Horses are grazing animals. They are happiest and healthiest eating small meals throughout the day, and grass and hay should form the basis of their diet. This is true whether they are kept at pasture or in work. However, horses who are in work, pregnant, nursing or standing at stud typically need more energy, or calories, than can be provided by hay and grazing alone, even a high-quality legume hay such as good alfalfa. These horses need grain; corn and oats are excellent for this purpose.
Determine your horse's feeding requirements. Generally speaking, adult horses should eat about 1.5 to 2 percent of their body weight a day, about 15 to 20 pounds of food. Hay and grass are the most important sources of food for a horse; grain is only a supplement.
Monitor your horse's condition. Generally speaking, you should not be able to see your horse's ribs but you should be able to feel them, although race and endurance horses are very lean and muscular. If your horse is getting fat, cut back slowly on feed to avoid causing emotional and physical distress, including colic, and ride your horse more.
Add oats and corn to your horse's diet if your horse is getting thin or lacks energy to work. Feed corn for its fat, or energy value and oats for their protein content as well as for the roughage that they add to your horse's diet. This is extremely important if your horse is in heavy work and eating a lot of grain.
Feed whole corn and oats because cracked corn and oats are more expensive and less nutritious. If you find a lot of whole grain in your horse's manure, your horse either needs his teeth floated (the vet literally files off sharp hooks and edges) or he's bolting his food. Either call your vet or put large rocks or a salt block in the grain pan to slow him down.