. When trotters were tested cise probably account for most of the energy utilized. How- on a treadmill and on a track, heart rate at a given speed was ever, as exercise becomes more intense, the anaerobic com- lower on the treadmill, suggesting that exercise on a tread- ponent of energy expenditure will increase. Although the mill may not precisely replicate exercise on a track, even contribution of anaerobic metabolism to total energy utiliza- though speed is the same (Couroucé et al., 1999). Compared tion is relatively small at light to moderate workloads, it may to horses on the track, horses on the treadmill did not pull a be as high as 30 percent during maximal exercise (Eaton et sulky, which probably accounted for most of the differences al., 1995a). observed. Other differences between the treadmill and the track, such as ground reaction forces, wind resistance, and Energy Requirements of Exercising Horses psychological factors, were also suggested. In addition, it is possible that biomechanical factors or conformational fac- The daily energy requirements of an equine athlete may tors could alter the relationship between speed and oxygen be partitioned into at least two compartments, which include utilization. In a study comparing oxygen utilization of small the amount of energy required for the daily exercise effort ponies (171 kg), medium ponies (319 kg), and Thorough- and the amount of energy required for maintenance. A third breds (487 kg), ponies reached 40, 60, 80, and 100 percent compartment may be the amount of energy required for all of maximum oxygen consumption at slower speeds than the other activities associated with a performance horse’s daily Thoroughbreds (Katz et al., 2000). Hoyt and Taylor (1981) life (such as transportation to and from events). This com- demonstrated that the gait of the horse affected oxygen uti- partment could be accounted for separately, or it could be lization at a given speed. Their data indicated that horses ap- added to the maintenance or exercise components, but it pear to select gaits to maximize energy efficiency, and that should be considered. Doherty et al. (1997) have suggested shortening or lengthening of a stride may be energetically that energy use during transportation in a trailer may be sim- quite expensive. Eaton et al. (1995b) concluded that the ilar to the amount of energy used during walking. Therefore, most efficient speed for Thoroughbred horses exercising on several hours of transportation a week could add signifi- a level or an inclined treadmill was 3 to 6 meters/second cantly to total energy requirements. (m/s), and horses generally changed from a trot to a canter As noted previously, the amount of energy utilized dur- at 5 to 6 m/s. Other factors influencing oxygen utilization at ing any exercise bout will depend on factors such as incline, a given speed include level of warm-up (Tyler et al, 1996; speed, and ground resistance. It is relatively simple to cal- McCutcheon et al., 1999), conditioning (Eaton et al., 1999; culate the energy utilized by an unloaded horse trotting or Katz et al., 2000), and possibly body composition (Kearns et cantering on a level treadmill, but it is much more difficult al., 2002a). In summary, it is clear that it may be difficult to to calculate the energy utilized by a loaded (ridden) horse precisely predict oxygen utilization from speed of travel for trotting and cantering up and down hills or jumping obsta- a specific individual. Nonetheless, within an individual, cles. Attempts to convert theoretical estimates of oxygen oxygen utilization, and thus energy use, usually increases consumption to practical estimates of dietary energy need with speed. are fraught with pitfalls. Factors that can potentially affect Oxygen utilization is increased by 25–35 percent when the reliability of the process include differences among horses exercise on a slight incline compared to a flat surface horses, level of training, type of exercise, rider weight and (Eaton et al., 1995b). At steeper inclines, the effect on oxy- experience, climate, and ground conditions. An excellent re- gen use is much greater. At a slope of 6.25 percent, oxygen view of the energy costs associated with a 3-day event has consumption increased 76 percent in Standardbreds at the been published (Jones and Carlson, 1995), and models to es- trot (Thornton et al., 1987). Oxygen utilization of horses timate uphill and downhill running have also been published

