The oil can be soy, canola, or corn oil, although corn oil is the most expensive. Buying vegetable oil in bulk from a restaurant supply can save a lot of money. Another source of fat is powdered animal fat, which can be purchased in large quantities at a relatively low price. Be aware, however, that most of the animal fat products are designed to be fed to ruminants, and horses will not be able to digest them fully. There are, however, some powdered animal fat products designed to be fed to horses and pigs, and these are thought to be more digestible.
Rice bran can help to add some fat and also to soak up added oil, though.
There is no problem with mixing rice bran and oil, no matter what anyone might try to tell you. Some horses will not eat alfalfa pellets, at least not in high volumes, and for these horses we have looked to feeding some of the commercial diets that are higher in fat. Purina Strategy , Purina Athlete , Nutrena Compete , and some of the Blue Seal feeds have been used successfully for many EPSM horses. The manufacturers of horse feeds have recognized the value of feeding increased levels of fat to horses, and many new products are coming on the market that have an increased fat level. Maintaining a moderately high level of protein may also be important, especially in horses with muscle wasting.