Horses eat grass, hay and grains with the occasional carrot, apple or sugar cube as a treat. Meat products or too many sweet treats can cause indigestion or colic. Horses also need supplements that contain salt and minerals such as a salt lick.
Although wild horses feed almost exclusively on grass and other wild plants, domesticated horses should not eat only fresh grass. Often pastures do not contain the quality of plant-life necessary to keep a horse healthy or the silica to keep its teeth strong. For this reason, horses need hay and grain supplements. An owner needs to use quality horse hay to supplement pasture feeding. Also, the horse should not have unlimited access to the hay.
Grains are another good supplement, as horses have natural access to seed grasses in the wild. The best grains for a horse include oats and small amounts of corn or grain mixtures that include bran and flax seed. An owner needs to avoid giving a horse wheat, wheat bran, rice brown or lawn clippings, as these can't be digested well and can even be poisonous to the animal. Grains, like treats, need to be fed to the horse sparingly to prevent teeth and digestive problems.