Equine Digestive Physiology

The horse has been identified as a non-ruminant herbivore, an animal without a rumen that eats forage. When a horse is out in the wild with thousands of acres of free-roam grazing, and the only external demand is to maintain itself and occasionally run from predators, this configuration serves it quite well, most of the time. However, when a horse is confined to small acreage or total confinement, where the selection of forages is limited, or it is placed under performance demands seen in modern competitions, the non-ruminant herbivore meets with some nutritional challenges. First of all, confinement practices prevent the horse from benefiting from his ability to be a highly selective grazer, grazing only on young, immature forages that his system can digest and utilize efficiently. Second, the modern demands placed on the horse increase his nutrient requirements and horsemen must figure out how to meet those higher nutrient needs in an amount of feed a horse can safely consume and utilize.

An understanding of the horses' digestive tract, where feedstuffs are digested and how that impacts the end products of digestion, is necessary to help the horse meet these challenges. The digestive tract of the horse is divided into two sections, the foregut and the hindgut. The foregut has similarities to the prececal digestive systems of a monogastric animal such as a dog, man or pig. The hindgut acts more like the rumen of a cow. Both sections contribute to the nutritional status of the horse, but in very different ways. The foregut, more specifically the small intestine, is the primary site of nutrient absorption in the horse. Digestion in the small intestine is accomplished by enzymatic degradation of starches, fats and proteins. The efficiency of this enzymatic digestion is affected by factors including meal size and frequency, particle size, and interactions between diet constituents. These factors affect rate of passage (ROP) of feedstuffs through the small intestine and the surface area of substrate accessible to enzymatic action, both of which impact prececal digestibility coefficients.

Starches

In discussing absorption and storage of starches, one clarification should be made. Many times, the terms starch and carbohydrates are used interchangeably. Is starch and carbohydrate the same thing? Basically, all starches are carbohydrates but not all carbohydrates are starches. Carbohydrates include sugars, starches and fibers. The term carbohydrate arises from the fact that most substances in this class have empirical formulas suggesting they are carbon "hydrates", with the ratio of carbon to hydrogen to oxygen being 1:2:1. For example, the empirical formula of D-glucose, the precursor to all carbohydrates, is C6H12O6.  There are three major classes of carbohydrates: monosaccharides, or simple sugars; oligosaccharides, which are short chains of monosaccharides joined together by covalent bonds and are considered sugars as well; and polysaccharides, which consist of long chains having hundreds or thousands of monosaccharide units and are considered non-sugars. The most abundant polysaccharides, starch and cellulose, consist of recurring units of D-glucose, but differ in how the D-glucose units are linked together. In starch, the monosaccharides are linked together by (-linkages, which are subject to mammalian enzymatic digestion. Cellulose, on the other hand, contains monosaccharides linked together by (-linkages, and can only be broken by cellulase, a microbial enzyme. Often, in the horse's diet, sugar and starch are referred to as soluble or hydrolysable carbohydrates and cellulose is referred to as an insoluble or fermentable carbohydrate. This is in reference to whether the carbohydrate is hydrolyzed or digested prececally, as with starch, or fermented in the hindgut, as with cellulose. Concentrates and forages consumed by horses contain primarily starch and fiber (cellulose, hemicellulose and lignin), which require breakdown to monosaccharides before absorption. Starch will be digested primarily in the duodenum and proximal jejunum, while the fiber component will be digested in the cecum and large intestine.

Common feedstuffs eaten by horses contain only low levels of simple sugars. The best-recognized source of dietary sugar for the horse is molasses. However, the most important source of dietary sugar for the horse is pasture grasses; molasses in feeds is an insignificant source. Consider the following example: Beet molasses contains 700-750 g DM/kg and of this DM, about 500 g is soluble carbohydrate. Typical molasses inclusion in a mixed sweet feed is 10% or 100g/kg. At that inclusion rate, the net contribution of sugar would be 50 g/kg of feed from the molasses1.  Compare that to perennial ryegrass, which contains 200 g/kg DM as soluble carbohydrate2.

