The average modern horse on good hay or pasture with light work usually does not need supplements; however, horses subjected to stress due to age, intensive athletic work, or reproduction may need additional nutrition.
Extra fat and protein are sometimes added to the horse's diet, along with vitamin and mineral supplements. Soybean meal is a common protein supplement, and averages about 44% crude protein. The protein in soybean meal is high-quality, with the proper ratio of dietary essential amino acids for equids.
Cottonseed meal, Linseed meal, and peanut meal are also used, but are not as common. Vegetable oil is a common fat source added to a ration. Corn oil is particularly popular, but other oils are used as well. Rice bran is a very good fat supplement that contains 20% fat as well as fiber and other nutrients.
Flax seed is another good source of fat, though it must be ground up for horses to digest it. Some commercial feed manufacturers now make products containing both flaxseed and rice bran. There are hundreds, if not thousands of commercially prepared vitamin and mineral supplements on the market, many tailored to horses with specialized needs.