24 NUTRIENT REQUIREMENTS OF HORSES (Schroter and Marlin, 2002). However, estimates of energy where HR = heart rate in beats/min. (Note: this is not the utilization during many equine events are not available. Per- equation published in the original reference, which had a ty- haps more importantly, the average daily energy require- pographical error). This equation (Coenen, 2005) provides a ment of an individual horse is more closely related to the ac- better estimate of oxygen utilization at lower heart rates than tivities performed during training than the energy expended the equation of Eaton et al. (1995b). in a single event. Table 1-8 shows the estimated oxygen utilization and en- There is a strong relationship between heart rate and oxy- ergy utilization at various heart rates for a 500-kg horse with gen utilization (Eaton et al., 1995b; Coenen, 2005); there- a 50-kg rider. By using heart rate, rather than speed alone, a fore, it may be possible to estimate energy expenditure from more comprehensive estimate of work intensity is provided. heart rate (Coenen, 2005), particularly at submaximal exer- Also, heart rate can be used to estimate oxygen consumption cise intensities. Oxygen utilization is related more closely to for some activities where speed might be irrelevant (such as percentage of maximal heart rate than to actual heart rate. cutting), or where the relationship between speed and oxy- The maximum heart rate of an individual horse may vary gen consumption might not be known, such as for horses with age and breed, and, therefore, to estimate the percentage performing specialized gaits. The values in Table 1-8 have of heart rate maximum achieved during a specific work bout, not been corrected for resting oxygen utilization, and there- it is necessary to know the maximal heart rate of an individ- fore slightly overestimate the energy cost of exercise above ual horse. However, it is difficult for many equestrians to im- maintenance. This overestimation would be of minor impor- pose a workload sufficient to elicit maximum heart rate. It is tance to horses exercising for short periods of time, but more practical to use heart rate as a guide to oxygen con- could be of significant importance when calculating the en- sumption than percentage of heart rate maximum achieved ergy requirements of horses exercising for extended periods. during various exercise bouts. At the very least, heart rate can In addition, at high heart rates, the energy expenditure cal- be used to assess the relative intensity of a work bout for a culated from oxygen utilization will underestimate the true given horse. The availability of on-board heart rate monitors energy utilization because oxygen utilization does not ac- for horses makes it possible to determine heart rate during count for the anaerobic energy component. the various stages of an exercise bout and even calculate a In order for heart rate measurements to be a useful indi- mean heart rate that could be used to estimate mean energy cator of exercise intensity, the heart rate must be determined expenditure. Eaton et al. (1995b) derived the following equa- during exercise and not after exercise or at a rest break. tion to relate oxygen utilization to heart rate: Heart rate decreases rapidly (within a few seconds) after most exercise, and thus post-exercise heart rate is not a good Oxygen Utilization (ml O2/kg BW/min) = indicator of exercising heart rate. Heart rate measurements 0.833 × (HR) – 54.7 (R2 = 0.865) made when a horse is excited due to new surroundings or another situation may overestimate the effect of the actual where HR = heart rate in beats/min. This equation produces work performed. As it is not always possible to measure reasonable estimates of oxygen utilization at high heart rates heart rate under practical conditions, a brief review of se- but not at low heart rates. Coenen (2005) summarized oxy- lected studies that have measured heart rates in horses dur- gen consumption data from 87 studies