Digestion in the Horse
By Bradford G. Bentz, VMD, MS, Dipl. ACVIM, ABVP (equine) • Jan 18, 2014 • Article #33228
The horse’s evolution as a forage eater helps in understanding its digestive system, which is designed for continuous grazing of grass forages. The stomach and the small intestine can receive a nearly continuous flow of small amounts of food.
The large intestine has been adapted to extract extra nutrition from the fiber content of the forages that pass through the small intestine.
Domestication of the horse is at odds with an intestinal system well developed for continual grazing. Convenience to owners, modern equine athletic activities, and space limitations dictate modern feeding practices and force horses to receive more concentrated feeds at infrequent intervals, harvested and processed forages, and reduced access to pasture that permits natural grazing.
Cereal grains and fats have, therefore, been artificially increased in the diets of domesticated horses. Because the intestinal tract is not “designed” for this type of feeding, we see more digestive disturbances in horses receiving these modern management and feeding practices.
Although the anatomy of the equine intestinal tract is not dissimilar from that of other mammals, its organization and physiologic function differ. From the mouth to the beginning of the large intestine at the segment called the cecum, the digestive tract functions similarly to that of humans.
However, the horse has a comparatively reduced capacity for digestion. Salivary digestion of carbohydrates occurs in humans and other species, but such digestion is minimal in horses.
Beyond the cecum, the large intestine functions more like that of the forestomachs of a ruminant such as a cow. In the cecum and large intestine there is continual fermentation of dietary fiber. Normal function of the hindgut (intestinal tract beyond the small intestine) of the horse is highly dependent on an adequate source of dietary fiber, and without it the horse is at risk of developing various dietary imbalances.
As in humans, a horse’s digestion process begins in the mouth, where grasping of food and manipulation and chewing by the lips, tongue, and teeth allow grinding of feed into smaller pieces. This is particularly important for efficient digestion of fibrous feeds such as hay and for grinding and digestion of whole grains.
For this reason dental care is important to the horse in order to facilitate adequate digestion of feeds and to maintain body condition. Dental problems may lead to dropping of clumps of feed material from the mouth (quidding) and may predispose horses to choke (esophageal obstruction) and impaction colic.
Rate of digestion is determined by feed type. For instance, horses may take more than a half hour to eat one kilogram of hay, while the consumption of a similar amount of concentrated feed will take as little as 10 minutes. This translates into a significant difference in the amount of time the horse on a concentrated diet versus one on forage or a pasture diet spends eating.
Less time spent eating reportedly has been associated with increased boredom and development of vices. Chewing produces saliva. Because eating hay requires more chewing time, saliva production for hay is greater than for grains or concentrate. Saliva, which is high in bicarbonate, moistens the feed and helps to buffer acid secretions in the stomach. Therefore, diets containing adequate hay and/or pasture forage produce higher levels of salivary secretion and decrease the risk of developing gastric ulcers.
Saliva production is nearly two times greater for hay or grass than for grains and concentrates. Once the food enters the stomach, digestion begins. However, only a limited amount of digestion occurs in the stomach. In reality, the stomach primarily functions to liquefy the feed in preparation for passage into the small intestine.
The limited digestion that occurs is primarily for initial breakdown of proteins by an enzyme called pepsin. Because the stomach produces acid continuously, continual grazing permits increased protection from gastric ulcers by the bicarbonate introduced from salivary secretions. Continual grazing also permits absorption of gastric juices by feedstuff that is always in the stomach.
Horses that are fed concentrated dietsand do not graze in between the concentrate “meals” probably have long periods of time when there is little or no bicarbonate being introduced from the saliva. Nor do they have feed in the stomach to absorb the gastric juices.
This type of feeding can predispose horses to ulcer development. True extraction and absorption of nutrients begin in the small intestine. Ingesta (the liquefied feed material released from the stomach into the small intestine) passes through the small intestine rather quickly. Some ingesta may reach the cecum in one hour, and most will reach this site by three hours after ingestion.
Things such as meal size, type of feed, and activity level can influence the transit time through the small intestine, the primary site for digestion and absorption of sugar and starch (a complex sugar). The most important source of sugar in the horse’s diet comes from pasture grasses. A significant source of dietary sugar may also come from sun-cured hay, but hay forage has an overall lower sugar content relative to pasture grass.
Some sweet feeds contain up to 10% molasses; therefore, another major source of dietary sugar may also be in the form of the sugar present in molasses. Starch is a complex form of a carbohydrate in plants that is broken down to produce sugar.
A tremendous number of sugar (glucose) molecules make up the complex structure of the starch molecule in plants. Therefore, the breakdown (digestion) of starch in feed releases large amounts of sugar for absorption.
