Digestion From Start To Finish

This month we deal with an important nutrition fundamental--how the horse digests feed and makes use of the nutrients he eats. Although it is not necessary for you to become bogged down in the intricacies of equine digestive physiology, a basic understanding of how the horse digests feed is necessary for the selection of appropriate diets and feeding practices.

At the outset, it is useful to remind ourselves that horses evolved as forage eaters, grazing for upwards of 16-17 hours each day and traveling considerable distances as they grazed. The horse's digestive system is well suited to this feeding behavior--the stomach and small intestine are designed to cope with the almost continual entry of small amounts of food, while the large intestine is geared toward the extraction of maximum nutritional value from fibrous feeds.

Now consider how the pressures of domestication have dictated changes in a horse's diet and feeding behavior: Continual access to pasture is but a dream for most horses, and many spend a considerable part of the day in a stall. As well, our own work schedules dictate feeding programs. Rather than continual grazing, horses are often fed large meals in the morning and at night. The high energy requirements of the performance horse have necessitated inclusion of more energy-dense ingredients such as cereal grains and fats in horse diets. All of these factors can contribute to digestive upsets, some of which can be avoided by returning the horse to a more "natural" feeding situation.

Twists and Turns

The basic components of the digestive tract are similar in all mammals--the mouth (including salivary glands), esophagus, stomach, small intestine, cecum, and large colon. We can divide the horse's digestive system into two sections. The pre-cecal section (esophagus, stomach, and small intestine) in the horse functions much as it does in man, dog, and pig. On the other hand, the cecum and large intestine work like the forestomachs of a ruminant (e.g., cow or sheep)--there is continual microbial fermentation of dietary fiber. For this reason, horses are classified as hindgut fermenters. In fact, normal function of the hindgut is heavily reliant on an adequate supply of dietary fiber. This is a key point--without adequate dietary fiber, the horse is predisposed to nutritional imbalances and colic problems.

The Digestive Process

Digestion begins with the prehension of food--that is, food is grasped using the lips, tongue, and teeth. When eating tightly packed hay, larger muscles of the head and neck are also used to grab and pull feed into the mouth. After prehension, the food is chewed (mastication). This is an extremely important part of the digestive process. Digestion is most efficient when hay and other fibrous feeds are ground into small pieces rather than being bolted or swallowed in large chunks. Proper mastication of whole grains such as oats is also important to ensure optimal digestion in the small intestine. This is why it is so important that the teeth are in good working order.

Poor teeth, a common problem in older horses, will result in decreased feed intake and weight loss, particularly in horses on an all-forage diet. Quidding, the dropping of partially chewed feed from the mouth, is a sure sign of dental woes. Choke (the lodging of a food bolus in the esophagus) and impaction colic can also occur when a horse has poor dentition.

The type of feed also has a dramatic effect on the speed of ingestion. For dry hay, a horse will chew between 3,500 and 4,500 times per kilogram (one kilogram equals 2.2 pounds), taking about 40 minutes to eat each kilogram of hay. Therefore, if 26 pounds (12 kg) of hay is provided each day, the horse will spend at least eight hours eating.

When grains and other concentrate feeds are substituted for fiber in the diet, the total time spent feeding will be markedly re-duced. One kilogram of oats (2.2 pounds) can be consumed in 10 minutes or less, requiring only about 850 chews.

So, a diet of seven kilograms (15.4 pounds) of hay and five kilograms (11 pounds) of oats will decrease feeding time by two to three hours compared to an all-hay diet. Such reductions in feeding time are thought to cause boredom and other behavioral problems (e.g., stable vices). This is another reason why fiber is such an important part of the horse's diet. (For more information on stable vices, see "Stereotypic Behaviors" in the April 1997 issue of The Horse, online at http://www.thehorse.com/ViewArticle.aspx?ID=717.)

The horse produces saliva while chewing. Saliva moistens the ingesta (ingested feed), thereby easing the passage of food from the mouth to the stomach. Saliva is also rich in bicarbonate, which helps buffer the acid secretions produced in the stomach.

