The Equine Digestive System

The Equine Digestive System

The horse has a relatively small stomach, about the size of the football. The small stomach lends itself to the horse being a “trickle feeder”; horses prefer small meals often.

Photo: Photos.com

The horse has a complicated digestive system. It has a relatively small stomach, about the size of the football. The small stomach lends itself to the horse being a “trickle feeder”; horses prefer small meals often. Food does not stay in the stomach long, but a rapidly eaten large meal does have the potential to cause gastric rupture.

After leaving the stomach, food enters the small intestine (with segments of the small intestine being the duodenum, jejunum, and ileum). Here, a multitude of enzymes breaks most feedstuffs into small particles for absorption into the bloodstream. After leaving the small intestines, the particles that remain reach the large intestines. The horse has a unique section of the large intestine called the cecum. The cecum is approximately 3 to 4 feet in length and is important for fiber (cellulose) digestion. Also within the large intestine are the large and small colons. These are also important for fiber digestion as well as for water absorption.

From a digestive standpoint the small intestine easily digests carbohydrates (starches and sugars) but cannot digest more complex ones such as fiber. As introduced earlier, mammals do not have the enzymes required to break the special bonds between fiber’s monosaccharide units. Therefore, herbivores such as horses and cattle have formed symbiotic relationships with microbial organisms such as bacteria that possess these enzymes.

This microbial population resides within the large intestine (cecum and large colon primarily) of the horse, while they reside in the rumen of cattle and other ruminant animals. These microbes actually ferment fiber into volatile fatty acids (VFAs). These volatile acids include primarily acetate (acetic acid), propionate (propionic acid), and butyrate (butyric acid), but lactate (lactic acid) may also be produced.

After absorption, the metabolism of acetate and butyrate is similar to fatty acid chains, and propionate is metabolized to glucose in the liver. Ultimately all volatile fatty acids can be metabolized to adenosine triphosphate (or ATP). It should be noted that during the fermentation process, in addition to VFA production, some energy during the fermentation process is given off as heat (which is important during the cold winter months) or gas and therefore lost to the animal. Therefore, carbohydrates such as starch and sugar are digested more efficiently in the small intestine and ultimately provide more digestible energy (per unit weight) for the horse than does fiber (because some energy is lost during fermentation prior to producing the VFAs).

It should be pointed out that horses have an upper limit to the amount of simple starches and sugars their small intestines can digest. So, if a horse consumes a large amount of sugar and starch, there is a chance some starch and sugar would not be digested properly in the small intestine and would pass through to the large intestine. Here, the microbial organisms rapidly ferment the starch and sugar, producing excessive amounts of gas and acids (namely lactic acid) that can ultimately cause digestive upset, or colic, for the horse.

Beet pulp is unique in that it actually has high levels of fiber but is highly digestible, generating a moderate value of digestible energy per unit weight.

Photo: The Horse Staff

Feed Types

Fats are well digested by the horse, so that the digestible energy of something such as vegetable oil is very similar to its gross energy, making it a very good energy feed. Protein is variably digested, depending on the feed source. Feeds high in fiber, such as hay, have lower digestible energy values (or the amount of energy digested—and therefore available—by the horse after energy is lost through feces) compared to grains and fats. This is because fiber (cellulose) is fermented in the large intestine rather than digested in the small intestine, and fecal (or gas and heat) energy losses are high. Cereal grains such as oats, barley, and corn that are high in carbohydrates such as starch and sugar are easily digested in the small intestine and have lower fecal energy losses. Therefore, grains have more digestible energy per unit weight than forages.

Beet pulp is unique in that it actually has high levels of fiber but is highly digestible, generating a moderate value of digestible energy per unit weight. Oil is highly digestible and produces a lot of energy (ATP), giving it the highest value of digestible energy per unit weight.

Editor's note: See The Energy Content of Horse Feeds on TheHorse.com for more information on energy and horse feeds.

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