Digestive System of the Horse and Feeding Management

Agriculture and Natural Resources FSA3038 Steven M. Jones Associate Professor ­ Animal Science Animals have different types of digestive systems based on where and how they digest components. Nonruminant systems are character­ ized by enzymatic digestion of carbo­ hydrates, proteins and fats in the foregut, with limited fiber digestion in the hindgut. Digestive systems of man, pigs and dogs are examples of this type of digestion. Ruminants, such as cows, sheep and deer, have more complex digestive systems that allow fiber digestion in the rumen, enzymatic digestion in the foregut and relatively minimal digestion of fiber in the hindgut. The horse’s digestive system is somewhat intermediate between other nonruminants and ruminants in that high rates of enzy­ matic digestion occur in the foregut (mouth to ileum) plus high rates of fermentive microbial digestion occur in the hindgut (cecum to rectum). The horse is classified as a nonruminant herbivore – a roughage eater. Figure 1 shows the horse’s digestive tract with approximate lengths and capacities of various compartments of the tract. Accessory organs that aid in digestion (not shown) include the teeth, salivary glands, liver and pancreas. This diagram has been stretched out for demonstration purposes and thus is not anatomically correct. Figure 2 is a closeup of the major components of the digestive tract of the horse. Figure 2. Components of Horse Digestive Tract The foregut includes the mouth, esophagus, stomach and small intestine. Digestion begins in the mouth where feeds are chewed and wetted with saliva. The chewing process cracks the outer shell of grains, reduces the particle size of feeds and increases the surface area of food particles. Arkansas Is Our Campus Visit our web site at: http://www.uaex.edu Figure 1. Digestive Tract of the Horse University of Arkansas, United States Department of Agriculture, and County Governments Cooperating The horse’s stomach is small, relative to the total tract, and cannot accommodate large quantities of food at any one time, resulting in eating several times per day due to limited one-time capacity. Limited enzymatic digestion and some fermentive digestion from a small microbial population occurs in the stomach. Food remains in the stomach only about 15 minutes before it starts to pass into the small intestine. This limited capacity and any excess gas products in the stomach can cause the rupture of the stomach, other digestive upsets and death. The small intestine is the site for a major portion of nutrient absorption. Here soluble carbohydrates are digested to simple sugars and absorbed for use as energy. Efficiency of carbohydrate digestion in the small intestine appears to be important to increase the energy available to the horse and decrease the potential for colic or founder caused by excessive carbohydrates reaching the hindgut. The small intes­ tine appears to be the primary site for fat digestion and absorption. Diets containing 10 to 15 percent fat can be tolerated and used for energy. About 50 to 70 percent of the protein in grain-based diets is digested to amino acids and absorbed from the small intestine, but less than one-third of hay protein is absorbed from the upper tract. The fat soluble vita­ mins A, D, E and K are also absorbed in the small intestine along with B-vitamins, calcium and some phosphorus. Passage of feeds through the small intestine takes approximately 30 to 90 minutes. The horse’s hindgut includes the cecum, large colon, small colon and rectum. The cecum hangs against the right side of the abdominal cavity at the juncture of the horse’s body trunk and hind leg. During or immediately after a horse eats, gut sounds, which are mixing activities of the cecum, can be heard normally by placing an ear against the abdomi­ nal wall in the area of the cecum. Veterinarians rou­ tinely listen for these gut sounds when diagnosing digestive disturbances. Absence of gut sounds may indicate abnormal cecal activity. The horse’s hindgut contains an active population of bacteria and protozoa similar to that of the rumen in ruminants. Microbes break down fibrous feeds into short-chained volatile fatty acids. This microbial action allows the horse to efficiently utilize forages, either green or cured. Volatile fatty acids are an energy source for the horse, and the amount and proportion produced can be altered by composition of the diet. Starch that reaches the hindgut is fermented to volatile fatty acids plus lactic acid. Again, digestive disorders can occur when excessive amounts of soluble carbohydrate reach the hindgut, causing an excess of lactic acid to be produced. Therefore, maxi­ mizing starch digestion in the foregut is of the utmost