and developed the ing various activities is provided. Conditioned Arabian following equation: horses trotting on a level treadmill at 3.6 m/s had heart rates of 90 to 100 beats/min (Bullimore et al., 2000), whereas Oxygen Utilization (ml O2/kg BW/min) = conditioned Thoroughbred horses exercising at 6 and 8.5 0.0019 × (HR)2.0653 (R2 = 0.9) m/sec on a level treadmill had heart rates of 115 and 145 beats/min, respectively (Danielsen et al., 1995). Older unfit TABLE 1-8 Estimated Oxygen Consumption and Net mares had heart rates of 120 to 140 during free lunging in a Energy Utilization of a 500-kg Horse Ridden by a 50-kg round pen at a trot (Powell et al., 2002). (Ridgway) (1994) Rider at Various Heart Rates suggests that unfit horses ridden at a working trot (160 to 210 m/min) will have heart rates of 120 to 150 beats/min, Oxygen Oxygen Energy whereas conditioned horses performing the same work will Heart Rate Utilizationa Utilizationb Utilizationc (beats/min) (ml O2/kg BW/min) (liters/min) (kcal/min) have heart rates of 70 to 110 beats/min. The heart rates of French trotters on a sand training track were approximately 60 9 5.0 24 180 and 190 beats/min at 490 and 560 m/min, respectively 90 21 11.6 56 120 37 20.4 99 (Couroucé et al., 1999). Horses performing reining-type ex- 150 59 32.5 158 ercise have heart rates between 160 and 180 beats/min 180 86 47.3 230 (Howard et al., 2003), and horses performing cutting-type aWhere oxygen utilization (ml/kg/min) = 0.0019 × HR2.0653. exercise have been reported to have heart rates of 200 bFora 500-kg horse with a 50-kg rider (550-kg total weight). beats/min (Webb et al., 1987). Peak heart rates in 3-day cWhere 1 liter of oxygen utilized is equivalent to 4.86 kcal. eventing horses exercising on the beach at 450–500 m/min

ENERGY 25 have been reported to range from 126–151 beats/min, types of work. In addition, there are probably individual dif- whereas heart rates in a “paddock canter” at 350–400 m/min ferences among horses, as well as differences due to feed in- were 127–141 beats/min, and the peak heart rate of one take and diet composition that should be characterized. How- horse performing gallops on a hill was 205 beats/min (Ser- ever, in the absence of more specific data, it is estimated that rano et al., 2002). The peak heart rates represented the most the efficiency of DE use for exercise is 30 percent for horses strenuous portion of a workout. Heart rate in elite show engaged in strenuous activities and 40 percent or higher for jumpers is elevated prior to entering the arena and may in- horses engaged in moderate or mild exercise. crease to 200 beats/min by the end of the round (Clayton, The cost of exercise must be added to the cost of mainte- 1994). Racing (flat or harness) results in maximal heart rates nance to determine the daily DE requirement of perfor- (> 210 beats/min). mance horses. Maintenance estimates have usually been Although energy expenditure can be estimated on a daily made using sedentary animals. It is possible that athletic basis, it is more practical to evaluate energy expenditure horses have a somewhat higher maintenance requirement over a longer period in order to determine a mean daily en- than a sedentary horse. One factor that could influence the ergy expenditure. Table 1-9 shows a hypothetical example of maintenance requirement of a performance horse is total the estimated weekly energy expenditure of a horse utilized feed intake. Several studies have reported that the mean for recreational riding. daily dry matter intake of race horses exceeds 2.2 percent of The mean daily energy expenditure in Table 1-9 of 1.5 body weight (Zmija et al., 1991; Gallagher et al., 1992a,b). Mcal/day represents the NE used during exercise. In order to A horse’s response to its stabling environment may also af- estimate daily DE requirements, the efficiency of conversion fect maintenance requirements. A survey of Thoroughbred, of DE to NE during exercise must be known. Pagan and Standardbred, and pleasure horse stables found that approx- Hintz (1986b) originally estimated the efficiency of DE use imately 12 percent of horses exhibited some type of com- for exercise at 57 percent. However, much lower efficiencies pulsive behavior such