Digestive Anatomy and Physiology of the Horse
Equine Nutrition
Horses are non-ruminant herbivores, meaning they eat mainly plant material. The horse’s gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine and the highly developed large intestine composed of the caecum, large colon, small colon and rectum (figure 1). The Mouth Anatomical features of the mouth include the teeth, tongue and salivary glands.
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Equine Science
Horses are non-ruminant herbivores, meaning they eat mainly plant material. The horse’s gastrointestinal tract consists of the mouth, esophagus, stomach, small intestine and the highly developed large intestine composed of the caecum, large colon, small colon and rectum (figure 1).

The Mouth

Anatomical features of the mouth include the teeth, tongue and salivary glands. Digestion of feeds begins when food enters the mouth. The horse chews reducing feed particle size and mixing it with saliva to begin the digestive process. Saliva acts as a lubricant to provide easier passage through the esophagus and buffers acid in the stomach. Once swallowed the bolus of feed moves from the esophagus to the stomach.

The Stomach

The stomach of the horse is the smallest unit of the digestive tract with a capacity of approximately 2-4 gallons, comprising around 10% of the total volume of the horse’s digestive tract. The horse has the smallest stomach in relation to body size of all domestic animals. Due to the small capacity, smaller, frequent meals are recommended. The stomach’s main functions include mixing, storage and controlled release of feed into the small intestine; and secretion of pepsin to begin protein digestion. Very little absorption of nutrients occurs in the stomach. Once feed is released from the stomach it enters the small intestine.

Small Intestine

The horse’s small intestine is approximately 70 feet long, comprising 30% of the total digestive system. The passage of feed through the horse’s small intestine is rapid, moving at approximately 1 foot/min and delivering the digesta to the cecum in as little as 45 minutes after a meal. volume of feed consumed and rate of passage affect digestion and absorption of nutrients – larger volume and increased rate of passage will decrease digestion and absorption

In the small intestine a majority of non-structural carbohydrate (starch), protein and fat is digested by enzymes and absorbed. Starch is digested by amylase enzymes, oil is digested by lipase enzymes and protein is digested by protease enzymes. These enzymes, which are produced either in the pancreas or the small intestine, reduce starch into glucose, fats (oil) into glycerol and fatty acids, and protein into amino acids.

The digestion of oils and protein is extensive in the small intestine. The digestion of starch can often be incomplete due to the starch present in cereal grains being protected by the grains seed coat. If starch is not digested in the small intestine it will be delivered to the hindgut where it will be rapidly fermented by bacteria, causing lactic acid production and accumulation, hindgut acidosis and diseases such as colic, metabolic acidosis and laminitis/founder.

Hindgut

The hindgut of the horse comprises the cecum, large colon, small colon and rectum. The cecum consists of 12-15% of tract capacity and the colon 40-50% of tract capacity. The major functions of the hindgut are the microbial digestion (fermentation) of dietary fiber (structural carbohydrates primarily from forages in the horse’s diet). Important end-products of the fermentation are volatile fatty acids (acetic, propionic and butyric) which can serve as an energy source for horses fed mostly forages such as pasture or hay. Fermentation also produces methane, carbon dioxide and water, as well as most of the B-vitamins and some amino acids. Another function of the hindgut is water reabsorption.

The diet composition affects the makeup of the microbial population. When starch is delivered to the hindgut the starch fermenters (amylolytic bacteria) begin to rapidly ferment the starch, producing large quantities of lactic acid and volatile fatty acids (VFA). Because of the acidic nature of these products of fermentation, the pH in the hindgut begins to fall. A low pH favors pathogenic bacterial which can then contribute to serious diseases such as, laminitis or founder, colic, endotoxemia and metabolic acidosis.

Figure 1. Equine Digestive System

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