importance to horsemen. Microbes synthesize amino acids in the large intestine, but essential amino acids are not absorbed in any appreciable quantity from the hindgut. This means that, unlike the ruminant, the horse cannot eat low-quality protein feeds and then convert this protein into higher-quality protein for absorption and use in the body. Considerable amounts of B-vitamins are synthesized by the microbes in the hindgut and are absorbed. It appears that thiamine is not absorbed in sufficient quantities to meet the require­ ments of hard-working horses and, therefore, should be added to the rations of those horses classed as hard-working (e.g., jumping, roping, endurance riding, racing). Rate of passage through the cecum and large intestine is 36 to 72 hours. Many factors can influence the digestibility of nutrients in the complex digestive tract of the horse. These include type of feedstuff, level of maturity of forage, method of processing feedstuffs, quantity fed, frequency of feeding, rate of passage and age and individual differences among horses. Feeding Management Guidelines Horses require different amounts of nutrients in their daily diets, depending upon their nutritional class or status in life. Table 1 lists the nutritional requirements of horses as determined by the National Research Council. For this discussion, the classes are mature idle, working, growing and stage of repro­ duction. Dividing horses into classes relative to nutri­ ent requirements is the first step in designing a workable feeding management program. This approach helps a horse owner meet each horse’s nutrient requirements in the most manageable and economical fashion. The following suggestions should help in designing your feeding management program. 1. Feed horses according to body weight. Table 2 shows recommended daily feed intakes by horses as a percent of body weight. The most accurate method of determining body weight is to weigh the horse on a scale. Where weighing is impractical, weight tapes or body measurement formulas can be used. Probably the most commonly used technique for determining body weight is the heart girth tape, which is available from feed dealers, veterinarians and livestock supply companies. Another method for estimating a horse’s body weight is the body weight equation. One equation is: W = HG2 × BL 330 where W = weight in pounds, HG = heart girth in inches and BL = body length in inches (point of shoulder to point of hip). Table 1. Nutrient Concentrations in Total Diets for Horses and Ponies (90% dry matter basis) Digestible Energya (Mcal/lb) Mature Horses Maintenance Stallions Pregnant Mares 9 months 10 months 11 months Lactating Mares Foaling to 3 months 3 months to weaning Working Horses Light workb Moderate workc Intense workd Growing Horses Weanling, 4 months Weanling, 6 months Moderate growth Rapid growth Yearling, 12 months Moderate growth Rapid growth Long yearling, 18 mos. Not in training In training Two year old, 24 mos. Not in training In training Diet Proportions Conc. Hay (%) (%) Crude Protein Lysine (%) (%) Cal- cium (%) Vitamin A Phos- Magne- Potas- phorus sium sium (IU/kg) (IU/lb) (%) (%) (%) 0.80 0 100 7.2 0.25 0.21 0.15 0.08 0.27 1650 750 1.00 30 70 8.6 0.30 0.26 0.19 0.10 0.33 2370 1080 0.90 20 80 8.9 0.31 0.39 0.29 0.10 0.32 3330 1510 0.90 20 80 9.0 0.32 0.39 0.30 0.10 0.33 3280 1490 1.00 30 70 9.5 0.33 0.41 0.31 0.10 0.35 3280 1490 1.10 50 50 12.0 0.41 0.47 0.30 0.09 0.38 2480 1130 1.05 35 65 10.0 0.34 0.33 0.20 0.08 0.30 2720 1240 1.05 35 65 8.8 0.32 0.27 0.19 0.10 0.34 2420 1100 1.10 50 50 9.4 0.35 0.28 0.22 0.22 0.36 2140 970 1.20 65 35 10.3 0.36 0.31 0.23 0.12 0.39 1760 800 1.25 70 30 13.1 0.54 0.62 0.34 0.07 0.27 1420 650 1.25 70 30 13.0 0.55 0.50 0.28 0.07 0.27 1680 760 1.25 70 30 13.1 0.55 0.55 0.30 0.07 0.27 1470 670 1.15 60 40 11.3 0.48 0.39 0.21 0.07 0.27 1950 890 1.15 60 40 11.3 0.48 0.40 0.22 0.07 0.27 1730 790 1.05 45 55 10.1 0.43 0.31 0.17 0.07 0.27 2050 930 1.10 50 50 10.8 0.45 0.32 0.18 0.08 0.27 1620 740 1.00 35 65 9.4 0.38 0.28 0.15 0.08 0.27 2380 1080 1.10 50 50 10.1 0.41 0.31 0.17 0.09 0.29 1840 840 a Values assume a concentrate feed containing 3.3 Mcal/kg and hay containing 2.00 Mcal/kg of dry matter. bExamples are horses used in Western and English pleasure, bridle path hack, equitation, etc. cExamples are horses used in ranch work, roping, cutting, barrel racing, jumping, etc. dExamples are race training, polo, etc. Table 2. Recommended Daily Feed Intakes as Percent of Body Weights* Class Mature Horses Maintenance Working Light Medium Intense Late Gestation Lactating Mares Young Horses Yearling foal (12 months) Long yearling (18 months) 2-year-old (24 months) Forage Concentrate Total 1.5-2.0 0-0.5 1.5-2.0 1.0-2.0 0.5-1.0 1.5-2.5 1.0-2.0 0.75-1.5 1.75-2.5 0.75-1.5 1.0-2.0 2.0-3.0 1.0-1.5 0.5-1.0 1.5-2.0 1.0-2.0 1.0-2.0 2.0-3.0 1.0-1.5 1.0-2.0 2.0-3.0 1.0-1.5 1.0-1.5 1.0-1.5 1.75-2.5 Feed adjustments should be made according to condition scores so they can be fed to optimal condi­ tion and subsequently achieve maximum reproductive and performance efficiency. Table 3 is a condition scorecard. The ideal condition score for most horses is 5 to 7. 