Interestingly, legumes have a lower soluble carbohydrate content than improved pasture grasses. Drying the forage, as in making hay, especially conditioning or crimping the plant, reduces the soluble carbohydrate content as well3. Fructan is the largest constituent of soluble carbohydrate in some grasses, depending on grass variety, time of year and ambient temperature. There is no enzyme in the small intestine of the horse that can break down fructan to a monosaccharide for absorption. Therefore, fructan from grasses will pass into the cecum where it is rapidly fermented, causing a drop in cecal pH similar to that from grain overload.

Total tract digestion of dietary starches has been shown to be essentially complete. However, in horses, it is very important that the dietary starch is digested prececally, and does not reach the hindgut for microbial fermentation. Hindgut fermentation of soluble carbohydrates changes the end products of carbohydrate digestion and can be detrimental to the microbial population in the cecum.

Soluble carbohydrate that is digested prececally is used for the following purposes:
* Energy for cellular processes;
* Anabolic processes to form other compounds;
* Stored in liver, kidney and muscle as glycogen;
* Converted to fat and stored in adipose tissue.

Much work has been done to investigate the horse's ability to digest and utilize the starch component of the diet. Total tract digestion coefficients have ranged from 94.2%4 up to 99.6%5. These numbers are not affected by source of starch or level of intake. However, the digestion of starch in the small intestine is much more variable. Type of grain may be one source of variation in prececal starch digestion. At similar starch intakes (1.03 g/kg BW/feeding), oat starch has been shown to be more digestible prececally than corn starch, 91.1% vs. 78.2%6.  It also appears that the method of grain processing may be a significant factor in whether starch is digested in the small intestine or escapes into the large intestine of the horse. When crimping of oats and sorghum was compared with micronizing these grains, micronized oats had the highest prececal starch digestion and crimped sorghum the lowest4. In fact, the major site of starch digestion when horses were fed crimped sorghum was the large intestine. In this experiment, total starch intake was similar, although slightly lower for micronized oats (2.37g/kg BW/feeding) than for crimped sorghum (2.95 g/0kg BW/feeding). Although that appears to be a small difference, it has been demonstrated that feeding only 2.4 g starch/kg BW significantly (P<.05) reduced cecal pH (to 6.24) and acetate content7. The inference drawn from this was that undigested starch was reaching the cecum and being rapidly fermented by resident microbes. Starch intake on diets of micronized oats was below 2.4 g/kg and, and crimped sorghum, starch intake was above this level. It is possible that there was a compounding effect of source of starch, oats vs. sorghum, and amount of starch intake contributed to the reduced prececal digestion of crimped sorghum.

Another study attempted to quantify the upper limit capacity for prececal starch digestion in the horse. In this experiment, the daily ration was divided into 2, 3, or 4 meals at 12-, 8-, and 6-hour intervals, and starch intakes of 2.65, 1.77, and 1.33 g/kg BW/feeding, respectively8. The total tract starch digestion was about 98% regardless of the frequency of feeding, or total amount of starch per meal, and 74% was digested in the small intestine. Rations used in this study were 30% alfalfa and 70% commercial, corn and oat based concentrate. Clearly, the amount of starch consumed in the twice-daily feeding schedule in this experiment was not sufficient to overwhelm the capacity of the small intestine for starch digestion.

When ponies were fed chopped alfalfa and ground corn, with corn comprising 20, 40, 60 or 80% of the diet5, the highest starch intake achieved was approximately 3.4 g/kg BW/feeding. Total tract starch digestion was near complete in intake levels, and approximately 60% of the starch digestion occurred in the small intestine regardless of starch intake level. However, this means that the total amount of starch digested in the hindgut was significantly higher at the higher levels of corn intake. Based on data from this group of experiments, a recommendation was made that starch intake in the equine fed 2 - 3 meals per day should be limited to approximately 4 g/kg BW/feeding9. Another recommendation, based on the work revealing the change in cecal pH from different levels of starch feeding, suggests feeding not more than 2 g/kg BW/feeding. For a horse weighing 500 kg (1100 lbs), this translates into 3.15 and 4.2 lbs per feeing of corn and oats, respectively, for the lower recommended level and 6.29 and 8.4 lbs per feeding of corn and oats, respectively, for the higher recommended level.

