Very basically a horse gets most of his nutrition from forage (hay or grass) they should receive 2% to 2.5% of their body weight in forage. Any kind of grain is an artificial food that we use to make up for the extraordinary things we ask them to do (race, jump, herd cattle, etc) the harder they work the more they need within limits. A horses stomach is the size of a basketball but they have 100 ft of intestines, that makes it very easy to mess them up through food.
Horses need different levels of feed and nutrition in their diet depending on age, current weight, activity level and overall health. An equine's stomach is a delicate balance of getting the right proteins, grains, hay, supplements, water intake and proper digestions of these substances. With literally hundreds of different feed types available, this can be quite challenging for a new or even veteran horse owners.