Horses are non-ruminant, monogastric herbivores. This means that horses have a simple, one compartment stomach and eat primarily fibrous vegetation or plant material. Fibrous plants, such as grasses, should make up the largest percentage of a horse’s diet. The horses’ unique digestive system enables them to utilize most all the nutrients found in fibrous plant material. The soluble carbohydrates, proteins and fats are digested in the stomach and small intestines (foregut) with the help of enzymes while the insoluble carbohydrates or fiber portion is digested via bacterial fermentation in the cecum and colon (hindgut). This fermentation in the hindgut of the horse is what allows horses to utilize the fiber portion as a nutritional energy source.
Horses’ Unique Digestive System
The foregut is comprised of the mouth, esophagus, stomach and small intestine. Once a horse chews and swallows a bite of food, it travels down the esophagus and into the stomach. A horse’s stomach holds 8-17 quarts, depending on the size of the animal, and leads to the small intestine. The small intestine is an average of 70 feet long in the adult horse, and is the main site of protein, fat and soluble carbohydrate digestion as well as vitamin and mineral absorption. From the small intestine, any undigested components plus the fiber portion of the diet enter the large intestine, or hindgut.
The hindgut is comprised of the cecum (about 4 feet), large colon (about 10-12 feet), small colon (about 10-12 feet) and rectum (about 1 foot). The cecum and colon make up what is essentially a large fermentation vat containing numerous species of microorganisms, including bacteria, protozoa and yeast. While horses do not possess the digestive enzymes necessary for digestion of fiber, these microorganisms do, and through the process of fermentation are able to convert fiber into useful nutrients for the horse.
Fiber generally falls into two broad categories: soluble and insoluble. Soluble fiber consists of many non-woody portions of a plant, which may include sap, resin, pectin or gums. Insoluble fibers are structural components of a plant, such as lignin, cellulose and hemicellulose. With the exception of lignin, horses utilize most of the ingested fiber. The soluble fibers and the insoluble fibers cellulose and hemicellulose provide dietary energy to the horse through bacterial fermentation in the hindgut. The lignin is resistant to bacterial fermentation, yet it plays an important role in maintaining normal gastrointestinal motility and function.
Bacterial fermentation of fiber results in the production of volatile fatty acids (VFA’s) and lactic acid. VFAs are readily absorbed into the bloodstream and transported for direct energy use, or for synthesis and storage of glucose or fat. For some “easy-keeper” horses at maintenance, VFAs can provide all of the horses’ energy needs. Lactic acid can also be used in biochemical reactions to synthesize glucose for energy use. Production of VFAs and lactic acid result in a slower energy release, compared to the quick breakdown of soluble carbohydrates in the foregut. For this reason, continued intake of small, high-fiber meals helps to prevent large swings in blood glucose concentration (i.e. a “sugar rush”). In addition, the insoluble fibers may help to slow the intake of the rapidly digested soluble carbohydrates. Other by-products of bacterial fermentation that benefit the horse include B-vitamins, such as riboflavin, niacin, biotin, folate, B12 and B6.
Without the proper balance of bacteria in the hindgut, the horse would not extract much nutritional benefit from fiber. The balance and stability of this bacterial population in the hindgut is extremely critical because changes in the balance can result in digestive disorders. Since different bacterial species are required for fermentation of different feedstuffs, a small change in the diet can alter the balance. Should a large quantity of soluble carbohydrates reach the hindgut (commonly referred to as “starch overload”), excess lactic acid and VFA production occurs. Consequently, the environment of the hindgut becomes altered, resulting in hindgut acidosis. When this happens, the risk for digestive upset, colic, laminitis and/or founder increases. Thus it is very important to keep the diet constant, and to make any feeding changes, including hay or pasture, slowly.
Sources of Dietary Fiber
Dietary fiber for the horse can be found in a variety of plant sources or forages. These include pasture, hay, hay cubes, alfalfa pellets, shredded beet pulp, pelleted beet pulp, soy hulls, oat hulls and rice hulls. The long-stem plant sources (generally at least 1 inch in length) are important to facilitate proper gut motility and an appropriate rate of passage of digesta through the intestines. To maintain effective digestive health and function, long-stem plant material, primarily from hay or pasture, should make up no less than 50% of the diet and be fed at a minimum of 1.5% of the horse’s bodyweight per day.
The shorter-stem, ground or pelleted plant sources of dietary fiber are often found in commercial horse feed products. The fiber in these sources ranges in degree of fermentability. The benefit of having highly fermentable fiber in a horse concentrate feed is to maintain a healthy, active bacterial population in the hindgut and ultimately provide a safe and natural form of energy or calories to the horse. The benefit of having less fermentable fiber in a horse concentrate product is to provide bulk to the diet, helping to slow intake, encourage water consumption and ultimately prevent digestive upsets. High-fiber horse feed products are also beneficial when forage quality and/or quantity is questionable or variable. These products provide a consistent, guaranteed nutritional analysis and are a convenient way to supplement additional fiber to the horse’s diet. A high-fiber feed can be especially beneficial for horses that travel to help minimize digestive upsets associated with changes in diet or environment.
Fiber is an essential nutrient in the horse’s diet. The horses’ unique digestive system requires fiber for proper development, function and health. Research reports that insufficient fiber can lead to hindgut acidosis, colic, gastric ulcers, stable vices and behavioral problems. When developing or balancing the equine diet, the quantity, quality and type of forage or other high-fiber containing horse feed should be the first consideration. While some horses can maintain appropriate condition on forage alone, many others require the addition of a fortified grain concentrate. The ultimate goal is to provide the correct balance of forage and concentrates to provide all essential nutrients to meet requirements, while supporting the health and function of the equine digestive system.