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4 Proteins and Amino Acids INTRODUCTION circulating pool of amino acids in the blood. Nonprotein ni- trogen (NPN) sources, such as free amino acids and urea, Protein is a major component of most tissues in the body, are also absorbed in the small intestine. Protein and NPN es- second only to water. All tissues in the body are made of caping digestion in the foregut enter the hindgut where they protein along with enzymes, hormones, and antibodies. Pro- are available for degradation and the synthesis of microbial tein is made of chains of amino acids. Twenty so-called “pri- protein. Studies have reported an increase in plasma ammo- mary” amino acids make up most proteins. The types of nia concentrations when protein sources were infused di- amino acids incorporated into a protein chain as well as the rectly into the cecum (Reitnour and Salsbury, 1975), sug- length of the protein chain differentiate one protein from an- gesting that ammonia is the main nitrogen product absorbed other. Therefore, the horse’s requirement is actually for from the hindgut. There is no evidence that amino acids amino acids. Individual amino acid requirements (with the from microbial protein synthesis are absorbed in sufficient exception of lysine) have not been established for the horse. quantities to significantly contribute to the amino acid pool As a nonruminant species, there are 10 presumed essential for the horse. Cecal administration of 75 g of lysine did not amino acids for the horse: arginine, histidine, isoleucine, increase plasma lysine in horses, in contrast to a dramatic in- leucine, lysine, methionine, phenylalanine, threonine, tryp- crease in plasma lysine with a gastric dose of lysine tophan, and valine (NRC, 1998). These are amino acids that (Wysocki and Baker, 1975). If the hindgut were of signifi- cannot be synthesized by the body in sufficient quantities to cant importance in protein digestion, it would be expected meet the demand for them. All the necessary amino acids re- that cecal infusion of protein sources would result in im- quired for a protein to be made must be present at the same proved nitrogen balance, but this has proven not to be the time. One that is present in less than adequate amounts is re- case (Reitnour and Salsbury, 1972). Another study showed a ferred to as a limiting amino acid because it will limit pro- high correlation between blood amino acids and dietary tein synthesis. The challenge in feeding horses is to provide amino acids, but no correlation between cecal amino acid adequate quantities of protein that will allow for sufficient concentrations and blood amino acids (Reitnour et al., concentrations of circulating amino acids in the blood that 1970). These observations suggest that the horse is sensitive the body can draw on to synthesize tissues, enzymes, and to the quality of protein in the diet (amino acid profile), and hormones, as well as repair tissues. despite evidence that there is microbial synthesis of amino acids in the hindgut, these amino acids are not absorbed for the horse’s benefit. It seems evident that the quality of the PROTEIN DIGESTION AND UTILIZATION protein source should be considered carefully in the horse’s Dietary protein is digested mainly in the foregut of the diet. horse through enzymatic digestion in the stomach and small Feeding NPN sources, such as urea, is not useful to the intestine. Enzymatic digestion of protein in the stomach oc- horse in most circumstances. Hintz et al. (1970) reported curs via pepsin, which has specificity for peptide bonds in- ammonia toxicity and death of ponies (125 to 136 kg) when volving aromatic L-amino acids such as phenylalanine and 450 g of urea was fed orally. Inclusion of urea in the diet has tryptophan. Pancreatic proteases secreted into the small in- been reported to improve nitrogen retention in cases where testine continue protein breakdown and enable absorption of dietary protein appeared to be deficient, but not in cases amino acids and dipeptides in the small intestine. Dipeptides where other protein sources have been used such as soybean are hydrolyzed to amino acids in the gut wall and add to the meal (Slade et al., 1970). Inclusion of urea in the diet has 54

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PROTEINS AND AMINO ACIDS 55 consistently increased blood and urine urea concentrations, parent prececal protein digestibility of 48.1, 53.9, and 70.7 suggesting that the dietary urea contributed to excess nitro- percent for the three diets (corn, oats, and sorghum) and ap- gen in the body, resulting in the increase in blood urea and parent overall tract protein digestion of 98.1, 88.5, and 93.1 excretion of urea in urine (Slade et al., 1970; Hintz and percent, respectively (Gibbs et al., 1996). A similar trial was Schryver, 1972; Martin et al., 1991, 1996). Urea intake of as conducted to evaluate protein digestibility of protein supple- little as 0.9 percent of the diet (0.14 g urea/kg BW/d) was re- ments. Soybean meal (SBM) and cottonseed meal (CSM) ported to increase plasma urea nitrogen (PUN) over the con- were evaluated. Upper tract apparent protein digestibility trol ration (Hintz and Schryver, 1972), while the other stud- was 50.9 and 42.4 percent for SBM and CSM, respectively. ies fed urea at a rate of 2 to 3 percent of the diet and reported Endogenous nitrogen in the upper tract was estimated at 3.5 similar increases in PUN over control rations (Slade et al., mg N/g DM intake. True protein digestibility was calculated 1970; Martin et al., 1991, 1996). These studies support the to be 90.9 and 87.1 percent for SBM and CSM, respectively. conclusion that urea and other sources of NPN in the form Calculating the apparent digestibility of the protein supple- of urea are of little to no nutritional benefit to the horse. ment by difference results in prececal digestibility of 52.5 and 81.2 percent for SBM and CSM, respectively, and total tract digestibility of 92.2 and 85 percent for SBM and CSM, Protein Digestibility respectively. Apparent prececal digestibilities increased from The quality of the dietary protein should be considered 48 to 59.6 percent as SBM increased in the diet from 0.85 when selecting a protein source for the horse’s diet. Protein percent of DM intake to 2.85 percent of DM intake and total quality is a function of the amino acid profile and the di- tract digestibility varied from 77.9 to 87.2 percent with the gestibility of the protein source. The higher the digestibility same increasing amounts of SBM. This study also calculated (especially the foregut digestibility) of the protein source, the true prececal digestibility of SBM to be 72.2 percent the higher the absorption of amino acids to contribute to the (Farley et al., 1995). Potter et al. (1992) fed forage and con- amino acid pool for tissue synthesis and repair. centrate with a higher percentage of forage in the ration when Digestibility of nitrogen (N) or crude protein (CP) is cor- compared to other studies evaluating digestibility (50:50 related to dry matter (DM) intake as well as CP concentra- concentrate to forage) and reported apparent prececal di- tion in the diet. As DM intake and CP concentration in- gestibilities of 67.3, 70.3, and 75.9 percent for oats, SBM, crease, so does CP digestibility (Slade et al., 1970). and CSM, respectively, when fed in combination with Combining means of nitrogen digestibility from several Coastal Bermudagrass hay. Relative prececal digestion of studies (N = 16) provided evidence of this across a range of protein appears to be 25–30 percent of the dietary protein nitrogen intakes between 0.035 g N/kg BW/d to 0.57 g N/kg when diets contain only forage and approaches 70–75 per- BW/d. Total tract apparent digestibility of CP varies based cent when diets contain protein supplements such as SBM or on protein source (e.g., fishmeal vs. corn gluten meal) and CSM in combination with forage. Compiling means from components of the diet (e.g., forage vs. concentrate), as well studies that have reported nitrogen intake as well as fecal ni- as with the ratio of forage to concentrate in the daily ration. trogen (N = 16) resulted in an estimate of apparent nitrogen Within forages, apparent total tract CP digestibility varies digestibility of 79 percent for the total tract when linear re- from 73–83 percent for alfalfa, 57–64 percent for Coastal gression was applied to the data (r2 = 0.94) (Figure 4-1). Bermudagrass, and 67–74 percent for other grasses such as Comparing digestibility between diets that were a mix of fescue and bromegrass. Gibbs et al. (1988) examined the concentrate and forage to those containing only forage did difference in foregut and hindgut digestibility of forages. not result in a significant difference in digestibility (77 per- Apparent prececal digestibility was 28.5 percent for alfalfa cent for forage-only diets). The same approach of compiling and 16.8 percent for Coastal Bermudagrass. Endogenous means from studies (N = 4) that reported nitrogen intake as fecal nitrogen in the study was estimated to be 5.8 mg N/g well as foregut N disappearance was used to evaluate appar- DM intake resulting in true prececal digestibility of protein ent prececal nitrogen digestibility. It determined digestibility from the forages of 37 percent. to be 51 percent when including all diets (r2 = 0.83) (Figure Other digestibility studies have fed concentrates in high 4-2). Only one study (Gibbs et al., 1988) estimated apparent proportion to forage when determining nitrogen digestibil- prececal nitrogen digestibility with all forage diets and re- ity. Feeding corn, oats, or sorghum in combination with ported a slightly lower prececal digestibility (42 percent). Coastal Bermudagrass hay (3:1 concentrate to forage ratio) The studies reviewed to determine these estimates of pro- resulted in apparent total tract protein digestibility of 88, tein digestibility in the total tract, as well as prececal di- 82.8, and 84.6 percent respectively. Apparent prececal pro- gestibility, utilized sources of protein such as SBM, CSM, tein digestion was 38.5, 45.8, and 56.1 percent for corn-, linseed meal, brewer’s dried grains, fishmeal, milk byprod- oat-, and sorghum-based diets. This study determined the ucts, and corn gluten meal. If other sources of protein are apparent protein digestibility of the grain portion of the diet utilized that have not been included in this review, this may by difference (by subtracting the predetermined apparent di- affect overall protein digestibility and thus protein require- gestibility of the hay from a previous study), resulting in ap- ments when using that particular source of protein.

