What Does a Horse Need in His Diet?
By Karen Briggs • May 23, 2014 • Article #33925
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.
At A Glance
- Forage is the basis of the equine diet.
- Horses should ingest 1.5% to 3% of their body weight each day; at least half of their died should be forage, such as hay or pasture grass.
- The horse's gastrointestinal system is very sensitive; sudden changes in diet can put a horse at risk of colic.
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.
Inside the small intestine, enzymes go to work to break down food materials. Starch not already digested by saliva is converted to a simple sugar called maltose, and other complex sugars and carbohydrates are broken down to simple-sugar forms so they can be absorbed through the intestinal walls. (Capillaries transport them into the blood, and they eventually arrive at the liver, the horse’s major chemical processing plant. The liver also sorts amino acids and reorganizes them into proteins and binds water-soluble nutrients to their appropriate carriers for distribution throughout the body.)
The small intestine is also the primary site for the digestion and absorption of fats. Most animals use gallbladder secretions to break down fats, but horses have no gallbladder, another little evolutionary peculiarity. Nonetheless, horses seem able to use diets containing 10% to 15% fat very efficiently for energy and weight gain. Finally, the fat-soluble vitamins A, D, E, and K are absorbed in the small intestine, as are calcium, some phosphorus, and B vitamins. On average, it takes 60 to 90 minutes for food, now liquefied, to pass through the length of the small intestine.
The last portion of the small intestine, the ileum, leads to the final section of the gastrointestinal tour: the hindgut. This portion is made up of the cecum, large (or ascending) colon, small colon, rectum, and anus. Here’s where the bulk of digestion’s hard work is done. And instead of enzymes doing the honors, digestion in the hindgut is largely microbial—performed by a population of billions of symbiotic bacteria that efficiently break down plant fibers into simpler compounds called volatile fatty acids (VFAs), which can be absorbed through the gut wall. Not only are these bacteria a natural part of the digestive process, they’re essential to it.
The cecum, approximately four feet long and with a capacity of seven to nine gallons, is a huge factory for the first stage of hindgut digestion. The cecum originates high in the horse’s right flank area and extends down and forward toward the diaphragm. Plant fibers, composed of cellulose and other hard-to-digest molecules, pass through the stomach and small intestine unaffected by enzymes, but when they hit the “fermentation vat of the cecum,” the population of bacteria there makes short work of them, usually breaking them down in about five hours. The size and structure of the cecum (the physiological equivalent to our appendix, but far more useful) are such that it slows the passage of food in order for the microbes to do their job.
From the cecum, the partially digested food moves to the large colon, where fermentation continues. Almost 12 feet in length, on average, and holding an impressive volume of 14 to 16 gallons, or 50 to 60 liters, of food (about 38% of the GI tract’s total capacity), the large colon is also where food dwells longest—between 36 and 48 hours. It has a sacculated construction that resembles a series of pouches. This not only can facilitate the breakdown of large quantities of fibrous material but also can become a risk factor when the pouches become distended with gas during a bout of colic, as they seem custom-made for twisting and even strangulating their own tissues. Once the food has been thoroughly processed in the large colon, it moves to the small colon, another 10 to 12 feet long but smaller in diameter (about four inches). The vast majority of the nutrients have been absorbed by this time, and what’s left in the gut is whatever the horse cannot digest or use. The main function of this portion of the hindgut is to reclaim excess moisture from the remaining material.
By the time the food leaves the small colon, it has become solid again and has been molded into fecal balls. The small colon empties into the rectum and, some 36 to 72 hours after beginning its journey, the waste material from a horse’s meal is expelled as manure through the anus.
The equine gastrointestinal tract functions very well under normal conditions. But as every horseman knows, it’s also extremely sensitive and easy to upset. Any sudden change in diet, for example, can severely compromise the population of gut bacteria so essential for fiber digestion—and when these bacteria start dying off, the horse is at risk of colic or, at the very least, of not getting all the nutrients out of his feed. That’s why it’s always best to make feed changes over a couple of weeks rather than suddenly.
Another trigger for digestive upset occurs when the horse receives a large, carbohydrate-rich meal (typically, one that is light on forage and heavy on grain). Under these conditions, the small intestine might not be able to process and absorb completely all of the nutrients before the meal moves on to the hindgut. When excess amounts of soluble carbohydrates reach the fermentation vat of the cecum, they are processed to produce not only VFAs but also lactic acid. An increase in lactic acid lowers the overall hindgut pH level, which in turn can make the environment hostile for the gut bacteria. Bacteria begin to die off, and in the process can release endotoxins (poisons).
Between these endotoxins and the lactic acid itself, the stage might be set for colic or laminitis. Suddenly the old horseman’s rule of feeding small amounts often begins to make a lot of sense, particularly if your horse is on a high-grain diet.
So, what do horses actually need in their diets? The basics are fresh, clean water; access to salt; and forage—lots of it. As a general rule of thumb, horses should consume between 1.5% and 3% of their own body weight in feed every day—and at least half of that (and often much more) should be forage of some kind. Whether it’s pasture, hay, or some other form of roughage isn’t as important as the quantity because the horse’s gut literally needs that amount to stay in good digestive health.
Is grain necessary? Often, the answer is no. Remember that in the wild, horses have no access to concentrated forms of carbohydrates and little need for them because they are not doing “work” in the sense that we humans demand. When we domesticated the horse, we asked him to expend energy over and above what he would normally do in the course of his wild day; grains help provide the fuel he needs to perform for us. In addition, we bred horses to be larger, stronger, faster, more elegant—and, often, less hardy and more dependent on high-energy concentrates to maintain a healthy body weight. Nonetheless, grain always should be considered an optional add-on to the diet. It should be fed as necessary only to supplement the nutrition provided by the horse’s forage and in accordance with his condition, his metabolism, and the amount of work expected of him.