The nature of the feed is also important--on a dry matter basis, twice as much saliva is produced when horses eat hay or grass compared to grains and other concentrates. Diets high in grain and low in forage will therefore decrease saliva flow and result in lower gastric pH values (higher acidity), which is a risk factor for the development of gastric ulcers.

A limited amount of digestion occurs in the stomach. The stomach's main job is to further liquefy the incoming food and "feed" the ingesta into the small intestine, where digestion gets cranked up. However, gastric acid in the stomach helps break down some of the feed particles, and the enzyme pepsin initiates protein digestion.

In fact, the stomach produces gastric acid on a continuous basis--this works well when horses are grazing or nibbling on hay for much of the day because the incoming feed soaks up the gastric juices. However, if the horse is fed "meals" (morning and evening), the stomach will be empty for long periods of time. In that situation, the unbuffered acid can cause injury to the non-glandular portion of the stomach lining, with gastric ulcers being the end result.

The actual extraction and absorption of nutrients begins in earnest once ingesta enters the small intestine, a tube-like organ 60-70 feet in length. Despite this considerable length, the ingesta traverses the small intestine quickly. Some food enters the cecum within one hour, and much of the ingesta will reach this "fermentation vat" by three hours after eating. This rapid transit reflects the coordinated activity of the nerves and muscles contained within the walls of the small intestine.

Factors such as meal size, feed type, and exercise will influence transit time. Big grain meals result in rapid gastric emptying and intestinal transit and a reduction in the digestion of the available starch (more on this later). Exercise also results in a moderate acceleration of intestinal transit.

Sugar, Starch, Protein, and Fat

The small intestine is the primary site for the digestion and absorption of sugar, starch, protein, and fat. The fat-soluble vitamins (A, D, E, and K), calcium, and some phosphorus are also absorbed from the small intestine. Let's first deal with sugar and starch.

Molasses is perhaps the best-recognized source of dietary sugar for the horse--some "sweet feeds" are up to 10% molasses, although the current trend is for lower amounts. Pasture grasses are by far the most important source of sugar; a horse grazing full time could consume up to 4.4 pounds (2 kg) of sugar (nutritionists use the term water-soluble carbohydrate or WSC). Sun-cured hay has a lower WSC content, as there is loss following harvest.

Starch is the plant world's version of glycogen, the body's storage carbohydrate. A huge number of glucose molecules are linked by chemical bonds, forming a single structure. Starch is a major component of cereal grains--oats are about 50% starch, while corn is 65-70% starch.

The simple sugars in molasses and grasses are easily digested; glucose from them is absorbed directly into the bloodstream while enzymes located on the small intestine's lining make other sugars available to the body.

Starch is a slightly different story; the first step involves its breakdown to smaller sugars. Then, enzymes on the intestinal lining act on the smaller sugars until they are in an absorbable form. Amylase, an enzyme released by the pancreas when ingesta enters the duodenum, is the catalyst for the first step. Unfortunately, compared to other mammals, amylase is in short supply in the horse. As a result, the horse has a limited capacity to digest starch--the upper limit probably varies among horses, but as a general rule, a single grain or concentrate meal should contain no more than 2 grams of starch per kilogram of body weight.

Here is an example calculation: A horse weighing 1,100 pounds (500 kg) is to be fed oats. Based on our rule (2 grams of starch times 500 kilograms of body weight), the maximum amount of starch we want to feed this horse per meal is 1,000 grams. (Remember, 1,000 grams is the same as 2.2 pounds or 1 kg.) Since oats are 50% starch, a single meal for this horse can be up to 4.5 pounds (2 kg) of oats.

The starch story is further complicated by the fact that the digestibility of starch varies between the grains (see "Processing Effects on Starch" on page 72). For example, the starch in whole corn is not very digestible. Fortunately, most manufactured feeds contain grains that have been processed to greatly improve starch digestibility in the small intestine. Even so, with grain feeding (particularly with large meals), there is a risk that undigested starch will reach the large intestine (more on this later).

