This is a very good article on feeding fat to performance horses. Pretty much, fat stores allow the horse to work longer before the lactic acid kicks in, thus reducing and delaying fatigue. Fat for hard working horses can be fed in amounts up to 10-15% of the daily diet by weight. Many high quality feeds (like Ultium [12% fat] or Omolene 500 [8%fat] ) have high fat content already and don't need supplementation. When we are looking at having an intense ride season, I will probably begin adding 1/2-1 cup of vegetable oil (100% fat) to his diet because it takes about 4-6 weeks for the body to learn how to use the fat efficiently. In light of our impending move, our endurance season is probably going to be smaller than had been planned and Boomer will be getting plenty of fat from his Omolene 500.