2.0-2.5 1.0-1.5 2. Feed horses according to condition scores. *Air dry feed, 90% dry matter Source: NRC, 1989 3. Feed adequate long-stemmed forage. As a nonruminant herbivore, the horse innately has a need to forage or chew long roughage. A horse requires 1 percent of its body weight daily of long- stemmed roughage to allow for normal activity of the digestive tract. 4. Provide feed by weight, not volume. Standard volumes of feedstuffs do not weigh the same due to their density differences. For example, a Table 3. Condition Scorecard* Score Back Ribs Neck Shoulder Withers Tailhead (midbarrel) (forerib) poor 1 very prominent very extremely prominent prominent very vertebrae prominent thin prominent very thin 2 prominent prominent very thin very thin very thin very thin vertebrae thin 3 vertebrae - fat see easily thin thin thin prominent half way up moderately 4 negative see slight moderately moderately moderately some fat thin crease outline thin thin thin moderate 5 level not see; blend into blend smoothly rounded moderate (no crease) easily feel shoulder into body fat moderately 6 slight not see; little fat little fat little fat moderate fleshy crease feel fat fleshy 7 average barely average average average fleshy crease feel fat fat fat fat fat 8 obvious difficult fat flush fat filled fat crease to feel behind extremely 9 very obvious not feel bulging bulging bulging bulging fat crease (patchy fat) fat fat fat fat *Adapted from NRC-Nutrient Requirements of Horses. standard 3-pound coffee can containing 32 pounds per bushel oats will weigh 21⁄2 pounds; 38 pounds per bushel oats will weigh 4 pounds; and the same can filled with corn will weigh 5 pounds. Hays will vary just as grains and concentrates. Always check feed weight per unit volume, especially when new or different feeds and hays are purchased. 5. Never feed concentrates at a level more than 0.75 percent of body weight at any one feeding. For example, a 1,000-pound horse should never be fed more than 7.5 pounds of concentrate at any one feeding. If more than 7.5 pounds of concentrate is required by an individual horse, split the amount equally and feed two or more times per day. Space multiple feedings throughout the day and feed daily at set feeding times. Horses are creatures of habit and respond positively to a regular schedule. 6. Avoid abrupt ration changes. Ration changes become necessary in the normal production cycle. Typically, these changes are in amounts, types or forms of feedstuffs. When changing a ration, some changes can be made almost immedi­ ately, some require a few days and others will require a week or longer. For example, changing from one textured concentrate to another with equal energy densities is only a slight change and can be done in a few days. However, changing horses from a ration high in oats to one high in corn represents a signifi­ cant change in energy and will require a week or more to safely make the transition. When changing horses from grass hay to lush pasture, turn horses out for only a few hours for two to three days, then half a day for two to three days and then out to the lush pasture continuously. If this gradual changeover is not feasible, fill the horses up on dry, bulky grass hay prior to turning them out on lush pasture. A simi­ lar management scheme should be followed when changing from grass hay to lush legumes. Selecting the Correct Feed For the majority of horse owners, the best way to feed horses is to choose the best available forage, either pasture or hay, and then purchase a complete, balanced commercial grain concentrate. Commercially prepared feeds are practical for feeding a small number of horses. Hay should be tested to determine the amount of essential nutrients it can provide. Many of the commercially prepared feeds from reputable companies are balanced for protein, calcium and phosphorus; contain trace mineralized salt; and are fortified with vitamins. The nutrients provided in the commercial concentrate may be determined by reading the feed tag, with one exception. That excep­ tion is energy. It is not required by law to print the energy content of a feed on the tag. However, energy content may be estimated by its relationship to percent crude fiber (Table 4). Owners with a large number of horses may find it more economical and practical to custom mix a ration and have it delivered in