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56 NUTRIENT REQUIREMENTS OF HORSES Nitrogen Digestibility Prececal Nitrogen Digestibility All diets 0.40 0.50 slope = 0.79, r 2 = 0.94, N = 85 0.30 0.40 slope = 0.51, r 2 = 0.83, N = 23 N Digested, g/kg BW/d N Digested, g/kg BW 0.20 0.30 0.10 0.20 0.00 0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 N Intake, g/kg BW/d 0.00 FIGURE 4-2 Regression of means from nitrogen digestibility 0.00 0.10 0.20 0.30 0.40 0.50 0.60 studies evaluating foregut vs. total tract digestibility of nitrogen. The slope of the fitted line indicates a 51 percent digestibility for N Intake, g/kg BW dietary protein (Gibbs et al., 1988, 1996; Potter et al., 1992; Farley FIGURE 4-1 Regression of means from nitrogen digestibility et al., 1995). studies for sedentary horses. The slope of the fitted line represents a 79 percent digestibility for dietary protein (Slade et al., 1970; Hintz et al., 1971; Hintz and Schryver, 1972; Harper and Vander provided data are available regarding the DP concentration Noot, 1974; Reitnour and Salsbury, 1976; Meyer, 1985; Freeman et of feedstuffs. In France, protein requirements and protein al., 1986, 1988; Gibbs et al., 1988, 1996; Potter et al., 1992; Farley content of feeds are expressed in terms of MADC (Matières et al., 1995; Martin et al., 1996; van Niekerk and van Nierkerk, Azotées Digestibles Cheval). This system was developed to 1997a; de Almeida et al., 1998a; Olsman et al., 2003). account for differences in digestibility of various sources of protein and availability of protein digestion products in the small vs. large intestine. Estimates of true ileal digestibility of protein have been made for hays, concentrates, and mixed A study by de Almeida et al. (1998b) determined indi- diets. Adjustments for total tract digestibility have been vidual amino acid digestibilities using foals and reported in- made in this system due to the fact that nitrogen of endoge- creasing apparent digestibility of amino acids as the con- nous origin and nitrogen of microbial origin cannot be dif- centrations increased in the diet. An increase in amino acid ferentiated in the feces (Martin-Rosset et al., 1994). This digestibility with increasing amino acid intake is consistent system provides insight into considering available protein with the increase in protein digestibility with increasing pro- for the horse rather than CP. Improvement of this system and tein intake. Apparent digestibilities ranged from a low of more research can assist in expressing the protein require- 52.8 percent for glycine to a high of 86.3 percent for iso- ments for the horse in the future in terms of DP rather than leucine. Apparent lysine digestibility was 63.8 percent in CP. The lack of information regarding DP and amino acid this study. Slightly higher digestibilities were reported by availabilities in feedstuffs prevents doing this at this time. van Niekerk and van Niekerk (1997a) for lysine (79 per- More information about estimating DP in feedstuffs for cent), threonine (79 percent), isoleucine (80 percent), leu- horses is given in Chapter 8. cine (82 percent), methionine (81 percent), and arginine (90 percent) in pregnant mares. Protein Bioavailability Nitrogen balance studies have resulted in various esti- mates of the amounts of nitrogen needed to maintain zero Proteins that are digested in the foregut are potentially nitrogen balance. The differences in estimates may be ex- available to the horse to contribute to the amino acid pool in plained by variation in nitrogen digestibility. Therefore, 100 g the body, whereas those that pass to the hindgut are not. of CP from SBM may be different from 100 g of CP from Thus, proteins that are largely digested in the equine foregut fishmeal nutritionally to the horse, because of differences in are of higher quality than those that are mainly degraded in the amino acid profile and digestibility of the protein source. the cecum and colon. In addition to its foregut digestibility, Understanding protein digestibility in the horse enables pro- the quality of a protein supplement is further determined by tein requirements to be expressed in terms of digestible pro- its amino acid profile rather than its crude protein content. tein (DP) instead of CP. When protein requirements are ex- As a consequence, nitrogen balance studies found that dif- pressed in terms of DP, diets can be balanced in terms of DP, ferent amounts of protein were required for zero nitrogen re-

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PROTEINS AND AMINO ACIDS 57 tention when different protein sources were fed. Thus zero Available Protein 1.20 nitrogen balances were observed for fishmeal, SBM, and corn gluten meal at intakes of 0.57, 0.8, and 1.18 g N/kg AP = 0.61(DP) + 0.068 Calculated Available Protein Intake, kg BW/d, respectively. Such results suggest that different pro- r 2 = 0.94 N = 20 0.90 tein sources have varying foregut digestibilities and/or amino acid profiles, and thus differ in their biological value to the horse. 0.60 A number of studies have determined DP values for a limited number of feeds in horses, but as yet insufficient 0.30 data are available for the horse in order for DP to be the stan- dard expression for describing either the protein require- ments of horses or the protein quality of the wide range of 0.00 feedstuffs given to them. Crude protein is simply nitrogen × 0.00 0.30 0.60 0.90 1.20 Digestible Protein Intake, kg 6.25. Not all CP is available to the animal and adjusting CP content for protein that is not available to the horse could FIGURE 4-3 Relationship between calculated available protein allow estimation of digestible or “available protein” (AP). (AP) intake and digestible protein (DP) intake (AP = CP – (NPN + Available protein can be estimated by subtracting NPN and ADIN)) (Hintz and Schryver, 1972; Reitnour and Salsbury, 1972; acid detergent insoluble nitrogen (ADIN), which represents Reitnour and Salsbury, 1975; Glade et al., 1985; Gibbs et al., 1988, “bound” protein, from CP values for the feeds. Thus, AP is 1996; Farley et al., 1995; Crozier et al., 1997; LaCasha et al., a calculated estimate of protein that may be available to the 1999). horse, while DP is based on whole tract digestibility studies that measured the amount of protein apparently digested in vivo. For more information on protein analysis in feeds, see Chapter 10. Studies that evaluated protein digestibility in the When protein needs are evaluated, energy intake must be horse were used to evaluate the concept of AP. Digestible adequate. Horses fed only 700 mg CP/kg BW/d (350 g/d for protein intake was calculated using the digestibility for pro- a 500-kg horse) lost weight even when energy intake was tein observed in the respective study. The AP of the diet in adequate. These horses continued to lose weight when en- each study and the respective AP intake was then calculated ergy intake was deficient despite CP intakes of 1,300 mg/kg using the diet and intake data provided in the study. The in- BW/d (650 g CP for a 500-kg horse) (Sticker et al., 1995). take of AP was compared to the DP intake. There was a high Therefore, when CP is deficient, weight loss results; how- correlation between the two variables (r2 = 0.91) (Figure ever, when energy is deficient despite CP being adequate, 4-3). Additional research is required to evaluate this concept weight loss still results. in vivo, but it could be the next step in progressing from CP Endogenous urinary and fecal nitrogen have been evalu- to DP in terms of expressing protein requirements for ated in several studies in an attempt to estimate the minimal horses. protein needs of the horse. Based on these evaluations, 400 Data from appropriate studies were subjected to linear mg DP/kg BW/d has been determined to be the minimum and/or nonlinear regression to obtain estimates of require- protein need (Reitnour and Salsbury, 1976; Patterson et al., ments in the following sections. This approach is different 1985). Higher estimates have been reported by others: 440 from the previous edition and results in changes in the cal- mg DP/kg BW/d by Harper and Vander Noot (1974), 545 culation of protein requirements for horses in various phys- mg DP/kg BW/d by Olsman et al. (2003), 580 mg DP/kg iological states. BW/d by Slade et al. (1970), and 631 mg DP/kg BW/d by Hintz and Schryver (1972). Meyer (1985) estimated en- dogenous losses (fecal, urine, and cutaneous) to be approxi- MAINTENANCE mately 57 g N/kg BW/day, suggesting a minimum of 500 Nitrogen balance studies in mature horses and ponies mg DP/kg BW/d, but also recommended 714 mg DP/kg have concluded that between 400 and 800 mg DP/kg BW/d BW/d to allow for some reserves of nitrogen in the body. is necessary to achieve nitrogen balance in the sedentary The recommendation was based on a 20 percent increase in horse. However, different protein sources have been used in DP over the minimal nitrogen need to build up nitrogen re- the various studies. Taking into account true digestibility serves and assumed 50 percent efficiency of use for the ni- may reduce the variation. Linear regression of means from trogen. Intakes of < 400 mg DP/kg BW/d are inadequate. studies (providing adequate data to calculate nitrogen bal- Horses being fed 264 and 310 mg DP/kg BW/d were in neg- ance) using fishmeal, SBM, and corn gluten meal resulted in ative nitrogen balance (Martin et al., 1996; Olsman et al., zero nitrogen balance at intakes of 0.57, 0.80, and 1.18 g 2003). The NRC (1989) CP requirement for maintenance N/kg BW/d respectively. (656 g CP/d for 500-kg horse) assumes a 46 percent di-