as cribbing, weaving, or stall-kicking have been suggested (Harris, 1997; Pagan et al., 2005a). A (Luescher et al., 1998). The performance of a compulsive number of studies (Webb et al., 1987; Freeman et al., 1988; behavior could increase the maintenance requirement. Other Bullimore et al., 2000; Graham-Thiers et al., 2000) that re- stabling factors that have not been assessed but could influ- ported body weight, weekly exercise program, and either ence maintenance requirements include amount of turnout, feed intake or DE intake were reviewed to examine the effi- activity in the stable, and even comfort associated with stall ciency of DE use for exercise. When maintenance is esti- size, ventilation, temperature, or bedding. mated at 33.3 kcal/kg BW, the estimated efficiency of DE use Most researchers who have measured oxygen utilization for exercise ranged from approximately 20–50 percent. during work have not accounted for the energy costs of an el- Based on estimates of feed intake and typical training pro- evated post-exercise metabolic rate (or of walking during a grams, the efficiency of DE use for exercise by race horses recovery period) in their calculations of energy use. For 1 would be 20 percent or lower. If the elevated maintenance es- hour after a short-term, moderately intense exercise bout, timate is used in the equations (36.3 kcal/kg BW), somewhat oxygen utilization was elevated by about 11 percent above higher efficiencies of DE use for exercise can be calculated, the pre-exercise value (Dunn et al., 1991). Body tempera- but for most studies, the values would still be below 50 per- ture, heart rate, and respiration will remain elevated after ex- cent. Additional research is needed to determine the effi- ercise in horses for various lengths of time, depending upon ciency of DE use for exercise in horses performing different the fitness of the horse, the type of exercise performed, envi- TABLE 1-9 Hypothetical Weekly Net Energy Expenditure (above maintenance) of a 500-kg Horse Used for Recreational Riding Average HR Energy Utilizationa Day of Week Type of Exercise during Exercise for Exercise (Mcal/d) Monday 45 min riding in arenab 100 3.1 Tuesday 45 min on trail (mostly walking) 60 1.1 Wednesday No exercise 0 0 Thursday 45 min riding in arena 100 3.1 Friday No exercise 0 0 Saturday 2 hours trail riding (mostly walking) 60 2.9 Sunday No exercise 0 0 Mean daily energy 1.5 Mcal/d expenditure above maintenance aFor calculations see Table 1-8. bIncludes walk, trot, and canter.

26 NUTRIENT REQUIREMENTS OF HORSES ronmental conditions, and other factors. A post-exercise ele- Heavy work: vation in metabolic rate could increase the maintenance re- DE (Mcal/d) = (0.0333 × BW) × 1.60 (1-7c) quirement. It is possible that the average maintenance com- ponent of 33.3 kcal/kg BW/d is appropriate for some Very heavy work: performance horses, but that the elevated maintenance re- DE (Mcal/d) = (0.0363 × BW) × 1.9 (1-7d) quirement discussed in the maintenance section (36.3 kcal/ kg BW/d) is appropriate for others, particularly if horses are The assignment of the DE increments above maintenance transported long distances to competitions. (20, 40, 60, and 90 percent) to the words “light,” “moder- Given the discussion and equations reported above, it is ate,” “heavy,” and “very heavy” are arbitrary. Other cate- possible to calculate an estimate of DE requirements for an gories and descriptors could be used instead. Table 1-10 individual horse, based on body weight, weight of rider, and provides estimates of weekly workloads for each category. individual work program. However, as this approach is not The estimated workloads should be used only as a guide, as practical for every situation, general categories of exercising there are many combinations of work intensity and work du- horses have been devised. The previous edition of this doc- ration that could not be included in a single table. In addi- ument used three categories of work (light, moderate, and tion, the heart rates that are associated with each category in intense). Other publications have listed as many as seven the table should not be used to define that category. The categories (Lewis, 1995). Four categories of work are