Starch is a major component of cereal grains. Oat sare about 50% starch and corn may be up to 70% starch in content. The simple sugars in molasses and grasses are easily digested by the horse. However, starch, because of its molecular complexity, requires breakdown into less complex sugars that can be further broken down into simple sugars before they can be absorbed, such as the simple sugars found in molasses and grasses.
Amylase is an enzyme that is released into the duodenum from the pancreas (through the pancreatic duct) that initiates the digestion of complex sugar molecules such as starch. However, amylase is produced in limited amounts in the horse, relative to other species. Therefore, the small intestine of the horse can become overwhelmed by excess dietary starch.
As a general rule, a single grain or concentrate meal should be no greater than 5 pounds in weight. Furthermore, the digestibility of starch also varies among different types of grain. For instance, starch in corn is rather poorly digestible. However, grains in most commercially produced feeds are processed to improve the digestibility of starch within them.
Despite this processing, there is always significant risk that with large grain meals undigested starch may reach the large intestine. This can be associated with digestive disturbances in the large intestine. Furthermore, heavy grain meals result in rapid transit through the stomach and small intestine. Increased rapidity of transit through the small intestine reduces time for the small intestine to digest and absorb available starch. Therefore, in addition to the increased levels of starch in heavy grain meals, the transit of starch to the large intestine is further facilitated by reduced time for starch digestion.
Pelleted and ground feeds move through faster than hay and grass feeds. The fat-soluble vitamins (A, D, E, K), calcium, and phosphorous are also absorbed in the small intestine and a horse’s daily requirements of vitamins and minerals are usually met when it ingests a minimum of 3 pounds of a commercially produced concentrated feed. However, for many horses these requirements can be met simply by feeding high-quality hay or having access to good pasture.
Fat and protein digestion also occurs predominantly in the small intestine. Enzymes from the pancreas and in the lining of the small intestine are capable of digesting proteins to their individual amino acids, permitting their absorption into the bloodstream. The horse’s diet is usually relatively low in fat, yet horses do have the capacity to digest and absorb large quantities of this nutrient.
Studies of fat in the equine diet have indicated that horses can tolerate up to 10% of their total diet as dietary fat. Once ingesta has passed through the small intestine, the material moves into the large intestine. This begins with the cecum. Attached to the cecum is the remainder of the large intestine (described in the anatomy section).
Similar to the rumen of a cow, the cecum and large colon are “fermentation vats” where microorganisms including bacteria and protozoa perform much of the digestion by producing enzymes capable of breaking down fiber. This digestion, which does not occur in humans, enables any fermenting species to breakdown structural sugars in the fibrous portion of the diet.
The process takes much more time than the digestive process described for the small intestine. Ingesta that enter the large intestine may remain there for up to 48 hours before being passed as fecal material. The dietary fiber in the feed is not capable of being digested by the horse’s own digestive enzymes.
Dietary fiber is primarily made up of the structural components of plant material; mammalian digestive processes cannot use this energy source. However, because of the symbiotic relationship between the microorganisms and the horse (or other fermenting species), cellulose and hemicellulose that exist in plants are broken down and available for use as energy.
Lignin, another form of fiber, cannot be broken down by fermentation. Therefore, it is passed in the feces. For this reason the type of dietary fiber influences its nutritional value. Overly mature hay will have relatively high amounts of lignin, which reduces its digestibility and, therefore, its value as a dietary source of nutrients and energy. However, young hay, beet pulp, and soy hulls have much less lignin and much more digestible fiber. They are, therefore, much more valuable to the horse as an energy source.
The fermentation process leads to the production of a group of compounds called volatile fatty acids. They are produced by the digestion of the digestible dietary fiber. These volatile fatty acids are acetate, butyrate, and propionate. In addition to these compounds, heat, water, and gas are also produced. These volatile fatty acids can be absorbed into the bloodstream, where they are an extremely important source of energy for the horse. Vitamin K is a by-product of the activity of the microorganisms in the large intestine. It becomes available to horses for absorption by their activity. Horses, therefore, seldom require vitamin K in their diet.
The microorganisms in the large colon and cecum also break down protein that enters the large intestine. However, this protein is not used by the horse; rather, the end product of this break down is ammonia. Ammonia is then used by the bacteria to produce protein required for the bacteria’s growth and survival— thus the symbiotic relationship (both the bacteria and the horse benefit). The digestive process essentially ends at this point in the large intestine.
The remainder of absorption that takes place is primarily absorption of water in order to recover the fluid secreted to aid digestion and passage of ingesta. The end result is the formation of concentrated fecal balls of waste left over from the digestive processes.
Having reviewed some of the anatomically and physiologically significant factors that are important in the process of digestion and intestinal transit of feedstuff through the equine gastrointestinal system, you will be better able to understand many factors that may be controlled to help prevent colic and some of the processes that may become disrupted when a horse experiences an episode of colic.