While you may believe this is a easy thing to do - feed your horse - you’d be surprised at the amount of horse owners that don’t know about the fundamental principles. There is no real rule of thumb for feeding, as all horse’s nutrition requires will vary depending on age, size and level of activity. Grass is the most fundamental part of a horse diet. Being one of the most essential factors to keep its digestive system to function optimally, grass ordinarily means natural pasture and cut hay.