sug- heart rates given in each category are consistent with the gested here: light, moderate, heavy, and very heavy. The DE amount of work performed over the duration specified. requirements for horses in these categories are calculated These suggested DE intakes (Table 1-10) appear to be using the following equations: consistent with estimates from several studies. Several studies have reported that horses in race training (450–500 Light work: kg) have DE intakes above 30 Mcal/day (Zmija et al., 1991; DE (Mcal/d) = (0.0333 × BW) × 1.20 (1-7a) Gallagher et al., 1992a,b; Southwood et al., 1993). Mc- Gowan et al. (2002) reported that polo ponies were fed ap- Moderate work: proximately 11 kg of hay and 2 kg of concentrate per day, DE (Mcal/d) = (0.0333 × BW) × 1.40 (1-7b) which would result in a DE intake of about 26 Mcal/day, TABLE 1-10 Example Weekly Workloads of Horses in the Light, Moderate, Heavy, and Very Heavy Exercise Categories Exercise Category Mean Heart Ratea Descriptionb Types of Eventsc Light 80 beats/min 1–3 hours per week; 40% walk, 50% trot, 10% canter Recreational riding Beginning of training programs Show horses (occasional) Moderate 90 beats/min 3–5 hours per week; 30% walk, 55% trot, 10% canter, School horses 5% low jumping, cutting, other skill work Recreational riding Beginning of training/breaking Show horses (frequent) Polo Ranch work Heavy 110 beats/min 4–5 hours per week; 20% walk, 50% trot, Ranch work 15% canter, 15% gallop, jumping, other skill work Polo Show horses (frequent, strenuous events) Low-medium level eventing Race training (middle stages) Very Heavy 110–150 beats/min Various; ranges from 1 hour per week speed work to Racing (Quarter horse, Thoroughbred, Standard- 6–12 hours per week slow work bred, Endurance) Elite 3-day event aMean heart rate over the entire exercise bout. bThese are general descriptions based on weekly totals of work and do not include all combinations of work intensities and duration. The hours of work per- formed per week in any particular category could be much more than the estimate given, if the work intensity was much lower. For example, horses in the light category could be exercised for more than 3 hours per week if the work intensity was much lower (see Table 1-9); and horses in the moderate category could be exercised for more than 5 hours per week if the work intensity were lower than the mean heart rate given. cFor additional discussion of the sources of variation in energy requirements, see explanation in the text.

ENERGY 27 which is similar to a previous estimate for 475-kg polo house and Burrill, 1998). In that study, condition scores of ponies (Hintz et al., 1971). Crandell (2002) reported that 4, 4.5, or 5 were most frequent, and no horses with a body endurance horses are fed approximately 24 Mcal DE/day, condition score below 3 completed the race. Mean condition but noted that many horses receive pasture that could not be score for endurance horses had previously been reported to quantified. For a 425-kg endurance horse, 24 Mcal/day be 4.67 (Lawrence et al., 1992b). Some studies have shown would exceed maintenance requirements by 57–73 percent no disadvantages to very high condition scores in brood- (using a maintenance requirement of 36.3 or 33.3 kcal/kg mares (Henneke et al., 1983; Kubiak et al., 1989; Cavinder BW, respectively). The DE requirements for equitation et al., 2005). However, Hoffman et al. (2003) suggested that horses were estimated at about 140 percent of maintenance very fat horses may have disturbed metabolic and endocrine (Hintz et al., 1971). Webb et al. (1987) reported that cutting regulation, although specific consequences to animal health horses (470 kg) in a lean body condition consumed about were not reported. Therefore, at this time the optimal body 19 Mcal/day, which would be about 25 percent above condition for a horse is not known. maintenance and much lower than the estimate suggested When horses have body condition scores outside the de- here. However, it was not clear whether these horses were sired range, energy balance may be manipulated to produce actively engaged in competition; therefore, cutting horses weight loss or weight gain. Although condition score is should probably be