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58 NUTRIENT REQUIREMENTS OF HORSES gestibility and an all-forage diet. This equates to approxi- N balance for maintenance horses mately 600 mg DP/kg BW/d. 0.25 Applying linear regression to means from studies (N = 12) Zero intercept = 0.13 g N × 6.25 = that measured nitrogen intake and nitrogen retention resulted 0.813 g CP/kg BW × 0.79 = r2 = 0.9, N = 39 0.20 0.619 g DP/kg BW in 619 mg DP/kg BW/d (813 mg CP/kg BW/d) for zero ni- N retention, g/kg BW/d trogen retention (r2 = 0.76). Thus, based on these data, the 0.15 Broken line 0.202 g N/kg BW minimum digestible protein intake for horses in maintenance or 1.26 g CP/kg BW 0.10 should be > 620 mg DP/kg BW/d. Because nitrogen balance • • can underestimate true nitrogen loss from the body due to •• •• 0.05 • • • measuring error as well as nitrogen losses from hair, skin, ••• • •• •• • • • •• • • •• • and sweat, some allowances for nitrogen retention greater –0.00 • than zero should be made when determining the mini- •• • • • • mum requirement. This would help compensate for errors in –0.05 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 nitrogen balance data as well as allow for some nitrogen re- serves in the body. Fitting the same data to a broken-line N intake g/kg BW/g model estimates the requirement to be 0.202 g N/kg BW/d, FIGURE 4-4 Linear and nonlinear regression for nitrogen bal- resulting in a CP equivalent of 1.26 g/kg BW/d (Figure ance for horses at maintenance. Linear regression determined zero 4-3). Thus, the equation for determining the CP for the N retention to be 0.126 g N/kg BW and broken-line analysis esti- horse in average maintenance would be BW × 1.26 g/kg mated the requirement to be 0.202 g N/kg BW (Slade et al., 1970; BW/d. For a 500-kg horse this would equate to 630 g CP/d. Hintz and Schryver, 1972; Reitnour and Salsbury, 1972, 1976; Using the 95 percent confidence interval for the data deter- Harper and Vander Noot, 1974; Meyer, 1983a, 1985; Freeman et mines the requirement to be between 1.08 g CP/kg BW/d to al., 1986, 1988; Gibbs et al., 1988; Martin et al., 1996; Olsman et 1.44 g CP/kg BW/d, which would provide three levels of al., 2003). maintenance similar to those described for energy require- ments (Chapter 1), based on the assumption that horses that are more active without forced exercise would have more lean tissue to support. Therefore, minimum CP needs for the lysine per day for the 500-kg horse. With a CP requirement maintenance horse can be calculated using the equation: of 630 g for the 500-kg horse, the lysine requirement of 27 BW × 1.08 g CP/kg BW/d and the need for horses deter- g/d is equal to 4.3 percent of the CP requirement. Therefore, mined to be in elevated maintenance can be calculated by the the requirement for lysine for horses in minimal and ele- equation: BW × 1.44 g CP/kg BW/d. vated maintenance can be calculated by multiplying the CP Crude protein requirements for the horse can be calcu- requirement by 4.3 percent. It is important to emphasize that lated using the following equations: the relationship between CP and lysine should result in ly- sine being 4.3 percent of the CP requirement. If the protein Minimum: BW × 1.08 g CP/kg BW/d sources utilized in the ration do not provide this amount of lysine, this could alter the CP requirement as well. Average: BW × 1.26 g CP/kg BW/d The lysine requirement for horses in maintenance can be calculated as follows: Elevated: BW × 1.44 g CP/kg BW/d Lysine (g/d) = CP requirement × 4.3 percent Studies evaluating lysine requirements of sedentary adult horses have not been conducted. The lysine requirement for GROWTH maintenance has, in the past, been based on the average ly- sine content in most protein sources fed to horses. Using Hintz et al. (1971) studied 4-month-old growing horses means from studies (N = 7) that reported diet composition, and reported maximal nitrogen retention and average daily intake, and N retention, linear regression was applied and gain (ADG) when horses consumed 4.25 g CP/kg BW/d resulted in an intake of 0.036 g lysine/kg BW/d for zero N from a diet in which the supplemental protein was provided retention, representing the minimum need for lysine at by “milk product blend” (2.77 g DP/kg BW/d). These young maintenance. Broken-line analysis (Figure 4-4) identified a horses had greater ADG when fed milk protein compared to plateau in N retention for maintenance horses at an intake of linseed meal (Hintz et al., 1971). Other studies have re- 0.054 g lysine/kg BW/d. The equation to calculate the min- ported greater ADG for young horses (< 8 months of age) imum lysine requirement for maintenance would be BW × fed milk protein over SBM (Borton et al., 1973) or barley 0.036 g/kg BW/d, while the optimum could be calculated (Saastamoinen and Koskinen, 1993), while SBM proved su- with the equation: BW × 0.054 g/kg BW/d. This equates to perior for ADG compared to brewer’s dried grains (BDG) a minimum of 18 g lysine per day and an optimum of 27 g (Ott et al., 1979). However, when canola meal was fed as the

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PROTEINS AND AMINO ACIDS 59 protein source to horses 6 months of age, ADG was equal to amino acid profile of the diet has resulted in improved ADG that produced by SBM (Cymbaluk, 1990). Urea as a dietary as well as an ability to lower the overall quantity of CP in source of nitrogen did not improve ADG in growing horses the diet. Ott and Kivipelto (2002) concluded through regres- younger than 8 months of age (Dubose, 1983; Godbee and sion analysis that lysine was the most important factor af- Slade, 1981). This reinforces the idea that the horse is sen- fecting growth, and Ott et al. (1981) earlier concluded that sitive to protein quality. This is especially true in the grow- CP could be reduced in the diet if lysine intake was ade- ing horse where lysine has been found to be the first limit- quate. Studies have concluded that lysine intake for wean- ing amino acid. lings (4 to 10 months of age) should be 33 to 42 g/d (151 to Average daily gain was maximized for weanlings (6 179 mg lysine/kg BW/d) to improve ADGs (Breuer and months of age) when 5.05 g CP/kg BW/d was consumed Golden, 1971; Ott et al., 1979; Ott and Kivipelto, 2002). Ly- SBM, and a reduction in ADG was reported when intake sine supplementation of linseed meal (Hintz et al., 1971) reached 5.45 g CP/kg BW/d (Yoakam et al., 1978). If a 79 and brewer’s dried grains (Ott et al., 1979) yielded ADG percent protein digestibility is assumed, this equates to max- similar to those reported with milk protein and SBM re- imal growth at 3.95 g DP/kg BW/d. Other studies have re- spectively. Using the data (means) from studies that reported ported improved ADG when weanlings (4 to 6 months of ADG as well as diet, body weight, and feed intake, estimates age) consumed at least 4.1 g CP/kg BW/d (Pulse et al., of amino acid intake can be made. The broken-line model 1973), 4.57 g CP/kg BW/d (Jordan and Myers, 1972), and estimated the lysine requirement to be 168 mg/kg BW/d. For 3.37 g CP/kg BW/d from SBM (Schryver et al., 1987). the 4-month-old 168-kg weanling, this would be 28 g ly- Combining