Oats are considered to be the top choice for horses because they are the most nutrient balanced grain and have the least amount of starch of all whole grains.
Whole oats have about 11-13% protein and 5% fat and are low in vitamins D, E, K, and several B vitamins, horses should be able to supplement vitamin D from the sun or hay. Horses manufacture vitamins B and K in their hind-gut. Research has shown that oats contain high levels of other antioxidants that act to fortify the Vitamin E.