Though exercising will help to build muscle, a key component is amino acids; in particular Lysine, Methionine and Threonine. These three amino acids are limited in the equine diet and in proper amounts have proven to increase muscle size by up to 10%. Also, keep in mind that as you increase your horses workload you will increase his caloric requirement and need to feed a more calorie dense diet. The average 1,000lb. Maintenance horse requires approximately 16,500kcal/day(including forage sources). When you start riding them, a moderately exercised horses requirements jump to approximately 23,000.