We are commonly asked how much weight can a horse carry? Certainly this is a concern as you do not want to injure a horse by making her carry too much weight. The amount of weight a horse carries depends not only on the horse's size but also conformation, condition, age, and duration and speed of the work to be performed. A starting point is the U.S. Cavalry Manual of Horse Management: published in 1920, it indicated a horse should not carry more than 20% of the weight of the horse including rider, equipment and tack. However, until now, there was no scientific study to prove or disprove this statement.
A study was recently published from Ohio State Technical Institute that looked at eight light riding horses and had them carry from 15% to 30% of their body weight while they were exercised using a submaximal mounted standard exercise test. Horses were fitted with a wireless heart rate monitor, and blood samples were taken before and after exercise. The study revealed a weight load of 25% to 30% was found to influence work rate, heart rate, and blood tests. However, horses that carried only 15% to 20% of their body weight did not experience a difference in heart rate, respiratory rate, or work rate compared to riderless horses at exercise. This would indicate the 1920 U.S. Cavalry estimate of 20% of a maximum carrying capacity of a horse printed 90 years ago was pretty accurate. So this would indicate that a 1,000-pound horse should only carry 200 pounds and that includes saddle, rider, and equipment. The only concern is that if a horse is overweight, it does not indicate they can carry more weight because their bones and tendons are the same size, so the 20% should be figured on the horse's normal body weight.