One must keep in mind that cecal pH has been shown to decline within 4 hours following a meal of all hay as well as meals of corn or oats7. While the changes in cecal pH may be greater on all grain diets than on hay diets, it is obvious that any meal feeding scenario results in a change in cecal pH. This is different from what would occur in a continuous grazing scenario. Obviously, further study is needed to fine-tune any feeding recommendation, as it is apparent that total intake and source of starch fed, as well as other components in the diet, will affect these results significantly. One of the biggest variants in research trials appears to be type and amount of hay fed with the experimental grain diets. Hay has been long known to decrease ROP through the total tract in the equine, however it has also been shown to increase ROP through the small intestine. Prececal retention time for hay stems was 1.94 hr, compared to 3.07, 7.35 and 3.4 hr for hay fines, oat hulls and oat groats, respectively10. Therefore, when grain and hay are fed, the type and amount of hay fed, as well as the timing of hay feeding, may impact starch digestion in the small intestine. All things considered, the general recommendation of limiting the amount of concentrate fed to a maximum of 6 lbs per meal would appear to be reasonable for most horses and most grain mixes.

Prececal starch digestion yields simple sugars, mainly glucose. These simple sugars are absorbed into the blood stream, where the rise in plasma glucose concentrations triggers the release of insulin and the subsequent removal of glucose from the bloodstream by the liver. Glucose storage in the body is in the form of glycogen, either in the liver or muscle tissue. The liver is 3.6% glycogen by weight, whereas muscle tissue is 1-2% glycogen. However, muscle makes up 50% of body mass, and is therefore has 3-4 times the glycogen storage capacity as the liver. Muscle glycogen cannot contribute to blood glucose levels, being used only for intracellular energy demands. During starvation, muscle will catabolize and liberate amino acids, which will be transported to the liver and converted to glucose.  There are 2 grams of water stored with every gram of glycogen, making it a "soggy" or heavy molecule. Fat, on the other hand, is six times more compact for the energy yielded per unit weight.

On a practical note, some horsemen and some researchers are promoting high fat/high fiber diets for performance horses. While that type of diet does address concerns with starch digestion and metabolism, it may very well fall short of providing sufficient starch for adequate muscle glycogen stores to support top level performance. In an attempt to create a muscle glycogen loading effect in horses through diet manipulation, a study was designed with a conditioning phase, followed by a depletion phase and a repletion phase11. The conditioning phase, lasted 28 days on a control diet (40% alfalfa, 40% corn, 18.5% oats), the depletion phase, lasted 5 days and involved both exercise and a low starch diet (78% alfalfa, 15% corn oil, 5% casein), while repletion lasted 3 days and involved a very high starch diet (15% coastal, 82.5% corn). The notable results from this study were that in a period of 5 days on the low starch diet, muscle glycogen fell from 19.31 g/kg to 5.63 g/kg and time to fatigue fell during this time from 53.4 minutes to 34.30 minutes. This would indicate that a high fat, high fiber diet, with very low starch content, may not be able to support the glycogen needs of a performance horse, particularly a horse doing anaerobic work.

Fats

Fats are included in the family of chemicals called lipids. Other lipids in the diet, which are not considered simple fats, include fat-soluble vitamins (A, D, E, K), phospholipids and cholesterol. Simple fats are usually present in the form of triglycerides, three fatty acids attached to a glycerol backbone, and are the major fuels for most organisms.  Fatty acids vary in length, ranging from 4 to 24 carbons, usually containing even number of carbons and are classified as saturated, containing no double bonds or unsaturated which contain double bonds. Fats are insoluble in water but very soluble in organic solvents. In the small intestine, bile salts assist in breaking fat globules into smaller sizes and then pancreatic lipase hydrolyses the triglyceride molecules, successively splitting off one fatty acid at a time. Ultimately this yields three free fatty acids and the glycerol backbone. These products are absorbed into the intestinal mucosa, where triglycerides are resynthesized. The resynthesized triglycerides, along with other lipids and some free fatty acids are combined with protein to form chylomicrons. Chylomicrons leave the mucosal cells, enter the lymph system and finally enter circulation. Chylomicrons are essentially cleared from circulation and taken up by the liver or adipose tissue within two to three hours following a meal.

