I often hear from horse owners that a high-protein diet makes their horse hot or too reactive. It is important to realize that protein is a poor source of energy so it is not the high protein but the high-energy in a diet that may cause the horse to behave differently. The most common energy sources for the horse are starches and fat. A recent study done in England tried to determine if a high-starch diet resulted in changed behavior compared to horses fed a high-fiber diet. The study involved eight mature horses. One group was fed a high-starch diet consisting of grain plus hay for 28 days, and the other group was fed a high-fiber diet with hay and alfalfa. After 28 days, the rations were switched for another 28 days. Horses were evaluated once weekly.
Results found horses on the high-starch diet had higher than average heart rates during handling. Also, horses on the high-starch diet showed more interruptions in eating than those on the high-fiber diet when an unfamiliar object was presented while they were eating. High heart rates can indicate fear or excitement and this is what many people consider making their horse hot. So it is possible that feeding high-starch diets could make your horse hot, but not high-protein diets. Also, horses are designed to eat forage as their major source of food, not grain. We feed grain because we ask them to compete athletically but a better plan is to feed mostly hay and only feed enough grain for extra calories. An even better option is to increase your horse's energy by feeding fat instead of starch because it is safer and less likely to make your horse hot.