We think of hay as the quintessential horse food, like birdseed for birds or peanuts for elephants. But the natural foods of horses are grasses and herbs plucked fresh from the soil. The last 50 million years have seen the genus Equus evolve into a top-class browser and grazer with the long, supple neck and large, flat molars necessary for reaching and masticating ground vegetation.
Horses are non-ruminant herbivores, which means that unlike cows, they have only a single stomach.
A horse's digestive system is designed to handle a regular intake of small quantities of fibrous food. In order to keep its delicate gut functioning efficiently, domestic horses should be fed in a manner as similar to Nature's own as possible. This sounds like an easy solution. Unfortunately, it isn't so simple as this. Not just any old grass will do.