When feeding a horse that is housed in a stable, or stall, there are a few considerations to keep in mind before you decide on the food types you are going to give.
The first is body score, or weight. A body score of 3 is generally considered healthy. This means that you can feel the horse's ribs underneath his skin, but you cannot see them. This also means that the horse is not obese. The next thing to think about is the amount of exercise the animal is getting. If he does a lot of work he needs higher energy foods. If he stays in his stall all day, you will want to give him foods that have little caloric value, otherwise he will be fat and climbing the walls of his stall.
Now that we have determined the horse's basic food needs, we need to decide what to give him. There are two general types of food that are given to horses. The first is roughage, or hay. There are many hay types that fill different nutritional needs. Alfalfa hay is given most often, but it is also highest in energy. If your horse is not exercised frequently he doesn't need much of this. Other hay types that have less calories, such as orchard hay or grass hay, are usually given in conjunction with alfalfa to add to the volume the animal gets. Because horses are grazing animals, it is healthiest for them to eat all day long, instead of in two or three concentrated meals.
The other type of food that you can give your horse is termed "grain." This can range from actual grain, oats, bran, to beet pulp. Speak with your trainer and your veterinarian to determine if your horse needs these extra calories. It is usually only animals that are worked frequently that need these types of add-ons to their diet.
Other options that you can add to your horses diet fall under the general term supplements. These can be vitamins or oils that help to keep your horse healthy. Most horse owners give some sort of supplements, generally in the form of a powder that works as a multivitamin. But speak with your veterinarian before you give your horse any supplements, to make sure that they will be healthy for him and his workload and stage of life.
Good luck and happy feeding!