included in the moderate category. often reported in equine nutrition studies, the amount of Although the guidelines above should be helpful in de- weight loss or gain necessary to achieve a change in body termining whether an individual horse should be classified condition score has not been well studied. Heusner (1993) in the light, moderate, heavy, or very heavy work category, found that weight gains of 33 to 45 kg were associated with it is likely these categories will not apply to all horses. There an increase in condition score of approximately two units are numerous factors that could potentially affect the energy (from 4 to 6) in mature horses (approximately 480–580 kg). needed by a performance horse including level of fitness, Consequently, it appears that each unit of condition score in- skill, and weight of the rider, climate conditions, breed, and crease requires about 16–20 kg of weight gain. The amount housing conditions. Therefore, these recommendations of weight needed to change condition score by one unit will should be applied in combination with other criteria such as vary with the mature weight of the horse. body condition, performance, and maintenance of body To alter energy balance, either energy expenditure or en- weight. ergy intake can be manipulated. The most common means of increasing energy expenditure is to increase activity, usually by imposing a regular exercise program. Energy expenditure EFFECTS OF ENERGY DEFICIENCY AND EXCESS may be decreased by reducing the amount of regular exer- In most cases, energy deficiencies and excesses are eas- cise. For horses that are not receiving forced exercise, en- ily identified by weight loss or weight gain. Because it is ergy expenditure may be reduced by limiting activity or by difficult for many horse owners to regularly weigh horses, reducing environmental stresses impacting the horse (i.e., use of a body condition scoring system may help determine providing shelter during cold wet weather). whether a horse is gaining or losing weight. A number of When it is not desirable or practical to alter energy ex- condition scoring systems have been developed for horses penditure, energy intake may be manipulated. The relation- and even for donkeys (see Chapter 13). Some systems have ships among energy intake, change in body weight, and used a 9- or 10-point scale, whereas others have used a change in body condition score have not been well described 5-point scale. Regardless of the details of any one method, in horses. One recommendation suggested that a change in to be most useful the system must be applied consistently condition score could be achieved if DE intake were in- over time. A body condition scoring system for horses that creased or decreased 10–15 percent above or below mainte- has been widely used for many years (Henneke et al., 1983) nance (NRC, 1989). Recent information suggests that this is shown in Table 1-7. This system scores horses on fat cover recommendation may not be realistic for many situations in at several locations on the body, with a low score of 1 being which weight gain is desired. The amount of DE required applied to an extremely thin horse and a high score of 9 per kilogram of gain will depend on several factors includ- being applied to a very fat horse. A condition score of 5 is ing composition of gain and composition of diet. The considered moderate. Condition scoring has been used to as- amount of DE required per kilogram of gain typically in- sess the condition of horses in various physiological states. creases with maturity. In a study by Heusner (1993), mature As mentioned previously, reproductive efficiency is opti- horses required approximately 24 Mcal of DE (above main- mized with a body condition score of at least 5. Similarly, tenance) per kilogram of gain. Martin-Rosset and Vermorel the body condition score of select Thoroughbred yearlings (1991) estimated that 0.99 Mcal ME/d was required above at public auction was between 5 and 6. For performance maintenance for mature Standardbred geldings consuming a horses, lower body condition scores may be appropriate. mixed diet to gain 5.6 kg over a 3-month period. This Body condition scores for endurance horses completing a amount of ME is approximately equivalent to 18 Mcal race have been reported to range from 3 to 5.5 (Garling- DE/kg gain. Using two different methods of determining

