Horses, despite being herbivores that graze on plants, are not ruminants. Their digestive system operates on fundamentally different principles compared to animals like cows or sheep. While both types of animals derive energy from plant matter, the location and mechanism of microbial fermentation within their digestive tracts vary significantly.
What Defines a Ruminant
Ruminant animals possess a specialized digestive system characterized by a multi-compartmented stomach, typically with four chambers: the rumen, reticulum, omasum, and abomasum. The rumen, the largest compartment, acts as a fermentation vat, housing a population of microbes. These microbes break down tough plant fibers, like cellulose, into volatile fatty acids (VFAs), which serve as a primary energy source.
The process of rumination, often called “chewing cud,” is a hallmark of ruminant digestion. Partially digested plant material is regurgitated from the rumen back to the mouth for further mechanical breakdown, increasing its surface area for microbial action. This pre-gastric fermentation allows ruminants to efficiently extract nutrients from fibrous diets. The microbes, rich in protein and B vitamins, are then digested in the abomasum (the true stomach) and absorbed in the small intestine, providing a significant protein source.
How Horses Digest Their Food
Horses are monogastric, with a single-chambered stomach, similar to humans. Their digestive process begins in the foregut, encompassing the mouth, esophagus, stomach, and small intestine. In the stomach, initial enzymatic digestion of proteins and some carbohydrates occurs, despite its small capacity. The small intestine handles most enzymatic digestion and absorption of non-fibrous nutrients like starches, proteins, and fats.
Equine digestion uniquely occurs in the hindgut, including the cecum and large colon. Horses are hindgut fermenters, as extensive microbial breakdown of fibrous material occurs here. Billions of microbes reside in these chambers, fermenting cellulose and complex carbohydrates into volatile fatty acids. These VFAs are absorbed through the hindgut wall, providing substantial energy. This post-gastric fermentation distinguishes horses from ruminants, as microbial protein and B vitamins synthesized here are largely unabsorbed by the small intestine.
Implications for Horse Nutrition
Understanding hindgut fermentation is important for proper nutritional management. Since microbial protein produced in the hindgut is not efficiently absorbed, horses must obtain adequate protein directly from their diet. The continuous nature of hindgut fermentation means horses require a consistent forage supply.
Feeding practices should support the hindgut’s microbial population. Large meals, especially those high in starch, can overwhelm the small intestine and lead to undigested carbohydrates reaching the hindgut. This can cause a drop in pH, leading to hindgut acidosis, which disrupts microbial balance and contributes to issues like colic or laminitis. Therefore, a forage-based diet with frequent, small meals is recommended to maintain a healthy, efficient digestive system.