Physiological FactorsBreed plays greatly into the horse's natural body condition. For example, you would most likely have a difficult time ever getting a Thoroughbred be as thick as a Percheron. The reason being that Thoroughbreds are usually a taller, racier-looking breed. Along the same lines, comes temperament. Thoroughbreds are usually going to be a lot more energetic than Percherons, and therefore they are going to burn more energy. For these reasons, your Thoroughbred is going to need a higher energy intake (per pound of body weight) than your Percheron to gain and maintain weight.
Age also plays a huge factor in your horse's ability to gain and maintain weight. As a horse ages, his or her ability to digest feed decreases. You can't expect a 30-year-old horse to metabolize his hay and grain as well as a horse that is 15 years old. Therefore, you should provide him with high-quality hay, more concentrates, and higher levels of protein. Providing your elderly horse with probiotics and live yeast cultures will aid in digestion and allow him to get more out of his feed. Metabolizing more of the high-quality feeds will help your older horse gain and maintain weight better.
Maintenance FactorsWhen helping your horse gain and maintain weight there are many maintenance issues that you need to consider. First, have you had your veterinarian check his teeth lately? If his teeth are bad, then he cannot chew his feed efficiently; the first step in breaking down the hay or grain to be metabolized. Teeth should be checked once to twice a year. The next maintenance step is feeding your horse a complete vitamin-mineral supplement, which is crucial in addressing vitamin and/or mineral deficiencies so he is better able to gain weight. Vitamin E is an example of a vitamin that affects weight, as it is a fat-soluble vitamin. If there is not an adequate amount of fat stores in the body, vitamin E cannot be stored at adequate levels. Another issue that is crucial in reaching an ideal body condition is a routine deworming program recommended by your veterinarian. If your horse has parasites in his gut, they are taking nutrition from your horse and not allowing him to get the needed nutrients for weight gain. A proper deworming program will allow your horse, and not the parasites, to get the nutrients he is consuming.
When considering feed for your thin horse, energy density is key to weight gain. Feed your horse the highest-quality forage possible. Cool-season grass hays and alfalfa are usually higher in energy and protein than warm season grass hays. Also, allow him access to all the hay he can eat. This will allow him to get the most calories from hay as possible. When choosing a concentrate, consider high-fat feeds or adding a cup or two of vegetable oil to their grain because fat has more calories ounce for ounce than carbohydrates. If you are feeding your horse a lot of grain, make sure to give him no more than 5 pounds in one feeding. This will help ensure the carbohydrates won't bypass the small intestine and wreak havoc on the microorganisms in the hind gut.