Despite having very little fat or oil in their natural diets, horses are able to digest relatively large amounts of dietary oil and diets of up to 15% oil are accepted and safe to feed long term. Time should be given for adaptation of digestive enzymes and gut function, however, so oil should always be introduced and increased gradually. Up to 20% of dry matter intake of oil doesn’t appear to suppress the digestibility of other nutrients in horses.
Researchers have found corn oil the most palatable for horses, but soya, linseed (flaxseed), sunflower, rapeseed (canola), safflower, coconut, peanut and blended vegetable oils are all acceptable. Oil-rich feed by-products such as copra (coconut) meal, whole processed linseed, and ricebran are also palatable for horses. Fish oils such as cod liver oil are suitable only to feed in small quantities for their omega-3 and vitamin content, and should not be fed as a substantial amount of energy intake.
For skin, hair and hoof health, linseed (flaxseed) oil is best due to its high content of the omega-3 fatty acid alpha-linolenic acid. Rapeseed and soya oils also contain alpha-linolenic acid, in lesser amounts. Feed at least 100 ml per day to a typical horse, and increase for those with sweet itch (feed a mugful or two mugfuls of micronized whole linseed).
For joint health, cod liver oil is the best choice due to its content of the omega-3 fatty acids DHA (docohexaenoic acid) and EPA (eicosapentaenoic acid), and there is plenty of evidence of its anti-inflammatory effects on joints in other species. If you prefer a vegetarian diet for your horse, then linseed (flaxseed) is the best choice, because the alpha-linolenic acid it contains is converted to DHA and EPA in the body.
For extra dietary energy for either work or weight gain, any vegetable oil will do but make sure its palatable because you need to add much more than a couple of tablespoons. For weight gain you need to add at least 250 ml per day, and you could increase up to 400 ml (for a typical 500 kg horse).
Do not feed any more than 50 ml of oil per day to an overweight horse and 25 ml per day to an overweight pony, and limit to similar amounts for horses or ponies with metabolic syndrome. Oils are calorific and increase glucose intolerance and insulin resistance in ponies (but are unlikely to in working horses).
Extra vitamin E is sometimes recommended if high levels of dietary oil are added, but if your horse’s diet has good levels of vitamin E, this is not necessary. If vitamin E is short (less than 1000 mg in the total diet of a 500 kg horse), then add 200 mg of vitamin E per 100 ml oil added.
Although some research showed performance-enhancing benefits of high-fat diets, other research did not, so more research is necessary to confirm whether or not adding oil can boost performance. Replacing starchy concentrates with vegetable oil is very useful for horses who tie up (suffer from exertional rhabdomyolysis), horses who get over-excitable, during times of stress e.g. weaning, and for endurance horses exercising in hot, humid conditions.