The digestion of protein and fat is more straightforward. Enzymes from the pancreas and those present on the intestinal lining digest proteins to their constituent amino acids, which are absorbed into the bloodstream. Even though the "natural" equine diet is very low in fat, horses can digest fairly large quantities of it. Studies have shown that horses can tolerate a 10% fat diet (total diet), although if fat supplementation is needed, there should be a gradual increase to this level to allow the digestive system to adjust.

The Boiler Room

The large intestine begins with the cecum, a structure that lies in the right flank area. This organ is three to four feet long and holds up to 15 gallons (57 liters) of fluid and ingesta. Adjoining the cecum is the large colon, the largest single structure in the digestive tract (about 40% of total capacity). Like the rumen of a cow, the cecum and large colon work like a fermentation vat. Billions of microorganisms (bacteria and protozoa) do the digestive work, producing enzymes that are able to break down the fibrous portion of the diet. This process is much more time-consuming compared to digestion in the small intestine, as ingesta dwells in the large intestine for 36-48 hours.

Dietary fiber is the portion of the ingesta not affected by the horse's own digestive enzymes. There are many (confusing) chemical and physical definitions of dietary fiber, but basically we are talking about the structural components of plant material. Some of this fiber can be digested by microbial enzymes, particularly cellulose and hemicellulose. On the other hand, lignin--another fiber form--is not digestible and will be passed in the feces. The type of dietary fiber greatly influences its nutritional value.

For example, over-mature grass hay will be relatively high in lignin, which depresses digestibility of the fiber. Other fiber sources such as young grass, beet pulp, and soy hulls are highly digestible.

The products of fermentation are the volatile fatty acids (VFAs)--acetate, butyrate, propionate--heat, water, and gas. The VFAs are absorbed into the bloodstream, providing an important source of energy for the horse. Microbial enzymes also break down undigested proteins that then enter the large intestine, although these proteins are not used by the horse. Instead, the main end product of this process--ammonia--is used by the bacteria to produce proteins needed for their own growth and survival. On the other hand, vitamin K, another product of microbial activity, is absorbed in the horse's bloodstream. As a result, in most circumstances the horse does not require vitamin K in his diet.

Another very important function of the large intestine is the absorption of water. Each day, a huge quantity of water is secreted into the small intestine during the digestive process--about 30 gallons (114 liters) for a 1,100-pound (500-kg) horse. As the ingesta moves through the large colon, much of this fluid is reabsorbed, allowing the formation of semi-solid fecal material. The final step in digestion occurs in the small colon, where waste material is formed into fecal balls that are evacuated through the rectum and anus.

The Starch Problem

Proper function of the horse's "fermentation vat" is highly dependent on the health of the microbial population. Things work well when the horse is in its natural environment eating pasture, or is fed an all-forage diet. Heavy grain feeding can upset this delicate environment, sometimes with disastrous consequences. When undigested starch enters the large intestine, it undergoes fermentation, the end product of which is lactic acid. The build-up of lactic acid decreases the pH of the cecum and colon (makes it more acidic).

A large drop in pH can radically change the makeup of the microbial population, killing organisms that cannot survive at a low pH, which might then release endotoxins by the dead bacteria. There can also be damage to the large intestine's lining, allowing absorption of the endotoxins. In the worst-case scenario, the horse can develop diarrhea, colic, and/or laminitis.

The bottom line is that the horse's digestive system functions best when it is fed a predominantly forage diet on an almost continuous basis. Problems are more likely when a horse is fed a high-concentrate, low-forage diet, particularly when given two (or even one) large meals per day. Yes, the performance horse needs more energy than can be supplied by an all-forage diet, but try to reduce the amount of dietary starch by using highly digestible fiber sources (such as beet pulp) and by adding fat to the diet. If possible, spread the daily grain allotment over more meals. Finally, allow the horse to nibble on hay (or better, pasture) as much as possible.

About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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