data from several studies, fitting it to a broken- sine/d. This amount of lysine is equivalent to 4.3 percent of line model estimate, and extrapolating to different mature the horse’s CP requirement. Therefore, the lysine require- body weights, the requirement becomes 4 g CP/kg BW/d for ment for weanling horses between 4–10 months of age is 4.3 weanlings between 4 and 10 months of age with an expected percent of the CP requirement. mature body weight of 500 kg. This equals 672 g CP/d for Yearlings (11–17 months of age) have also responded to the 4-month-old (168-kg) weanling. Subtracting the mainte- lysine supplementation. Studies have concluded that be- nance requirement (1.44 g CP/kg BW/d) from the total CP tween 48–50 g lysine/d (154–175 mg lysine/kg BW/d) for results in a 50 percent efficiency of use of the remaining CP improved growth was necessary. In yearlings, lysine supple- for gain (0.84 kg/d), assuming the gain is 20 percent protein. mentation has improved ADG reported from CSM- and This efficiency of use of CP for gain can be used to calcu- BDG-based rations (Potter and Hutchon, 1975; Ott et al., late the CP requirement for gain with an adjustment for di- 1981). Graham et al. (1994) evaluated threonine as a poten- gestibility and adding this amount of CP to the daily main- tially limiting amino acid for yearling horses. Improved tenance requirement. growth and reduced serum urea nitrogen concentrations De Almeida et al. (1998b) estimated total endogenous ni- were reported in yearlings fed 45 g of lysine (127 mg/kg trogen losses in yearling horses to be 588.8 mg CP/kg BW/d BW/d) and 39 g of threonine (110 mg/kg BW/d) per day and found nitrogen retention was maximized at a protein in- compared to yearlings receiving no amino acid supplemen- take of 3.2 g CP/kg BW/d (2.4 g DP/kg BW/d). This study tation or supplemental lysine of 42 g/d (116 mg/kg BW/d). also compared prececal and postileal digestibility and deter- A reduction in serum urea nitrogen would provide evidence mined that the majority of protein digestion took place in the of a reduction in excess amino acids presumably from an foregut. Yearlings (315–333 days of age) have had greater improvement in amino acid balance and utilization for tissue ADGs when fed at least 3.3 g CP/kg BW/d when fed SBM synthesis. This study also supported the idea that improve- and alfalfa (Ott and Kivipelto, 2002), and 3 g CP/kg BW/d ment of protein quality can allow reduction of the overall when fed SBM and Bermudagrass hay (Ott and Asquith, concentration of CP in the diet. 1986). Analysis of means from studies providing intake data Most studies have only utilized sedentary growing horses as well as ADG reported improved ADG with increasing CP to evaluate CP requirements. Orton et al. (1985) compared intake up to 3.3 g CP/kg BW/d for yearlings between 11 and groups of 2-year-olds fed low and high amounts of dietary 17 months of age. Calculating the efficiency of use of CP for protein as well as combinations of exercise and control gain (after subtracting the CP needed for maintenance) in groups. Exercise improved the utilization of the dietary pro- studies reporting CP intake, BW and ADG resulted in aver- tein in the low-protein group (1.45 g CP/kg BW/d). Average age efficiency of only 30 percent for horses over 11 months daily gain was equal to that of the group consuming 3 g of age. A 321-kg yearling (12 months old) gaining 0.45 kg/d CP/kg BW/d with or without exercise. This increase in di- would require 462 g CP/d for maintenance needs (1.44 g etary protein utilization was also reported in 9-month-old CP/kg BW/d) and 380 g CP/d for gain after adjusting for ef- weanlings (1.76 g CP/kg BW/d with exercise yielded simi- ficiency of use and digestibility assuming gain is 20 percent lar ADG as 3.1 g CP/kg BW/d with or without exercise) and protein. yearlings (2 g CP/kg BW/d with exercise had ADG similar Young horses (< 10 months of age) have also responded to consumption of 3.4 g CP/kg BW/d with or without exer- to amino acid supplementation of diets. Improvement of the cise) (Orton et al., 1985). Certainly it seems that exercise

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60 NUTRIENT REQUIREMENTS OF HORSES improved digestibility in the low-protein exercise group al- The slow return to cycling in these mares is possibly due to lowing for improved efficiency of use of the dietary protein. low progesterone concentrations that were reported in all of The decrease in activity observed with stabled horses as well the mares in the protein-deficient group. Progesterone is as horses with minimal exercise may be at a disadvantage produced by the corpus luteum, which is critical to the main- for optimal protein utilization. tenance of early pregnancy (van Niekerk and van Niekerk, Nutrient requirements for growing horses less than 4 1998). Holtan and Hunt (1983) reported a positive linear re- months of age and greater than 18 months of age have re- lationship between dietary protein and progesterone concen- ceived very little attention in published research. Studies in- trations. This study did not report intakes but fed CP con- volving horses less than 4 months of age have reported ADG centrations of 8.6, 11.4, and 17.2 percent CP and found 6.5, between 0.86 and 1.57 kg/d feeding between 300 and 7.9, and 10.3 ng/ml progesterone, respectively. Pregnant 700 g CP/d (Lawrence et al., 1991; Cymbaluk et al., 1993; mares generally have circulating progesterone concentra- Breuer et al., 1996). These ADG fall into the normal growth tions of 7 to 10 ng/ml after day 60 of gestation (Terblanche curve reported in Chapter 1 for horses with an expected ma- and Maree, 1981). A study evaluating nitrogen balance in ture weight of 500 kg. Body weights were not always re- mares in early pregnancy reported mares in positive nitrogen ported in the studies so it is not possible to express CP on a balance with 1.86 g CP/kg BW/d (930 g CP/d for the 500- BW basis. Rations also varied from nursing foals receiving kg horse) (Boyer et al., 1999). This study also evaluated CP only milk for a period of time, foals nursing and receiving digestibility and determined it to be only 51.7 percent for the creep feed, and foals receiving milk replacer and creep feed, all-forage diet, which equates to a DP amount of only 950 making it difficult to make comparisons. This is an area of mg DP/kg BW/d for these mares in early to mid-gestation research that needs to be addressed. (4.5–9 months of pregnancy). Boyer et al. (1999) concluded It is important that lysine amount to 4.3 percent of the CP that approximately 1,100 g CP/d for mares in early preg- requirement for growing horses. If protein sources in the ra- nancy to be appropriate. However, an amount of protein in- tion do not provide this relationship between lysine and CP, take that created negative nitrogen balance was not evalu- the overall CP requirement of the animal may need to be in- ated. The lower amount of acceptable protein intake for creased compared to recommendations made here. mares in early to mid-gestation has not been investigated The protein requirements for growing horses can be cal- thoroughly. In the study by van Niekerk and van Niekerk culated as follows: (1997a) the CP:DE ratio was < 35 in mares that lost weight and had higher incidence of fetal loss compared to the other CP requirement = (BW × 1.44 g CP/kg BW) plus groups with (CP:DE) ratios ≥ 38. The study by Boyer et al. ((ADG × 0.20)/E)/0.79 (1999) had a CP:DE ratio of 40. It can be concluded that feeding mares in early to mid-gestation at an average main- where E = efficiency of use of dietary protein, which is esti- tenance level of protein intake (1.26 g CP/kg BW/d) is ade- mated to be 50 percent for horses 4–6 months of age, 45 per- quate, but more research needs to be done to determine more cent for horses 7 and 8 months of age, 40 percent for horses precisely the needs of the mare in early to mid-pregnancy. 