It is not necessary to feed a fat source as a horse supplement. However, understanding what fat is and where the horse gets fat in a grass field helps.
Other words for fat include triglycerides (TAG), fatty acids (FA), omega fatty acids, MCT oil, saturated (SFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids, Omega fatty acids (Omega-3, Omega-6) and essential fats.
Fats comprise a chain of carbons with hydrogens and oxygens attached to the chain. The length of the carbon chain determines if it is a short-chain (6 or fewer carbons), a medium-chain (6 to 12 carbons and called medium-chain triglycerides or MCT oil in humans), long-chain (13 to 21 carbons including EPA) and very-long-chain (more than 22 carbons including DHA). Horses and humans can make long chains by adding together shorter chains. An example is adding to alpha-linolenic acid (18 carbon oils from seeds) 2 carbons into EPA (eicosapentaenoic acid – 20 carbons) or 4 carbons into DHA (docosahexaenoic acid – 22 carbons).
Humans have 2 “essential fatty acids” because they cannot synthesize them. They are alpha-linolenic acid (ALA – an Omega 3’s) and linoleic acid (LA – the Omega 6 fatty acids). Humans supplement their diet with flax seeds, chia seeds, hemp and several other plants to get these essential fatty acids, but there is no evidence that horses need to consume them. The current movement is to supply ALA and LA to horses in their diet via supplementation. But there is little research indicating the need, nor are there diseases showing a deficiency in these fatty acids. As far as I can understand, these “essential fatty acids” in humans are constructed in a yet unknown pathway from the short-chain fatty acids derived from bacterial digestion of cellulose. I believe any problem in horses is where they are fed grain diets throughout the year, disrupting the bacteria normally living in the hindgut, which reduces the formation of short-chain fatty acids.
The fatty acids can attach their hydrogens to each carbon (a saturated fat), to all by one carbon (an unsaturated fat), or they can skip several carbons (a polyunsaturated fat). The first carbon in a chain is called the “alpha,” and the last carbon in the chain is called the “omega.” Some carbons attach, holding the longer chains together with two bonds rather than the usual one (called “double bonds”). The location of the 1st double bond from the last carbon determines the naming. If it occurs at the 3rd carbon from the end, then the fat is called an “Omega 3 fatty acid,” and if it occurs at the 5th carbon from the end, it is an “Omega 6 fatty acid.” If there are 20 carbons with five total double bonds, then it is called eicosa- (20) -penta (5) -enoic acid or EPA. 22 carbons with 6 double bonds is called docosa- (22) -hexa- (6) enoic acid or DHA. Both of these are Omega 3 fatty acids.
Cellulose is the structural carbohydrate of plants. It is created by linking glucose (sugar) molecules into a long chain with every other glucose molecule upside down compared to the ones on each side. When the horse eats cellulose, the bacteria in the hindgut (the colon) break this chain down to form short-chain fatty acids (butyrate, pyruvate and acetate). These fats can be stored, used as fuel, or built upon to create longer chain fatty acids (fats). Feeding your horses cellulose provides them with a fat diet and is essential to their existence, assuming a healthy gut microbiota.
When used as a fuel, fatty acids yield 20 to 28 times MORE energy than glucose. The high fat from cellulose is why horses do so well on winter pasture. They are consuming plants low in starch (glucose) and high in cellulose (fat). By spring, these horses lose body fat on this diet but will be satiated, have restored the health of the cells through hormesis and will maintain their muscles. Therefore, there is no need to add any other fat to their diet.