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Much of the horse’s digestive anatomy is well adapted to the diet and eating style of a grazing life. Many of man’s management practices have taken the horse far from the way it evolved. Therefore, a thorough understanding of the physiology of the digestive tract is important in order to grasp how it may influence the horse’s health and well-being.

Horses evolved as continuous grazers that nibble all day and are better equipped to utilize small frequent meals rather than large meals of concentrates. A horse’s stomach is relatively small when compared to the capacity of the entire tract. Rate of passage through the stomach is such that the food remains for only a short time in this organ.

The equine stomach has two distinctive regions, the nonglandular and glandular, each of which serves a different function. The area that delineates the two regions is called the margo plicatus. The nonglandular region has a surface similar to that of the esophagus and does not have the ability to secrete stomach acids. This region of the stomach is the first one to be encountered by swallowed material. It depends on mucus and the buffers in saliva to protect it. The glandular region is where all of the stomach acids are produced.

The pH of the stomach is quite acidic because it constantly secretes digestive acids. Saliva has buffers to help raise the pH and neutralize some of the acid. Although the major impact of the stomach on food is acid hydrolysis and enzymatic digestion of protein, a small amount of lactic acid is produced from the fermentation of soluble sugars by microbes located in the glandular region of the stomach.

Because a horse cannot burp or regurgitate, anything offensive in the stomach has only one way to get out: through the rest of the digestive tract. If it is gas, the horse can be in great pain during the gas’s passage, possibly resulting in serious colic. If it is a toxic substance, it can cause considerable damage on its way through.

Most problems of the equine stomach arise from difficulties in adjusting to the feeding management humans have chosen. A significant problem with the equine stomach is a high susceptibility to gastric ulcers. Several studies have found very high incidences of gastric ulcers among certain populations of horses.

For example, 80-90% of racehorses in training have been found to be affected. The major cause has been proposed to be prolonged exposure of the nonglandular region to gastric acid, a situation caused by the way racehorses are managed. Most horses in race training are confined for much of the day and fed large grain meals. Since the saliva production from a grain meal is considerably less than from a forage meal, buffering from saliva will be minimal. Often, racehorses are fasted for an extended period before exercise, allowing gastric acid to accumulate in the stomach. Intense exercise further exposes the nonglandular stomach to gastric acid when acid splashes in the stomach during work. The majority of the ulcers found in racehorses are in the nonglandular region.

Treating ulcers involves either inhibiting gastric acid secretion or neutralizing the acid produced. Since the natural buffering mechanism in the horse is from saliva production, the most effective way to treat ulcers is simply to turn the animal out on pasture, allowing it to eat and produce saliva continuously. However, if this is not possible, try to base the diet around as much forage as possible, with grain being fed in small frequent meals. Ulcers can be treated with drugs and antacids until they are healed, but without permanent dietary or management changes, the problem may recur.

Because of the limited capacity of the stomach, it is recommended not to offer more than 5 lb (2.25 kg) of grain per feeding. Try to limit the amount of grain being fed to a minimum in order to keep an adequate weight and energy level for the horse. Make any changes in the diet gradually and keep a careful watch on the quality of the feed (both forage and grain) offered.

Want to backtrack a bit? Brush up on your knowledge of the equine mouth and esophagus here.

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