9 and 10 months of age, 35 percent for horses 11 months of Therefore, the equation to determine CP requirements for age, and 30 percent for horses 12 months of age or older. pregnant mares from conception through the 4th month of Lysine requirements can be calculated by multiplying the gestation is: CP requirement by 4.3 percent. CP = BW × 1.26 g CP/kg BW/d PREGNANCY Studies utilizing in utero measurements as well as Protein requirements for pregnant mares have received aborted fetuses have reported fetal weight gain in the third little attention. Pregnant mares fed 1.86 g CP/kg BW/d had trimester to be related to gestational age (Y = –20.7 + no apparent ill effects (Boyer et al., 1999); however, preg- 0.00067X2 , r2 = 0.84, where Y = fetal body weight in kg and nant mares that were fed < 2.0 g CP/kg BW/d (1,000 g CP/d X = gestational age in days; Platt, 1984). Therefore, at the for the 500-kg horse) lost weight and had a higher incidence start of the last trimester (day 240), the fetus would be 17.9 of early fetal loss than mares fed ≥ 2.8 g CP/kg BW/d kg, 28.1 kg at day 270, 39.6 kg at day 300, and 52.3 kg at (1,400 g CP/d for the 500-kg horse). Digestibility of CP in day 330. Fetal birth weight is estimated to be approximately van Nierkerk and van Nierkerk’s 1997a study was approxi- 9.7 percent of the mare’s body weight. Therefore, this equa- mately 80 percent, resulting in DP intakes of 1.6 and 2.2 tion appears to work well for the 500-kg mare. The rates of g/kg BW/d for the apparently deficient and adequate groups growth estimated from the equation equate to approximately mentioned previously. Mares fed the apparently deficient 0.38 kg of weight gain/d for the fetus. Fowden et al. (2000) amount of dietary protein were slower to begin ovulating used ponies in studies that reported fetal weights. Plotting after the anovulatory period than mares fed higher amounts linear regression of mean weights of fetuses of known ges- of dietary protein (van Niekerk and van Niekerk, 1997c). tational age results in a fetal growth equation of Y = 0.13x –

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PROTEINS AND AMINO ACIDS 61 24.8 where Y = fetal weight in kg and x = gestational age in support to the hypothesis that mares may store reserves dur- days (r2 = 0.98). From this equation, at the start of the last ing the earlier stages of pregnancy and mobilize reserves in trimester (day 240), the fetus would be 7.2 kg, 10.3 kg at day the last trimester. 270, 14.2 kg at day 300, and 18.1 kg at day 330. This equa- The amino acid requirements of the pregnant mare have tion would presumably work well for the 200-kg pony. The not been addressed. Van Niekerk and van Niekerk (1997b) rate of gain in the fetus from these data is 0.13 kg of weight reported lower fetal loss during pregnancy in mares fed gain/d. Meyer (1983a) reported that newborn foals were ≥ 110 mg lysine/kg BW/d. No other information regarding 17.1 percent CP. If the neonatal foal is also similar in com- lysine needs of the pregnant mare is available at this time to position in utero, this would equate into 65 g CP deposited the best of the available knowledge. Without any available per day in the horse mare and 22.5 g CP deposited per day data regarding the amino acid needs of the pregnant mare, in the pony mare. This does not take into account other pro- the lysine requirement for pregnancy will be estimated to be tein needs of the uterus. Using only fetal protein deposition 4.3 percent of the CP requirement. Therefore, the lysine re- and the 50 percent efficiency of use of protein in pregnancy quirement for pregnancy can be calculated by multiplying for fetal growth estimated by Meyer (1983a) results in an the CP requirement by 4.3 percent. Protein sources should additional need of 130 g CP/d and 45 g CP/d over mainte- be of high quality and allow the CP relationship with lysine nance for fetal growth in the horse and pony, respectively. to have lysine at 4.3 percent of the CP. If this is not achieved Using a digestibility of 79 percent, in order to provide 130 g because of lower-quality protein sources, the CP require- and 45 g of protein for fetal growth, an additional 165 g and ment may need to be increased. 57 g of CP must be provided in the diet for horses (500kg) The protein requirements for pregnancy can be estimated and ponies (200kg), respectively. Bell et al. (1995) deter- as follows: mined body composition of calves of known gestational age. Nonlinear regression was applied, and an equation estimated Early pregnancy (conception through the 4th month): total fetal and uterine protein deposition. Fetal protein dep- Protein requirements = BW × 1.26 g CP/kg BW/d osition was much higher for calves than that estimated by Platt’s (1984) equation and Meyer’s (1983a) estimation of Pregnancy from the 5th month through parturition: fetal protein composition for horses. The difference between Protein requirements = BW × 1.26 g CP/kg BW/d fetal protein deposition and total uterine protein deposition plus ((fetal gain in kg/0.5)/0.79 (assumed to be placental protein need), however, was ap- proximately 20 g CP/d. Using 50 percent efficiency, this where 0.5 represents the efficiency factor and 0.79 repre- adds 40 g CP/d to the mare’s need during mid- to late preg- sents the digestibility of the protein. The lysine requirement nancy. Therefore, the 500-kg mare would need 802, 845, can be calculated by multiplying the CP requirement by 4.3 and 863 g CP for the 9th, 10th, and 11th month of preg- percent. nancy, respectively. Using the available data, a growth curve for the fetus was derived for the 5th through 11th months of LACTATION gestation. Based on estimated protein composition of the fetus, as well as the rate of fetal gain, estimates of protein Lactating mares fed < 2.8 g CP/kg BW/d lost weight and needs above maintenance were made with an allowance for produced less milk than mares fed at least 3.2 g CP/kg placental and uterine protein needs included. BW/d. Foals also gained more weight in this study when The difficulty in determining dietary protein needs of the mares were fed at least 3.2 g CP/kg BW/d (van Niekerk and pregnant mare is that the mare readily utilizes body reserves van Niekerk, 1997b). In Martin et al. (1991) mares fed only to support the fetus. Meyer (1996) speculated that the preg- 1.26 g CP/kg BW/d lost weight. Despite increasing the CP nant mare adjusts for marginal energy, protein, and mineral via urea supplementation to an amount of 2 g CP/kg BW/d, deficiencies by mobilizing body reserves and prolonging the mares still lost weight. Milk production in this study gestation time. Kowalski et al. (1990) weighed 10 pregnant was reduced; foals had slower growth and higher plasma Thoroughbred mares every 14 days for the last 3 months of urea nitrogen concentrations. Mares in the study had higher gestation. The diets provided 23 Mcal DE/d and the mares plasma urea nitrogen concentrations as well when fed urea. averaged a body condition score of 6. There were no signif- This suggests that urea is not appropriate for lactating mares icant weight gains in the last 3 months of pregnancy, leading and only high-quality protein should be fed (Martin the authors to conclude that the mares used body reserves to et al., 1991). Jordan (1983) reported that lactating mares fed support the pregnancy. Lawrence et al. (1992) also found no < 3 g CP/kg BW lost weight. Inclusion of SBM as the pro- significant weight gains for mares in the last trimester. In tein source in the diet reduced the daily weight loss com- this study, mares were evaluated throughout pregnancy, and pared to diets without SBM, but did not prevent weight loss it was determined that the significant increase in weight gain until CP intake reached an amount of 3 g /kg BW/d. Foal actually occurred in the second trimester of pregnancy rather ADG did not differ among any of the groups, suggesting than the third. All mares delivered normal size foals, adding milk production and composition were not affected by the

