It's important to remember that horses are grazing animals, programmed to chew on stemmy, fibrous plants for at least 12 hours a day.
Grazing is a full-time job for horses.
Given their druthers, they'd graze for 12 hours or more every day, their broad, flat teeth and sideways chewing motions making short work of the tough, stemmy grasses and weeds they favor. Like all true herbivores, horses get most of their daily energy requirements from eating plant fibers. Yet, ironically, horses can't digest fiber. In fact, no animal can digest fiber on its own. Animals don't produce the enzymes needed to break the beta bonds of polysaccharide fibers and make the nutrients within available for use.