Fat supplementation, usually in the form of oils, will supply large amounts of energy per weight of feed. Fat supplementation is one of the most effective ways to increase weight on poorly conditioned horses. Vegetable oils are the most frequently used sources of fat. Soybean oil and corn oil are the most frequent types of oils used for this supplementation. If there is any difference in soy or corn oil, it would be minor.
Palatability of oil supplemented in feeds may be more of a concern. If a horse doesn't desire to eat corn oil, try soybean or another oil, etc. Generally, commercially prepared fat-added feeds will incorporate oil into the ration so it is palatable and will stay fresh. Mimicking levels used in commercially prepared feed mixes means adding oil at 3% to 6% by weight of oil to the total weight of the mix. Start at the lower level of 3% inclusion of oil, watch intake, and if the horse allows an increase, you may increase the amount of oil to mimic levels in commercially prepared fat-added rations.