The use of fats and oils in equine diets was delayed for a time because it was assumed that the horse would have limited ability to digest and utilize fats, due to the absence of a gall bladder. However, the fact that mare's milk is around 15% fat, on a dry matter basis, was an indication that at least foals could digest and utilize fat. It has since been recognized that, instead of a gall bladder to store and release bile, the horse continuously secretes bile and therefore is capable of efficiently digesting fats in the small intestine. In the last 20 years, there has been much research investigating the palatability, digestibility and utilization of dietary fats by the equine. Because fats and oils supply 2.25 times the energy as an equal amount of starch, the initial goal of this research was to use added dietary fats or oils to increase the energy density in rations for horses with high energy requirements. The idea was to be able to meet the energy demands with a smaller amount of grain required in the diet. Benefits in addition to the simple caloric value have since been shown.

In cafeteria-style studies, horses have been demonstrated to prefer corn oil to blended fat, inedible tallow or peanut oil12, but they will consume soy oil equally as well as corn oil. Apparent digestibilities of different fats substituted into a ration at 10% have been reported to be 94, 74 and 70%, respectively, for corn oil, inedible tallow and blended fats13.  There is conflicting data regarding the effects of added fats and oils on the digestibilities of other diet components. Feeding a diet containing 14% added fat resulted in lowered digestibilities of dry matter, crude protein and neutral detergent fiber14. Other studies have reported no effect on apparent digestibilities of dry matter, protein, or neutral detergent fiber from adding feed-grade rendered fat15 corn oil, blended fat or inedible tallow16. There are also reports of increasing digestibility of neutral detergent fiber in diets with added fat17, 18. The negative effect of some fats on fiber digestion in ruminants has been widely published.  However, due to the site of digestion of fats in the equine, this should not occur. Fats and oils are digested in the small intestine and would not be present to affect fiber fermentation in the cecum of the horse the same way they would in the rumen.

Essential fatty acid requirements have not yet been established for the horse, although NRC recommends 0.5% linoleic acid in the diet. It is thought that linoleic, linolenic and arachidonic acids may be dietary essential fatty acids, though a fatty acid deficiency in the horse has not been reported in the literature. Further research is needed to determine the upper limit to fat digestion in the small intestine of the horse, and to quantify the essential fatty acid needs of the horse.

Amino Acids

Evaluating rations for horses is commonly centered around percent protein, primarily in the concentrate portion of the diet. However, percent protein in the concentrate is a very small piece of information in the total picture. First of all, horses do not have a protein requirement, but rather an amino acid requirement. Secondly, there is a need for an amount of protein and a quality of dietary protein source in order to supply the amino acid requirement. To simplify the evaluation of a horse's diet into a statement such as "A mature horse needs 10% protein" can be very misleading and totally miss the mark with regards to the horse's needs. There is increasing discussion regarding the relationship between energy and protein and there is an effort to fine tune feeding programs for horses on a nutrient:calorie ratio19. These ratios are expressed as grams of nutrient per megacalorie of digestible energy. To illustrate the value in these ratios, consider the following example:

Compare protein intake of a horse eating oats at 11.5% protein to Omolene 200 at 14% protein. If the horse requires 10 lbs of oats to maintain his body condition, he will need 7.5 pounds of Omolene to stay in the same body condition, due to the higher energy content of the Omolene 200 compared with oats.

Total protein intake: Oats  10 lbs  X  11.5%  = 1.15 lbs protein
 Omolene 200 7.5 lbs X  14%  =  1.05 lbs protein

The horse actually eats less total protein on the higher protein feed, due to the nutrient:calorie ratio. He eats less protein, but a better quality protein, providing a better amino acid profile from the Omolene 200 than the oats.

A horse's protein, or amino acid, requirement may appear to be met from a ration analysis standpoint, yet other factors come into play influencing whether amino acid requirements are actually met. These factors include site of digestion, feedstuff variation and biological value, total amount and rate of protein consumption and retention time in the digestive tract.
 