28 NUTRIENT REQUIREMENTS OF HORSES DE intake in Belgian and Percheron horses, Potter et al. perlipidemina that may progress to the clinical syndrome of (1987) suggested that 16–20.7 Mcal of DE were needed per equine hyperlipemia. In horses, hyperlipidemia is generally kg (gain). defined as serum triglyceride concentration in excess of 500 It seems reasonable to estimate that 16–24 Mcal of DE mg/dl, whereas hyperlipemia is characterized by lipemia (a are required per kilogram of gain for mature horses, based milky discoloration of the plasma), as well as hypertriglyc- on the results of the previously mentioned studies. The eridemia. In the clinical syndrome of equine hyperlipemia, amount of energy needed per unit of gain will be influenced affected animals have hyperlipemia as well as signs of he- by the composition of gain. The composition of gain could patic and/or renal failure. Severe liver and kidney damage be influenced by the body composition of the animal as well occurs due to excessive accumulation of lipid, and death rate as other factors. The efficiency of DE conversion to NE is high (Watson and Love, 1994). The onset of hyperlipemia varies among energy sources; therefore, less DE may be usually follows a period of negative energy balance that can needed per unit of gain when a high-fat diet is offered than arise from feed restriction, decreased feed intake due to dis- when a high-fiber diet is fed. Using the observations dis- ease, or increased energy demand due to pregnancy or lacta- cussed above, it is possible to develop an approximate esti- tion. Watson (1998) has proposed that hyperlipemia results mate of the amount of DE required above maintenance to in- from abnormal regulation of hormone sensitive lipase crease the condition score of a horse one unit. Table 1-11 (HSL), the enzyme responsible for stimulating lipolysis in shows the estimated amount of DE above maintenance that adipose. When HSL is stimulated during negative energy would be needed to change the condition score of a 500-kg balance or a stressful event in susceptible animals, excessive horse from a score of 4 to a score of 5 over different periods amounts of free fatty acids are released from adipose. These of time. These estimates have been derived from limited free fatty acids are taken up by the liver, re-esterified into data and have not been tested under controlled conditions. triglycerides, and then released into the circulation or re- Therefore, the estimates in Table 1-11 may under- or over- tained in the liver, eventually causing liver and kidney dam- estimate the amount of DE needed to change the condition age (Watson, 1998). Mares (particularly late in gestation or score of a horse, particularly when applied to animals at the early in lactation) appear to be more susceptible to equine extreme ranges of the condition scoring range. hyperlipemia than geldings or stallions, and obesity may As noted above, deprivation of calories will result in also increase risk (Jeffcott and Field, 1985; Watson et al., weight loss. The composition of weight loss in horses has 1992). As noted above, ponies and donkeys are more likely not been investigated, although it is likely that thin individ- to be affected than horses. For animals in high-risk cate- uals lose lean body mass as well as fat mass when calories gories, it may be prudent to avoid periods of extended or se- are restricted. Chronic calorie restriction resulting in severe vere energy restriction. Unfortunately, there do not appear to weight loss can result in starvation. 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ENERGY 33 Southwood, L. L., D. L. Evans, W. L. Bryden, and R. J. Rose. 1993. Nutri- Pp. 107–112 in Proc. 18th Equine Nutr. Physiol. Soc. Symp., East Lans- ent intake of horses in Thoroughbred and Standardbred stables. Austral. ing, MI. Vet. J. 70:164–168. Voss, B., E. Mohr, and H. Krzywanek. 