If your horse has been examined by your veterinarian and there is no reason for weight loss caused by his injury, any drugs that he may be receiving, or any other medical reason, then increasing calories should help with weight gain. Another reason horses sometimes lose weight when kept up is the expenditure of calories while engaged in stall vices such as weaving or stall walking.

One safe way to increase calories is to keep hay available at all times, which also gives the horse something to do and usually improves any stable vice behavior. Simply feeding more of what he is already eating would also boost caloric intake. I usually recommend gradually increasing the amount fed every 2-3 days to prevent any gastrointestinal upset. Fat can also be added to the diet as either rice bran or corn oil. Fats contain many more calories on a per-weight basis than carbohydrates. There are several commercially available rice bran-based pellets. Corn oil can be fed beginning at 1/2 cup twice a day for one week, then increased to 1 cup twice a day in the feed. Some horses will eat up to 2 cups of corn oil in their feed twice a day.

If the horse is not eating, it is possible that he has gastric ulcers, which can only be diagnosed with gastroscopy. A trial with omeprazole or other anti-ulcer drugs may also improve his appetite, if ulcers are present.

I recommend feeding to ideal body condition, a weight in which the horse's ribs can be easily felt but not seen. If there are no medical problems to affect weight gain, gradually increase calories with either more feed or by adding fat to get back to ideal body condition.