Amino acids are absorbed in the small intestine. No appreciable amount of amino acids from microbial digestion in the hindgut appear to be available for direct utilization by the horse20. Therefore, site of protein digestion is important in meeting amino acid requirements of the horse. Depending on the source of dietary protein, at least 60 - 70% of dietary protein may be digested and absorbed in the small intestine21. In one study, horses were fed a 50:50 concentrate:hay diet, where two thirds of the protein in the concentrates were supplied by either oats or sorghum grains, crimped or micronized. The remaining protein in the concentrate was supplied by a soybean meal-based supplement. On average, 67% of the nitrogen consumed was digested in the total tract. Approximately 70% of the digestible protein was digested in the small intestine22. Compare this to work where only 37% of the digestible protein was digested in the small intestine when ponies were fed an all hay diet23. In this study, ponies were fed 11.7 % protein coastal Bermuda hay, 15% alfalfa hay or 18% alfalfa hay. Dietary hay nitrogen was almost completely digested as the roughage passed through the entire tract. However, prececal digestibility was highest for the 18% alfalfa hay, next was the 11.7% coastal Bermuda hay and lowest prececal digestibility was reported for the 15% alfalfa hay. The difference here is due to the quality of the hay within its type, not percent protein on an analysis. Coastal Bermuda hay that tests at 11.7% protein is very leafy and relatively immature, as is 18% alfalfa hay, however, 15% alfalfa hay is much more mature and stemmy and is therefore less available for enzymatic digestion in the small intestine. Lower quality alfalfa hay, while higher in protein than grass hay, will contribute very little to the amino acid needs of the horse.

In order to evaluate common feeding practices for yearling horses, researchers fed one group of yearlings oats and alfalfa and another group were fed a balanced grain mix and alfalfa.  Both groups were fed a 65:35 grain:hay ratio. The diet with oats provided 1.15 times the NRC requirement for crude protein and 1.07 times the lysine requirement, more than meeting the protein requirement.  Upper tract digestible protein and lysine between the two treatments were 95% and 86%, respectively, for the oat diet and 124% and 140%, respectively, for the balanced grain mix. Yearlings in both groups showed similar gains in body weight, but those fed oats deposited nearly twice as much body fat and tended to gain less skeletal height than yearlings fed the balanced grain mix. Even though NRC requirements for protein were met when oats were fed, it was concluded that the higher protein:calorie and lysine:calorie ratios from the balanced grain mix appeared to support partitioning of available nutrients for skeletal growth as opposed to fat deposition24. Further work is obviously needed on a wider variety of feedstuffs to more clearly define protein digestion in the small intestine and its impact on meeting the amino acid requirements of the horse.

Designing and evaluating diets for horses is difficult because of the tremendous variability in feed ingredients. It is certainly not adequate to make an evaluation based on the total nutrient content of the diet, due to the tremendous variation in digestibility and bioavailability of feedstuffs. Even knowing total tract digestibility coefficients for starch, fat and protein are not adequate for evaluating the ability of a diet to meet a horse's requirement for these nutrients. Factors such as site of digestion and how diet components interact and effect rate of passage must be considered.

Editor's Note:

In the fall of 2001, equine veterinarians from across the United States gathered in St. Louis, Mo., to hear veterinarians and equine nutritionists share information at the Purina Mills Veterinary Equine Nutrition Conference. The group covered topics including digestive physiology, energy utilization, long-term soundness, and nutritional management of equine gastric ulcers. Purina has made the papers presented at this conference available to readers of The Horse Health E-Newsletter, and we will introduce these over the next several weeks. Here is the first in the series.

Karen E. Davison, Ph.D.
Purina Mills, Inc.


