It’s often been observed that horses are simple creatures. And while the phrase might have been unkindly intended to describe a lack of intellectual complexity, it’s pretty accurate when it comes to their nutritional needs.
Unlike us humans with our omnivorous tastes, horses are strictly plant eaters. Forage is the basis of the equine diet, and when the forage is of good quality and in plentiful supply, horses suffer few digestive difficulties. It’s only when we deviate from the “forage principle” that our horses run into trouble.
At first glance, however, the equine digestive tract seems to be something of an evolutionary mistake. Take the equine stomach, for example. It’s surprisingly small for an animal the size of the horse—with a capacity of only about two to four gallons (or 7.5 to 15 liters). In contrast, the small intestine can measure an amazing 70 feet (about 22 meters) in length, if uncoiled and stretched out, with a diameter of three to four inches and a capacity for 10 to 12 gallons of material. Compared with what we know about the physiology of other animals, the horse’s equine gastrointestinal (GI) tract seems strangely out of proportion. But from Nature’s point of view, everything’s just fine. In his wild state, the horse never expected to ingest large quantities of food at one sitting; his digestive system is optimally designed for his wandering, grazing lifestyle.
Let’s take a slightly more thorough tour through the equine innards and see what else we can discover about the link between his physiology and his diet. When a horse tears off a mouthful of grass with his teeth or uses his talented lips to pick up hay or grain from the ground or a feed tub, the tongue transfers the food to the back of his mouth.
There, the horse’s wide, flat molars grind it up and mix it with saliva (which almost immediately launches the digestive process by beginning to break down starches). When thoroughly chewed, a mouthful of oats will have absorbed its own weight in moisture while a mouthful of hay will have absorbed about four times its own weight.
From there, the base of the tongue pushes the food past the soft palate and into the pharynx, the opening to the esophagus, a flexible tube that leads down the neck to the stomach. Once in the esophagus, a series of muscular contractions pushes the food along. In the case of the horse, these contractions move only one direction—meaning that what goes down, for better or worse, stays down.
Surprisingly little digestion goes on in the stomach itself. A small microbial population initiates some fermentation, and there is also some enzymatic action—but because food remains in the stomach only 15 minutes, on average, before being pushed on to the small intestine, there is little time for any major food breakdown. As soon as the stomach reaches about two-thirds of its capacity, it typically starts to pass food (which, by now, has been liquefied by the stomach acids) on to the small intestine, and the process continues as long as the horse keeps eating.
Although food remains in the stomach for a very brief interval, its presence (or absence) has a direct bearing on the horse’s health. The upper, inner portion of the stomach’s lining is made up of a nonglandular, squamous cell layer that is vulnerable to the hydrochloric acid the stomach secretes. Having food in the stomach at frequent intervals tends to absorb the acid and keep it from splashing this upper layer. Horses fed infrequently (one or two large meals a day, rather than several smaller meals) are more at risk of gastric ulcers, which can result from exposure to stomach acids. And it’s worth noting, too, that forage does the best job of absorbing these acids. Horses fed a hay-only diet typically have a very low incidence of ulcers while those on a mixed diet are more at risk.
The next stop on the tour is the small intestine, a coiled and convoluted tube suspended from the loin region by a fan-shaped membrane called the mesentery. The first section of the small intestine—the duodenum—is shaped like a U-turn, which helps prevent food from being forced back into the stomach if the small intestine becomes distended. The small intestine is the primary site for protein digestion and the absorption of amino acids (although grains are processed more thoroughly here than is forage), and it can hold up to 30% of the GI tract’s total capacity.