As a draft horse owner I have switched my horses to the EPSM Diet. Draft horses are highly prone to Equine Polysaccharide Storage Myopathy (EPSM), meaning these horses aren't able to derive adequate muscle energy from carbohydrates and as a result end up "tieing up". As a result, they are put on low carb/high fat diets. In order to do this without over feeding on traditional grains, oil is added to the horse's diet.
Here's a link: Beth Valentine's Recommended EPSM Diets
I currently have my Belgian colt on Blue Seal Demand, a high fat, high protein, low starch pelleted grain. (this is daily)
4 lbs Demand
2 cups Canola Oil
1 oz GrowColt supplement
13 lbs Timothy hay