References

1. Cuddeford, D. 1999. Sugar is bad for my horse, isn't it? In: British Equine Vet. Assoc. Nutr. Proc. P. 69.
2. McGrath, D. 1988. Seasonal variation in water-soluble carbohydrates of perennial and Italian ryegrass under cutting conditions. Irish J. Agric. Res. 27:131.
3. McDonald, P., R. A. Edwards, J.F.D. Greenhalgh and C.A. Morgan. 1995. Animal Nutrition 5th Ed. Longman Sinapore Publishers Ltd. Singapore.
4. Householder D.D. 1978. Prececal, post-illeal and total tract digestion and growth performance in horses fed concentrate rations containing oats or sorghum grain processed by crimping or micronizing. Ph.D. Dissertation. Texas A&M University.
5. Hinkle, D.K., G.D.Potter, J.L. Kreider. 1983. Starch digestion in different segments of the digestive tract of ponies fed varying levels of corn. In: Proc. 8th Equine Nutr. Phys. Symp. P. 227.
6. Arnold, F.F., G.D. Potter, J.L. Kreider and G.T. Schelliing. 1981. Carbohydrate digestion in the small or large intestine of the equine. In: Proc. 7th Equine Nutr. Phys. Symp.
7. Radicke, S., E. Kienzle and H. Meyer. 1991. Preileal apparent digestibility of oats and corn starch and consequences for cecal metabolism. In: 12th Equine Nutr. Phys. Symp. P. 43.
8. Brown, K.M. 1987. Nutrient digestion in various segments of the gastrointestinal tract of ponies fed two, three or four meals per day. M.S. Thesis. Texas A&M University.
9. Potter, G.D., F.F. Arnold, D.D. Householder, D.H. Hansen and K.M. Brown. 1992. Digestion of starch in the small or large intestine of the equine. In: Pferdenheilekunde Sonderheft. P. 107.
10. Arnold, F.F., W.C. Ellis, G.D. Potter, J.L. Kreider and K.R. Pond. 1983. Prececal retention time of four feed fractions in ponies. In: 8th Equine Nutr. Phys. Symp. P. 240.
11. Topliff, D.R., G.D. Potter, T.R. Dutson, J.L. Kreider and G.T. Jessup. 1983. Diet manipulation and muscle glycogen in the equine. In: 8th Equine Nutr. Phys. Symp. P. 119.
12. Bowman, V.A., J.P. Fontenot, K.E. Webb Jr. and T.N. Meacham. 1977. Digestion of fat by equine. In: 5th Equine Nutr. Phys. Symp. P. 40.
13. Rich, G.A., J.P. Fontenot and T.N. Meacham. 1981. Digestibility of animal, vegetable and blended fats by equine. In: 7th Equine Nutr. Phys. Symp. P. 30.
14. Worth, M.J., J.P. Fontenot and T.N. Meacham. 1987. Physiological effects of exercise and diet on metabolism in the equine. In: 10th Equine Nutr. Phys. Symp. P. 145.
15. Meyers, M.C., G.D. Potter, L.W. Greene, S.F. Crouse and J.W. Evans. 1989. Physiologic and metabolic response of exercising horses to added dietary fat. J. Equine Vet. Sci. 9(4):218.
16. McCann, J.S., T.N. Meacham and J.P. Fontenot. 1987. Energy utilization and blood traits of ponies fed fat-supplemented diets. J. Anim. Sci. 65:1019.
17. Webb, S.P., G.D. Potter and J.W. Evans. 1987. Physiologic and metabolic response of race and cutting horses to added dietary fat. In: 10th Equine Nutr. Phys. Symp. P. 115.
18. Davison, K.E., G.D. Potter, J.W. Evans, L.W. Greene, P.S. Hargis, C.D. Corn and S.P. Webb. 1991. Growth, nutrient utilization, radiographic bone characteristics and postprandial thyroid hormone concentrations in weanling horses fed added dietary fat. J. Equine Vet. Sci. 11(2):119.
19. NRC. 1989. Nutrient requirements of horses. 5th Revised Ed. National Academy of Sciences. National Research Council. Washington D.C.
20. Reitnour, C.M. and R.L. Salsbury. 1976. Utilization of proteins by the equine species. Am. J. Vet. Res. 37:1065.
21. Hintz, H.F. 1975. Digestive physiology of the horse. J. S. Af. Vet. Assoc. 46(1):13.
22. Klendshoj, C., G.D. Potter, R.E. Lichtnewalner and D.D. Householder. 1979. Nitrogen digestion in the small intestine of horses fed crimped or micronized sorghum grain or oats. In: 6th Equine Nutr. Phys. Symp. P. 91.
23. Gibbs, P.G., G.D. Potter, G.T. Schelling, J.L. Kreider and C.L. Boyd. 1988. Digestion of hay protein in different segments of the equine digestive tract. J. Anim. Sci. 66:400.
24. Gibbs, P.G., D.H. Sigler and T.B. Goehring. 1989. Influence of diet on growth and development of yearling horses. J. Eq. Vet. Sci. 9(4):215.

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