If a horse loses weight rapidly due to illness or other reasons, it may be important to get it back up to weight in a hurry. An underweight horse is not likely to be very healthy, so knowing how to fatten up a horse when necessary is an important part of the knowledge required by any horse owner. It is important that horses stay reasonably active when you increase their weight so that they stay strong and healthy while they increase in size.
Have your horse thoroughly investigated by your veterinarian. It is always important to have a vet look at your horse before you increase its weight through feed, because its low weight may be due to a medical condition. The vet will look for conditions including gastrointestinal and dental problems. You need to keep up with a good deworming program as well, because if worms occur, they can prevent the horse from maintaining a proper body weight.
Give an underweight horse a feeding area that is separate from the other horses that you keep. Horses often compete for food, and if a younger, stronger or more aggressive horse wants the food, it can be difficult for a potentially weaker and underweight horse to get the share that it needs.
Add fats to the horse's diet. Corn oil is often used to supplement a horse's diet when weight gain is desired. When you supplement a horse's diet with oil, introduce it gradually. Starting with a large dose can make the horse sick. Start with as little as 1/4 cup, and then increase the dose gradually about once a week. As little as 1/4 cup twice a day can be helpful for weight gain. The maximum suggested dose is 2 cups a day, according to the Shadowood website.
Verify that you are feeding the horse high quality feed. One of the common causes of a horse losing weight is being fed low quality hay or not finding enough high quality feed in the pasture. If this is the case, you need to supplement the feed with a better quality hay, move the horse to a better pasture or supplement the diet with additional grains or feed, such as alfalfa.