Ok two opposing views, western vs holistic.
As already stated, the idea that you cannot feed oil (fats) because horses don't have gall bladders is horse poo-poo. Bile, the secretion made by the liver and stored in gall bladders by other species, is secreted somewhat continuously in horses. Besides other functions it helps digest fats and oils by emulsifying them.
It is an observed and measured fact that that horses digest and absorb fats efficiently. It is true that oils do not have other important nutrients so should be considered a form of supplemental energy after other essential nutreints have bee met. For more on all this see the article on Fats in the Horses Diet.
Holistic/equine nutritionists insight/reposted from barnmice: Horses And Oil Don't Mix - Barnmice Equestrian Social Community
I'm adding it because so many people are asking questions about "how to feed oil to a horse". This post is by Marijke van de Water, one of the most amazing equine nutritionists I have known. Her findings are both 'results driven' and controversial. This one in particular, goes against many of the articles you read on the web, especially the articles that the "big feed companies" put out. Remember everyone, those companies are responding to trends and marketplace, not the interest of the horse (regardless of what their pretty sites may say). If you want it, they will make it.
Now Marijke's post:
Question: I was told by an animal nutritionist that I should give my mare oil to help her coat and thin condition. I have also heard that oil is not good for horses? Why would that be? He did not recommend one over the other.
Answer from Marijke van de Water: Your animal nutritionist is recommending canola oil, corn oil, or some other vegetable oil to fatten up your horse. This recommendation is based on the fact that fats provide energy in a very concentrated form thus making it very difficult to burn off quickly. Fats are very slow to metabolize. One calorie of fat is equivalent to 3 pounds of oats or 6 pounds of hay. Very dense indeed. But is a daily feeding of 1/2 to 1 cup of oil from questionable sources a healthy cure-all to weight gain?
Vegetable oils in this form are poor-quality oils, all of which are polyunsaturated. This means that these oils are unstable with very poor resistance to heat and/or light. They easily release free electrons which are the culprits in free radical damage. The resulting cell damage is implicated in cancer, heart disease, and other chronic diseases related to aging and unless you buy organic many corn and canola crops are genetically modified as well.
It is significant that horses have no gallbladder – they don’t need one! The natural equine finds no puddles of concentrated fat in his foraging and if he/she did the reaction would be a “what in the heck was that?” lip curl. Can you imagine drinking a cup of oil without your gallbladder? I drank 1/2 cup of olive oil (twice), a relatively healthy oil, over 2 or 3 hours with my gallbladder intact and I can tell you my liver never worked so hard. The gallbladder acts as a timer that shoots bile (bile digests fat) within 20 minutes of ingesting fat. Without it the liver releases bile at random and thus becomes easily congested and overworked with excess dietary fats.
Fats slow down the normal rate that the stomach empties its food into the intestine so adding oil to grain means that the digestion of grains is abnormally slowed down, thus affecting stomach overload, gut motility, adequate enzyme activity, and energy. Horses have a small stomach designed to empty quickly. That is the reason they eat almost continuously. Replacing grain/hay calories with fat calories means a significant loss in protein, fibre, and minerals.
The key to healthy weight gain is finding the appropriate grain/hay combination for your particular horse based on breed, lifestyle, and biochemistry; correcting any nutritional deficiencies; and ensuring optimum digestive function with good enzyme activity with probiotics.