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62 NUTRIENT REQUIREMENTS OF HORSES apparent lack of dietary protein. This suggests the mare maintenance during the first month of lactation and approx- readily utilizes body reserves during lactation. imately 40 additional g of DP/kg milk must be provided The amount of milk production by mares has been docu- above maintenance in later lactation. Using a digestibility of mented in several studies and reported to vary between 79 percent results in 51 to 86 g CP/kg milk during the first 1.9–3.9 percent of the mare’s body weight throughout lacta- month of lactation, and 51 g CP/kg milk thereafter above tion (up to 6 months). The same studies reported a very con- maintenance needs of the mare (presumably elevated main- sistent protein content of milk: from 3.1 to 3.3 percent in tenance). The equation to determine the CP requirements for early lactation (generally colostrums) and gradually declin- lactation is BW × 1.44 g CP/kg BW/d plus milk production ing to 1.6 to 1.9 percent in later lactation (Doreau et al., 1986, (kg/d) × 50 g CP/kg milk. 1990; Smolders et al., 1990; Martin et al., 1992; Mariani et Wickens et al. (2002) estimated requirements for amino al., 2001). Broken-line analysis of means from these studies acids during lactation based on the ratios of amino acids to produced a linear decline in CP content of milk from day 1 lysine in milk. Assuming an average lysine content of 1.7 to day 22 of lactation and then the concentration of crude g/kg milk and 65 percent utilization efficiency, a dietary ly- protein in milk reached a plateau at 1.96 percent. Dietary sine requirement was estimated to be 2.62 g digestible ly- protein intake in these studies was not determined to affect sine/kg milk. If a 500-kg mare produces an average of 3.2 milk production or crude protein composition of milk. One percent of her body weight in milk/d, she will require exception reported that mares fed supplemental SBM (diet 41.9 g of digestible lysine per day above maintenance. The provided 1,674 g CP/d) compared to mares fed a diet without estimated amino acid requirements based on the amino acid supplemental SBM (diet provided 1,568 g CP/d) had higher profile of milk and the lysine requirement are as follows concentrations of amino acids in milk, with the exception of (grams of digestible amino acids/kg milk): arginine, 1.81; cysteine. The amino acid profile as expressed as a percent of histidine, 0.86; isoleucine, 2.04; leucine, 3.85; lysine, 2.62; milk protein was not different, however. The foals nursing methionine, 0.84; phenylalanine, 1.39; threonine, 1.88; va- these mares had higher withers height consistently through- line, 2.54. out the study, suggesting the protein quantity and/or quality Lysine requirements for lactation are estimated by the was better for their growth (Glade and Luba, 1990). equation: kg milk/d × 3.3 g lysine per kg milk plus the main- Lactating mares fed diets containing 129 g CP/kg DM or tenance requirement for lysine. Based on the amount of 142 g CP/kg DM (from SBM) produced between 2.6 to 3.9 lysine needed during lactation, it is important that high- percent BW in milk/d between weeks 1 and 8 of lactation. quality protein sources be fed to lactating mares. If poor- Based on the overall intake of CP, between 126 to 150 g CP quality protein sources are utilized and the same CP:lysine was needed per kg milk produced in the first week of lacta- relationship is not achieved as recommended here, CP re- tion. This declined to 110 to 120 g CP/kg milk in week 8 of quirements may need to be increased. lactation (Doreau et al., 1992). Using these values, a 500-kg The protein requirements for lactation can be calculated mare producing 3.2 percent of her BW (16 kg/d) would need as follows: 2,400 g CP/d in early lactation and 1,920 g CP/d in late lac- tation to support this amount of production. This is much CP requirement = BW × 1.44 g CP/kg BW/d higher than other estimates. Doreau et al. (1992) calculated plus milk production (kg/d) × 50 g CP/kg milk the daily output of CP from this study to be an average of 590 g, based on milk production at 3.2 percent of the mare’s Lysine needs can be calculated by multiplying milk produc- body weight. Adding this amount of protein, adjusting for tion (kg/d) by 3.3 g lysine per kg milk in addition to the 50 percent efficiency of use, to the assumed maintenance maintenance requirement for lysine. need of the mare would result in a requirement of 1,587 g CP/d. Average milk production, from the previous studies EXERCISE mentioned, was 3, 2.9, 2.8, 2, and 1.9 percent of the mare’s BW between 1 and 5 months of lactation, respectively. Lin- There is some evidence that the exercising horse requires ear regression of means from studies reporting milk CP con- additional protein per kilogram of body weight for develop- centrations prior to day 22 of lactation determined that the ing muscle and repair of damaged muscle. The increased percent of CP in milk can be calculated from the equation Y protein need is typically achieved by an increase in DM in- = 3.43 – 0.066x where Y = CP percent of milk and x = days take to increase energy intake. However, with an increase in in milk (r2 = 0.7). After day 22, milk protein concentration use of higher fat feeds, the increase in energy intake will not seems to plateau at approximately 1.96 percent CP. In gen- always result in a concurrent increase in protein intake. eral, the mare requires between 20 and 34 g CP/kg milk/d in Therefore, attention needs to be paid to amounts of protein the first month of lactation and approximately 20 g CP/kg consumed. milk produced/d for the remainder of lactation. Adjusting Freeman et al. (1988) reported an increase in nitrogen re- for efficiency of utilization (assumed to be 50 percent), an tention for exercising horses as exercise load increased. The additional 40 to 68 g DP/kg milk must be provided above increase in nitrogen retention was still evident after exercise

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PROTEINS AND AMINO ACIDS 63 had ceased during a deconditioning period, which suggests they were exercised. Those 2-year-olds fed similar amounts additional protein was needed for developing muscle mass of protein (but did not receive exercise) did not achieve sim- and repairing tissues. Horses retained an additional 0.37 g ilar ADGs compared to those fed higher protein amounts. CP/kg BW/d during exercise compared to rest periods. Interestingly, linear regression of means from studies (N = Working backwards using a 50 percent efficiency of use of 5) utilizing exercising horses that provided information to the available protein equates into a predicted additional need calculate nitrogen balance finds zero nitrogen balance to be of 0.74 g CP/kg BW for exercising horses over maintenance. at 0.813 g CP/kg BW/d, which is exactly the same as zero For the 500-kg horse, this would be 370 g CP in addition to nitrogen balance for horses at maintenance. maintenance; however, this study did not account for nitro- One source of nitrogen loss for the exercising horse, gen lost in sweat. Estimating the amount of protein lost in which has rarely been quantified, is nitrogen lost in sweat. sweat in this study accounts for 0.23 g of CP/kg BW out of Freeman et al. (1986) reported that water turnover rate in- the additional 0.37 g CP/kg that was apparently retained. creased in conditioning horses but urine output did not ac- This leaves 0.14 g CP/kg BW unaccounted for in nitrogen count for the losses, inferring that sweat losses increase losses and may represent the additional protein need over greatly with intense exercise. Hodgson et al. (1993) esti- maintenance for exercising horses. Freeman et al. (1986) re- mated sweat loss at 10–12 L per hour. McCutcheon and ported an increase in RNA concentration in biceps fermoris Geor (1996) reported losses of approximately 1 percent of muscle biopsies for horses in a conditioning program. An in- pre-exercise BW in training (50–60 percent of maximum crease in RNA concentration has been reported in hypertro- volume of oxygen [VO2max]) up to a loss of 2.6 percent of phied muscle in other species. This evidence in combination pre-exercise BW during a standard exercise test (SET) that with a decrease in body fat without a decrease in BW sug- approached 100 percent VO2max. Sweat contains between gests a gain in muscle and likely an increase in protein needs 1–1.5 g of N/kg sweat (Meyer, 1987). Sweat loss is gener- to support the muscle gain. ally correlated to increases in activity, and losses have been Wickens et al. (2003) fed exercising horses (moderate in- estimated to be as high as 5 kg/100 kg BW during intense tensity) various amounts of dietary protein (677 g/d, 790 exercise (Meyer, 1987). This equates to approximately 38 g g/d, 903 g/d, 1,016 g/d, and 1,129 g/d). Nitrogen retention of N or 238 g CP for the 500-kg horse. was maximized when horses were fed the diet providing The protein requirement for the exercising horse is there- 1,016 g CP/d compared to the diet providing 903 g CP/d and fore based on the fact that additional muscle appears to be the diet providing 1,129 g CP/d. The data from these studies gained during conditioning and