2002. Effects of aqua-treadmill ex- Staniar, W. B., D. S. Kronfeld, K. H. Treiber, R. K. Splan, and P. A. Harris. ercise on selected blood parameters and on heart rate variability of 2004. Growth rate consists of baseline and systematic deviation com- horses. J. Vet. Med. 49A:137–143. ponents in Thoroughbreds. J. Anim. Sci. 82:1007–1015. Watson, T. 1998. Equine hyperlipaemia. Pp. 23–40 in Metabolic and En- Sticker, L. S., D. L. Thompson, Jr., J. M. Fernandez, L. D. Bunting, and C. docrine Problems of the Horse, T. Watson, ed. London: Harcourt Brace L. Depew. 1995. Dietary protein and(or) energy restriction in mares: and Co. plasma growth hormone, IGF-I, prolactin, cortisol, and thyroid hor- Watson, T. D. G., and S. Love. 1994. Equine hyperlipidemia. Compend. mone responses to feeding, glucose, and epinephrine. J. Anim. Sci. Continuing Education 16:89–98. 73:1424–1432. Watson, T., D. Murphy, and S. Love. 1992. Equine hyperlipaemia in the Stillions, M. C., and W. E. Nelson. 1972. Digestible energy during mainte- United Kingdom. Vet. Rec. 131:48–51. nance of the light horse. J. Anim. Sci. 34:981–982. Webb, S. P., G. D. Potter, and J. W. Evans. 1987. Physiologic and metabolic Thompson, K. N. 1995. Skeletal growth rates of weanling and yearling response of race and cutting horses to added dietary fat. P. 115 in Proc. Thoroughbred horses. J. Anim Sci. 73:2513–2517. 10th Equine Nutr. Physiol. Soc. Symp., Fort Collins, CO. Thornton, J., J. Pagan, and S. Persson. 1987. The oxygen cost of weight Webb, G. W., S. P. Webb, and D. K. Hansen. 1991. Digestibility of wheat loading and inclined treadmill exercise in the horse. Pp 206–215 in straw or ammoniated wheat straw in equine diets. Pp. 261–262 in Proc. Equine Exercise Physiology 2, J. R. Gillespie and N. E. Robinson, eds. 12th Equine Nutr. Physiol. Soc. Symp., Calgary, Alberta. Davis, CA: ICEEP Publ. Westervelt, R. G., J. R. Stouffer, and H. F. Hintz. 1976. Estimating fatness Tyler, C. M., D. R. Hodgson, and R. J. Rose. 1996. Effect of warm-up on in horses and ponies. J. Anim. Sci. 43:781–785. energy supply during high intensity exercise in horses. Equine Vet. J. Whitehead, A. E., R. Foster, and T. Chenier. 2004. Placental characteristics 28:117–120. of Standardbred mares. Pp. 71–75 in Proceedings of the Workshop on Vermorel, M., and W. Martin-Rosset. 1997. Concepts, scientific bases, the Equine Placenta, University of Kentucky, Lexington, KY. structure and validation of the French horse net energy system (UFC). Williams, C. A., D. S. Kronfeld, W. B. Staniar, and P. A. Harris. 2001. Livest. Prod. Sci. 47:261–275. Plasma glucose and insulin responses of Thoroughbred mares fed a Vermorel, M., W. Martin-Rosset, and J. Vernet. 1991. Energy utilization of meal high in starch and sugar or fat and fiber. J. Anim. Sci. 79: two diets for maintenance by horses; agreement with the new French 2196–2200. net energy system. J. Equine Vet. Sci. 11:33–35. Wooden G. R., K. L. Knox, and C. L. Wild. 1970. Energy metabolism in Vermorel, M., W. Martin-Rosset, and J. Vernet. 1997a. Energy utilisation of light horses. J. Anim. Sci. 30:544–548. twelve forage or mixed diets for maintenance by sport horses. Livest. Zeyner, A., and E. Kienzle. 2002. A method to estimate digestible energy Prod. Sci. 47:157–167. in horse feed. J. Nutr. 132:1771S–1773S. Vermorel, M., J. Vernet, and W. Martin-Rosset. 1997b. Digestive and en- Zmija, G., H. Meyer, and E. Kienzle. 1991. Feeds and feeding in German ergy utilisation of two diets by ponies and horses. Livest. Prod. Sci. training stables of race horses. Pp. 85–90 in Proc. 12th Equine Nutr. 51:13–19. Physiol. Soc. Symp., Calgary, Alberta. Vernet, J., M. Vermorel, and W. Martin-Rosset. 1995. Energy cost of eating long hay, straw and pelleted food in sport horses. Anim. Sci. 61: 581–588. Vervuert, I., M. Coenen, A. Borchers, M. Granel, S.Winkelsett, L. Christ- mann, O. Distl, E. Bruns, and B. Hertsch. 2003. Growth rates and the incidence of osteochondrotic lesions in Hanovarian warmblood foals.

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Proper formulation of diets for horses depends on adequate knowledge of their nutrient requirements. These requirements depend on the breed and age of the horse and whether it is exercising, pregnant, or lactating.

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

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

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