If you are a pleasure rider or only work your horse hard occasionally, his diet will differ from that of a high-performance competition horse.
If your horse does need supplemental calories, consider adding soaked beet pulp or a small amount of vegetable oil or rice bran to his diet. These are dense in calories without the high starch content of cereal grains.
If you do incorporate grain into your feeding program, use only what your horse needs to maintain his weight, and never more than 5 pounds per meal. Grain mix formulated for horses has the advantage of including vitamins and minerals at the levels an adult horse requires.
If your horse is an easy keeper that tends to gain too much weight, ration balancer pellets are available to provide vitamins and minerals without the extra calories of grain.
If your horse is insulin-resistant, prone to laminitis, has Equine Metabolic Syndrome or any other problem requiring special nutritional management, talk to your vet about diets that are safer for horses with these needs.