Digestion is the process by which feedstuffs are broken down to their simplest forms. The resulting nutrients can then be absorbed into the bloodstream and provide fats, carbohydrates, amino acids, vitamins, and minerals for maintenance and/or growth, or they can be stored for future needs. Digestion takes place through a complex process that involves peristaltic muscular contractions, enzymatic action, and fermentation (digestion of fiber by microorganisms in the large intestine). The digestive process is completed when usable nutrients are assimilated and undigested feed residues and waste products are excreted. To feed a horse to achieve its full potential, a working knowledge of the digestive system is important.

Animals have different types of digestive systems based on how they digest components of feedstuffs. Non-ruminant systems (e.g., man, pig, dog) are characterized by enzymatic digestion of carbohydrates, proteins, and fats in the foregut (mouth to ileum), with limited fiber digestion in the hindgut (large intestine). Ruminants (e.g., cattle, sheep, deer) have more complex digestive systems which allow fiber digestion in the rumen, enzymatic digestion in the small intestine, and relatively minimal digestion of fiber in the hindgut. The horse’s digestive system is somewhat intermediate between non-ruminants and ruminants in that high rates of enzymatic digestion occur in the foregut; plus, high rates of fermentive microbial digestion occur in the hindgut (cecum to rectum). The horse is classified as a non-ruminant herbivore— a roughage eater.

The digestive tract of the horse consists of a muscular tube called the alimentary canal (digestive tract), which begins at the lips and terminates at the anus. The alimentary canal consists of the mouth, pharynx, esophagus, stomach, small intestine, cecum, large colon, small colon, and rectum. Associated organs which aid in the total digestive process are the teeth, tongue, salivary glands, liver, and pancreas. The alimentary canal is about 100 feet long in the mature horse. It changes diameter abruptly in several places, enlarging at the stomach, narrowing at the small intestine and enlarging again at the cecum. The tract is lined with mucous membranes, most of which contain glands to secrete digestive fluids.

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Figure 1 shows the horse’s digestive tract with approximate lengths and capacities of various compartments of the tract. The diagram does not show the exact anatomical location of digestive tract components; the tract is stretched out for ease of viewing. Accessory organs which aid in digestion, but are not shown, include the teeth, salivary glands, liver, and pancreas.

The digestive process begins in the mouth where teeth reduce the particle size of feed. The chewing process stimulates the flow of saliva, which lubricates the feed prior to swallowing. Feed which has been chewed and swallowed enters the esophagus, stomach, and the rest of the digestive system (see Figure 1). The horse has a small, simple stomach followed by a long, narrow small intestine. Once in the stomach and small intestine, digestive enzymes are mixed with the feed material. This allows the stomach and small intestine to digest and absorb most of the starch, protein, fat, fat-soluble vitamins (A, D, and E), and many minerals contained in feedstuffs.

Feed material which is not digested in the stomach and small intestine pass into two very large organs: the cecum and the colon. These two organs are known collectively as the large intestine or the hindgut. The horse’s large  intestine is a specialized organ which functions to utilize plant fiber (forage). The large intestine of the horse holds approximately 21 to 24 gallons of liquid and contains billions of bacteria and protozoa. These microorganisms produce enzymes that break down (ferment) plant fiber.

Hindgut microbes are absolutely essential to the horse, since the horse itself cannot produce enzymes to break down fiber. The end-products of microbial fermentation of plant fiber (which are absorbed and provide the horse with a source of energy and micronutrients) are volatile fatty acids, B vitamins, and vitamin K. In addition to fiber digestion, the hindgut is the primary site of digestion and absorption of phosphorus, an essential mineral needed for skeletal growth, muscle contraction, and energy utilization. The other main function of the equine hindgut is to serve as a reservoir—a reserve of electrolytes and water which are vital to sustain exercise performance.

With this condensed look at the equine digestive system, many things become obvious regarding correctly utilizing the digestive system to provide the horse’s nutrient needs:

• The basic design of the digestive system consists of a small simple stomach, which is certainly not intended for processing large, single meals. Instead, the functioning of the digestive system is more conducive to small, frequent meals.

• The well-developed hindgut, with its microbial population, is responsible for digestion of plant fiber. This indicates that horses are not only very capable of surviving on plant fiber, but thrive on it.

• Further, with a properly functioning hindgut serving as a reservoir (short-term nutrient absorption and resorption site) for energy (volatile fatty acids), B vitamins, electrolytes, and water, maximizing the use of the hindgut would certainly be helpful in maintaining exercise performance.

Taken together, a small simple stomach, combined with a large fiber-digesting hindgut, makes horses anatomically designed to ingest small forage meals in a continuous fashion. Therefore, by design, forage (hay, pasture) should be the mainstay of the horse’s diet; grain (starch), fat, and supplemental protein, vitamins, and minerals should play an important but lesser role in terms of feed volume.

Conclusion
While we continue to learn more about the science of feeding horses, we must also remember the art of feeding is equally important. Providing the right environment, schedule, sources of nutrients, as well as a well-trained eye, to constantly monitor the horse’s condition, are all important factors in a sound feeding program. Although rations differ with the age and class of horse, it should be remembered that the most important factor is the individuality of the specific horse, as every horse is a unique individual, and therefore should be fed as an individual. In other words, feeding should be designed around the training and growth requirements of each individual horse.

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