Numerous research studies have been carried out on horses' taste preference. What researchers have found is that the sense of taste in horses is probably not as important as the sense of smell, and it is difficult to separate behavioral responses that are due primarily to taste from responses caused by the olfactory sense. Horses use their sense of taste to tell one feed from another. When presented with a variety of feeds, horses will select certain feeds over others. In practical situations, such as under grazing conditions with multiple forage species present, the horse will select different types and species than either sheep, goats, or cattle.
Studies were conducted to determine if a horse will eat feeds that provide their nutrient needs. The research has shown that horses are not "nutrition wise." In most cases, horses are unlikely to balance their own rations when provided a variety of feeds. Horses also will consume feeds at a level far higher than necessary to meet their nutrient needs.
From eXtension HorseQuest