Horses have only one stomach, making them monogastric animals, like humans. The misconception of multiple stomachs often arises because their single digestive system processes food in distinct sections that function almost like separate organs. This anatomy is finely tuned for constant, small intakes of fibrous plant material, reflecting their natural grazing behavior.
Understanding the Horse’s Stomach
The horse’s stomach is relatively small, with a capacity of 8 to 12 liters (about 2 to 4 gallons) for an average horse. It is divided into two main regions: the upper, non-glandular (squamous) portion and the lower, glandular portion. The glandular region continuously produces hydrochloric acid to begin food breakdown.
The cardiac sphincter, the stomach’s entrance, is a strong, one-way valve that makes it nearly impossible for horses to vomit. This feature, combined with the acute angle of the esophagus, means that if pressure builds up, the sphincter tightens further. Horses cannot relieve stomach pressure by expelling contents, which can lead to serious health concerns if the stomach becomes distended.
The Horse’s Entire Digestive Journey
Beyond the stomach, the horse’s digestive system has two main phases: the foregut and the hindgut. The foregut includes the mouth, esophagus, stomach, and small intestine. After initial digestion in the stomach, food moves into the small intestine, a long tube measuring 19 to 25 meters (60 to 82 feet) in an adult horse.
In the small intestine, enzymatic digestion breaks down proteins, fats, and non-structural carbohydrates like sugars and starches. Nutrients are then absorbed into the bloodstream. Horses lack a gallbladder, so bile, which aids in fat digestion, is continuously secreted directly from the liver into the small intestine. Undigested material, particularly fibrous components, then proceeds to the hindgut.
The hindgut comprises the cecum, large colon, small colon, and rectum. The cecum, a large, comma-shaped organ with a capacity of about 35 liters, functions as a significant fermentation vat. Here, microbes, including bacteria and protozoa, break down complex fibers not digested in the foregut. This microbial activity produces volatile fatty acids (VFAs), a primary energy source for the horse, along with B vitamins and vitamin K. The large colon, approximately 3.5 meters long and holding up to 90 liters, continues this fermentation and absorbs water.
Implications for Horse Health and Feeding
The horse’s digestive anatomy and physiology dictate specific feeding practices. Since their stomach continuously produces acid and is relatively small, horses are adapted to consume small, frequent meals of forage throughout the day. This constant intake helps buffer stomach acid with saliva produced during chewing. Prolonged periods without food can leave the sensitive stomach lining exposed to acid, increasing the risk of gastric ulcers, particularly in the non-glandular region.
The hindgut’s reliance on microbial fermentation means that sudden diet changes, especially large amounts of starch or sugar, can disrupt the delicate balance of gut microbes. This imbalance can lead to an overproduction of lactic acid, lowering the hindgut pH and potentially causing health issues such as colic. Colic, a general term for abdominal pain, is a common problem in horses and can result from gas buildup, impactions, or displacements within the complex intestinal loops. Consistent access to forage and gradual dietary changes are important for supporting digestive well-being.