bulk. Those owners choosing to mix rations on the farm should take time to balance for protein, energy, minerals and vitamins. If individual grains are purchased for mixing, only the highest quality should be used. Some sample rations are included (Tables 5-10). The horse is dependent upon its owner to provide the proper nutrients and management system for optimum health and performance. Knowledge of the horse’s digestive system and nutrient requirements combined with a sound feeding management plan assures the horse owner that the horses in their care will be healthy and have the ability to reach maximum performance level. Table 4. Relationship of Crude Fiber to Expected Digestible Energy in Conventional and Fat-Supplemented Grain Mixes If the feed tag indicates crude fiber (%) of Then, the digestible energy But, if the feed also contains 4%-5% (Mcal/lb) of the feed added fat (tag shows 7%-8% fat), then the will be approximately digestible energy will be approximately 1.62 1.72 1.55 1.65 1.45 1.55 1.35 1.45 1.25 1.35 1.15 1.25 2 4 6 8 10 12 (and 3% to 3.75% fat) Table 5. Example Performance Horse Ration (designed to be fed with good quality grass hay or grazing) Ingredients Percent Cracked Corn 45.00 Whole Oats 42.50 Soybean Meal 7.50 Molasses 3.25 Calcium Carbonate .75 TM Salt 1.0 Vitamin A + Vitamin E + Pounds/Ton Calculated Analyses 900 850 150 65 15 20 + + Crude Protein = 12.0% Digestible Energy = 1.39 Mcal/lb Crude Fiber = 6.0% Crude Fat = 3.7% Calcium = .36% Phosphorus = .32% Important: See Table 2 on expected feed consumption, and always introduce new grain feeds gradually. This ration contains approximately 6% more energy than straight oats, so smaller amounts of this ration will usually maintain similar body condition. Table 6. Fat-Supplemented Performance Ration for Hard-Working Horses (to be fed with grass hay) Ingredients Cracked Corn Whole Oats Animal Fat Soybean Meal Molasses Calcium Carbonate Dicalcium Phosphate Brewer’s Yeast TM Salt Vitamin A Vitamin E Percent 40.00 37.50 5.00 12.25 2.00 .75 .25 1.25 1.0 + + Pounds/Ton Calculated Analyses 800 750 100 245 40 15 5 25 20 + + Crude Protein = 14.0% Digestible Energy = 1.50 Mcal/lb Crude Fiber = 5.7% Crude Fat = 8.25% Calcium = .42% Phosphorus = .38% Important: See Table 2 on expected feed consumption, and always introduce new grain feeds gradually. This ration contains 8% more energy than the ration shown in Table 5 and 13% more energy than straight oats, so smaller amounts of this ration will usually maintain similar body condition. Table 7. Brood Mare Ration to Be Fed With Good-Quality Hay or Grazing During Late Pregnancy and Lactation Ingredients Oats Cracked Corn Soybean Meal Molasses Ground Limestone Dicalcium Phosphate TM Salt Vitamin A Percent Pounds/Ton 40.00 800 40.00 800 15.00 300 3.00 60 .75 15 .75 15 .50 10 + + Calculated Analyses Crude Protein = 14.8% Digestible Energy = 1.4 Mcal/lb Crude Fat = 3.3% Calcium = .59% Phosphorus = .50% Table 8. Fat-Added Ration to Be Fed With Good-Quality Hay or Grazing During Late Pregnancy and Lactation Ingredients Oats Cracked Corn Added Fat Soybean Meal Molasses Ground Limestone Dicalcium Phosphate TM Salt Vitamin A Percent Pounds/Ton 35.00 175.00 35.00 175.00 5.00 25.00 20.00 100.00 2.00 10.00 .75 3.75 1.25 6.25 1.00 5.00 + + Calculated Analyses Crude Protein = 16.2% Digestible Energy = 1.51 Mcal/lb Crude Fat = 8% Calcium = .67% Phosphorus = .58% Table 9. Yearling Ration (to be fed with grass hay) Ingredients Cracked Corn Oats Soybean Meal Molasses Calcium Carbonate Dicalcium Phosphate TM Salt Vitamin A Percent Pounds/Ton 47.5 950 30.0 600 15.0 300 5.0 100 .5 10 1.5 30 .5 10 + + Calculated Analyses Crude Protein = 14.7% Lysine = .66% Digestible Energy = 1.42 Mcal/lb Calcium = .66% Phosphorus = .63% Table 10. Creep Feed and Weanling Ration (weanling ration designed to be fed with good-quality grass hay or grazing) Ingredients Cracked Corn Oats Soybean Meal Molasses Calcium Carbonate Dicalcium Phosphate TM Salt Vitamin A Percent Pounds/Ton 40.0 800 32.5 650 20.0 400 5.0 100 1.0 20 1.0 20 .5 10 + + Calculated Analyses Crude Protein = 16.5% Lysine = .80% Digestible Energy = 1.39 Mcal/lb Fat = 3.2% Fiber = 5.5% Calcium = .80% Phosphorus = .50% Printed by University of Arkansas Cooperative Extension Service Printing Services. STEVEN M. JONES is associate professor with the Department of Animal Science, University of Arkansas Division of Agriculture, Little Rock. FSA3038-PD-7-11RV Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Director, Cooperative Extension Service, University of Arkansas. The Arkansas Cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, gender, age, disability, marital or veteran status, or any other legally protected status, and is an Affirmative Action/Equal Opportunity Employer.