that nitrogen is lost in sweat. result in a recommendation of 1.9 to 2.1 g CP/kg BW/d for An additional need above maintenance is assumed with an moderately exercised horses. If 79 percent digestibility is adjustment for sweat loss added based on intensity of exer- assumed, this equates to 1.5 to 1.66 g DP/kg BW/d. For the cise. The additional protein needed for muscle can be calcu- 500-kg horse, this would result in a requirement of 950 to lated by: 1,050 g CP/d or a digestible protein need of 750 to 830 g DP/d. Wickens et al. (2005) used 3-methylhistidine (3MH) Light exercise: BW × 0.089 g CP/kg BW/d to estimate protein requirements for exercising horses by evaluating the breakpoint at which 3MH concentrations Moderate exercise: BW × 0.177 g CP/kg BW/d were minimized for horses fed various amounts of dietary protein. Muscle protein turnover can be estimated using Heavy exercise: BW × 0.266 g CP/kg BW/d 3MH. The 3MH method has limitations, particularly the fact that it measures muscle protein turnover and not whole body Very heavy exercise: BW × 0.354 g CP/kg BW/d protein turnover. However, the amount of muscle protein in the body far exceeds that of other proteins in the body. These If sweat loss is estimated to be 0.25, 0.5, 1, and 2 percent data predicted a crude protein requirement of 954 g/d (95 of pre-exercise BW for light, moderate, heavy, and very percent confidence interval: 823 to 1,085 g CP/d), which heavy exercise, nitrogen loss would be 1.56, 3.13, 6.25, and would equate to 1.9 g CP/kg BW/d or a range of 1.65 to 12.5 g, respectively, and equate to 9.75, 19.6, 39.1, and 78.1 g 2.2 g CP/kg BW/d. These three studies (Freeman et al., CP for the 500-kg horse. If a 50 percent efficiency of use is 1986; Wickens et al., 2003; Wickens et al., 2005) are in assumed for dietary CP to replace these losses (and assum- close agreement with each other. ing 79 percent digestibility of the dietary protein), the 500- Other researchers have reported much lower CP needs for kg exercising horse would need 24.7, 49.6, 99, and 197.7 g exercising horses. Patterson et al. (1985) determined mature CP to replace nitrogen lost in sweat during light, moderate, horses at three different work intensities required about 410 heavy, and very heavy exercise, respectively. The adjust- mg DP/kg BW/d (260 g CP for a 500-kg horse). Orton et al. ment for CP needed to replace nitrogen lost in sweat should (1985) reported improved utilization of dietary protein when be added to the additional need for CP for muscle develop- horses were exercised. Two-year-olds fed 1.2 g DP/kg BW/d ment and the horse’s maintenance requirement. had similar ADG to those fed 2.4 g DP/kg BW/d provided

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64 NUTRIENT REQUIREMENTS OF HORSES Plasma concentrations of branched-chain amino acids quirements of amino acids for mature exercising horses are (BCAA; valine, leucine, and isoleucine) increase during exer- not well defined and require further research. cise (Miller-Graber et al., 1990; Pösö et al., 1991; Assenza et Using means from studies (N = 3) that reported diet and in- al., 2004). These amino acids are oxidized for energy during take, lysine intake was calculated. Broken-line analysis of the exercise in the muscle. Trottier et al. (2002) reported de- means for lysine intake and nitrogen retention in exercising creases in plasma BCAA following exercise and during re- horses resulted in a recommendation of 0.068 g lysine/kg covery. Reports on muscle BCAA are contradictory: Trottier BW/d for the exercising horse. This would result in a recom- et al. (2002) reported no change in muscle BCAA after exer- mended lysine intake of 34 g/d for the 500-kg horse. The cise, whereas Essén-Gustavsson and Jensen-Waern (2002) re- horses in these studies participated in moderate to heavy ex- ported an increase in BCAA following exercise. There have ercise. Therefore, the requirement for lysine for the exercising been a few studies trying to demonstrate a benefit (such as de- horse in moderate to heavy exercise can be calculated from creased lactate) with BCAA supplementation for exercise, but the equation: BW × 0.068 g/kg BW/d. With a CP requirement results have been inconclusive or flawed in their design of 768 g CP/d for the 500-kg horse in moderate exercise, the (Glade, 1991; Casini et al., 2000). lysine requirement of 34 g/d represents 4.3 percent of the CP Several studies at the University of Illinois determined that requirement. Thus the lysine requirement for exercising there was no detrimental effect of high protein intakes (18.5 horses can be calculated by multiplying the CP requirement percent CP, > 1,700 g CP/d, 3.3 g CP/kg BW/d) on exercise by 4.3 percent. In order for lysine to be 4.3 percent of the CP performance, but did speculate that high protein intakes may in a ration, good-quality sources of protein must be used. If reduce glycogen (and thus available fuel for exercise) and ex- poor-quality sources of protein are incorporated in the horse’s ceed the capacity of the urea cycle (Miller and Lawrence, ration, lysine needs may not be met and CP requirements may 1988; Miller-Graber et al., 1991). Graham-Thiers et al. (1999, need to be increased compared to those recommended in this 2001) reported improved acid-base balance (higher blood pH discussion. and bicarbonate) during repeated sprints for horses fed lower Protein requirements for exercising horses can be calcu- CP (725 g/d, 1.45 g CP/kg BW/d) fortified with amino acids lated by the following equation: (lysine and threonine) compared to higher protein intakes (> 1,400 g CP/d, 2.8 g CP/kg BW/d). Combining this infor- BW × MG plus ((BW × SL × 7.8 g/kg)/0.50)/0.79, mation with the fact that excess amino acids increase urea for- plus the maintenance requirement for protein mation and water loss through urination (Meyer, 1987) may suggest that careful attention to dietary protein intake for ex- where MG = muscle gain and SL = sweat loss. Muscle gain ercising horses is needed. The concept of lowering the overall is estimated to be 0.089 g CP/kg BW for light exercise, 0.177 CP concentration with fortification (adding limiting amino g CP/kg BW for moderate exercise, 0.266 g CP/kg BW for acids) has been demonstrated to be successful with growing heavy exercise, and 0.354 g CP/kg BW for very heavy exer- horses (Graham et al., 1994; Stanier et al., 2001) and with ex- cise. Sweat loss is estimated to be 0.25, 0.50, 1, and 2 percent ercising horses (Graham-Thiers et al., 1999, 2001). of BW for light, moderate, heavy, and very heavy exercise re- Concentrations of certain plasma amino acids were re- spectively. Lysine needs can be calculated by multiplying the ported by McKeever et al. (1986) to decrease with horses in a CP requirement by 4.3 percent. conditioning program, suggesting that amino acid intake was inadequate for the demands from the body. These horses were IDEAL PROTEIN fed > 1,500 g CP/d (alfalfa hay and milo). The amino acid profile and the digestibility of the diet apparently influenced Protein quality has been mentioned in this discussion the available amino acids (which seemed to be inadequate), several times and should be of concern with the horse’s sen- despite presumably more than adequate amounts of CP (Mc- sitivity to amino acid profiles in the diet. The concept of Keever et al., 1986). In a study utilizing young and aged ex- ideal protein is used in feeding swine and poultry. Ideal pro- ercising horses (Graham-Thiers et al., 2005), dietary protein tein is based on formulating a diet with amino acids, not just amounts were fed either to provide current recommendations in the correct amounts, but also in the proper ratios to one of dietary CP or supplementary lysine and threonine at an another. Ideal protein is defined as a protein that includes the amount recommended for growth in an effort to evaluate the minimum quantity of each essential amino acid compatible horse’s ability to build and maintain muscle mass. The study with maximum utilization of the protein as a whole. The es- reported lower plasma urea nitrogen and 3MH concentra- timation of ideal protein in swine is based on results from tions, as well as greater plasma creatinine and subjective mus- experiments involving the removal of a single amino acid at cle mass scores for horses fed the amino acid fortified diets. a time from a casein diet and measuring nitrogen balance. The lysine intake of the control group was above the current The assumption is that the removal of the most limiting recommendation for lysine for exercising horses (Graham- amino acid results in the greatest reduction in nitrogen bal- Thiers et al., 2005). This study suggests that the balance of ance. There is also the assumption that ideal protein would amino acids was improved with supplementation and the re- closely resemble the amino acid profile of the likely end

