The fat portion of the concentrate ration can be about 10 percent. So about 6-7 percent fat can be added to the ration. Normally, it takes about 3-4 weeks for horses to adjust to added fat in the ration. Feed-grade corn oil is easy to add to a grain mix. Some commercial grain mixes contain added fat.
Rice bran is a source of added fat too.
Another advantage of fat in summer rations, in addition to less feed being eaten, is the fact that fat seems to aid thermoregulation or allows horses to more easily get rid of the extra body heat produced by exercise. Often owners mistakenly feed too much protein. Protein needs, as well as that of other nutrients, is related to energy intake. As energy needs increase, so does that of other nutrients. There is probably a small increase in protein required for more intense performance.