Feeding the backyard horse can be a delicate and demanding task. The horse's digestive system is an amazing yet poorly constructed vehicle for processing the amounts of food it needs. Although it may sound simple when you hear that grass, grain, hay, and water can provide the essentials of a horse's diet — energy, protein, carbohydrates, fatty acids, minerals, and vitamins — there's a lot more to know about what's good and what's not good for your horse.