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PROTEINS AND AMINO ACIDS 65 product of amino acid use in the body—largely muscle tis- and decreases in milk production in lactating mares. In ex- sue. The ratios for ideal protein for growth in swine are in ercising horses, a lack of protein will result in a loss of mus- close agreement with the ratios of amino acids in swine cle as would also be seen in sedentary adult horses. Other in- muscle tissue. Ideal protein ratios are normally established dicators of protein deficiency include reduced feed intake, by comparing all other essential amino acids to lysine. In poor hair growth, and reduced hoof growth (NRC, 1989). this approach, lysine is assigned a value of 100. Specific studies in horses have not been done to evaluate PROTEIN EXCESS ideal protein by removal of amino acids from the diet; how- ever, ideal protein for horses may be able to be estimated Not much evidence exists concerning the effect of excess from the ratios of amino acids in muscle of the horse. With protein consumption. Meyer (1987) pointed out that excess that in mind, the ratios of amino acids in muscle of the horse protein is degraded and results in an increase in urea, which are: lysine, 100; methionine, 27; threonine, 61; isoleucine, will be excreted in the urine. This will increase water loss 55; leucine, 107; histidine, 58; phenylalanine, 60; valine, 62; from the body and may increase the water need of the horse. and arginine, 76; with no information available for trypto- One study reported a growth reduction when protein was fed phan (Bryden, 1991). With this information and the as- at 5.45 g CP/kg BW/d (Yoakam et al., 1978). More recently, sumption that ideal protein in the diet should reflect muscle higher protein intakes in exercising horses resulted in lower tissue amino acid profiles, the formulation of the diet can at- blood pH at rest and during sprinting exercise. Therefore, tempt to achieve these ratios. Also, since the ratios are based protein in excess may interfere with acid-base balance dur- on lysine and the lysine requirement has been determined ing exercise (Graham-Thiers et al., 1999, 2001). An increase for the growing horse, estimates of the requirements for the in calcium loss with high protein intakes has been reported other amino acids can be made. in other species. Glade et al. (1985) reported an increased It would also seem logical that the amino acid profile and calcium and phosphorus loss when weanling horses were corresponding ratios to lysine found in milk would corre- fed > 1,000 g CP/d. Other studies have not supported these spond reasonably well to the dietary amino acid needs of conclusions. The effect of high protein intakes on calcium growth in the foal. Several studies have evaluated amino balance needs further investigation. This, along with other acid composition of a mare’s milk. The ratios of the amino reasons to avoid excess protein, should be considered. Un- acids in milk relative to lysine (set to 100) are as follows: like many other species, longevity is a concern for horses arginine, 70 to 82; histidine, 29 to 37; isoleucine, 53 to 79; and skeletal development in the young horse (which is af- leucine, 127 to 147 methionine, 29 to 35; phenylalanine, 53 fected by calcium balance) is crucial to its usefulness. to 59; threonine, 53 to 68; and valine, 64 to 97. These ratios Concern for the environment has grown in recent years are in close agreement with the ratios in muscle with the ex- and led to regulations regarding waste management in cat- ceptions of histidine, leucine, and valine (Davis et al., 1994; tle and swine. One of the concerns is nitrogen excretion Doreau et al., 1990; Wickens et al., 2002; Stamper et al., from the animal due to excess protein in the diet. Lawrence 2005). Stamper et al. (2005) used these ratios to evaluate et al. (2003) reviewed studies reporting nitrogen excretion foal milk replacer and reported arginine, isoleucine, and in sedentary and exercising horses. Regression analysis leucine lower in relation to lysine than in mare’s milk. This determined, on average, horses excreted (via manure and information may help to improve formulation of milk re- urine) 89 g N/d at rest and 99 g N/d if the horses were placers in the future. exercising. Synthetic amino acids such as lysine and threonine have been utilized in rations of growing horses (Ott et al., 1979; SUMMARY—FEEDING PROTEIN Graham et al., 1994; Stanier et al., 2001) and exercising horses (Graham-Thiers et al., 2001, 2003, 2005) as a means Total tract and prececal digestibility vary with protein to improve protein quality in the ration. It should be noted source and protein concentration in the diet. It is important that the addition of synthetic amino acids to rations alters to consider the amino acid profile and prececal digestibility the balance of amino acids. The balance created by supple- of feedstuffs in addition to total crude protein, especially mentation may or may not lend itself to the concept of ideal in rations fed to growing horses and those in high states of protein. production. Several factors can affect amino acid digestion in horses. As summarized by Gibbs and Potter (2002), these include PROTEIN DEFICIENCY site of digestion, feedstuff variation, biological value of pro- Reduced intake of protein results in decreased growth in tein, protein intake, amount consumed, and transit time horses despite adequate energy; however, energy is normally through the digestive tract. In addition to emphasizing the the first limiter for growth (Ott and Asquith, 1986; Stanier et need to evaluate amino acid content and availability in al., 2001). As discussed earlier, a protein deficiency results growing horse rations, Gibbs and Potter (2002), using re- in weight loss in adult horses, fetal loss in pregnant mares, sults from several trials, provided supportive evidence of the

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66 NUTRIENT REQUIREMENTS OF HORSES need to supply amino acids in smaller, more frequent meals Doreau, M., S. Boulot, W. Martin-Rosset, and J. Robelin. 1986. Relation- per day as compared to twice-daily feedings of rapidly ship between nutrient intake, growth and body composition of the nurs- ing foal. Reprod. Nutr. Dev. 26:683–690. growing horses. Doreau, M., S. Boulot, J. P. Barlet, and P. Patureau-Mirand. 1990. Yield and Rations should be evaluated for amino acid content and composition of milk from lactating mares: effect of lactation stage and availability, especially for those fed to growing horses and individual differences. J. Dairy Res. 57:449–454. those in states of production. If using low-quality protein Doreau, M., S. Boulot, D. Bauchart, J. P. Barlet, and W. Martin-Rosset. roughages, growing horses and lactating mare diets should 1992. Voluntary intake, milk production and plasma metabolites in nursing mares fed two different diets. J. Nutr. 122:992–999. be formulated so that 60 percent or more of the total protein Dubose, E. 1983. Utilization of urea and lysine diets for growth by young is supplied by a high-quality protein supplement. Splitting equines. P. 107 in Proc. 8th Equine Nutr. Physiol. Soc. Symp., Lexing- high-protein rations meal fed to growing horses into three or ton, KY. more feedings per day may enhance amino acid absorption Essén-Gustavsson, B., and M. Jensen-Waern. 2002. Effect of an endurance in the small intestine, although more research is necessary to race on muscle amino acids, pro- and macroglycogen and triglycerides. Equine Vet. J. Suppl. 34:209–213. more accurately define influences of diet, individual horses, Farley, E. B., G. D. Potter, P. G. Gibbs, J. Schumacher, and M. Murray- and feeding schedules. Gerzik. 1995. Digestion of soybean meal protein in the equine small and large intestine at varying levels of intake. J. Equine Vet. Sci. 15:391–397. REFERENCES Fowden, A. L., P. M. 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