Why must horses eat large amounts of vegetation despite having relatively small stomachs?

Horses must consume large amounts of vegetation because their digestive systems are adapted for a process known as hindgut fermentation, which is less efficient at breaking down high-fiber plant material than the ruminant system found in animals like cattle.1

While the horse's stomach is relatively small—accounting for only about 8.5 percent of its total digestive system—the majority of its digestion occurs in the cecum and the large, sacculated colon. In these areas, bacteria ferment the fibrous portions of the vegetation to extract nutrients. Because this process requires a high volume of intake to meet the animal's energy needs, horses are natural grazers that forage almost constantly under natural conditions. This anatomical adaptation allows horses to maintain a high level of athleticism, as they do not carry the heavy, bulky digestive organs required for ruminant fermentation.213