They have evolved to utilize grasses and other forage plants as their primary source of nutrition.
Horses are most content when they can nibble almost constantly.
Although it's not always possible to let our domesticated friends graze to their hearts' content, one way to satisfy their urge to chew and provide essential nutrients is to feed high quality hay. Horse hay is often a mixture of the two. What is readily available and most cost effective generally depends on the part of the country in which you live. Alfalfa is more commonly fed as hay than is clover, although clover may be a component of a mixed hay. Legumes tend to be higher in protein, energy, calcium and vitamin A than grass hays.