The B Complex vitamins are many different vitamins; such as Thiamine, Niacin, Folacin, Panthothenic Acid, Riboflavin and a few others. Like many of the other vitamins, of the little that is in the hay and grain, much of it is lost in the curing, milling and storage. Therefore it is essential to supplement most of the B vitamins to the horse's feed. The best known source is brewers yeast.
Vitamin E is one of the most talked about of the vitamins and has been proven to have a lot of effect on many things in life and health. It has been proven many times that it is definitely associated with an animal's reproduction ability, like increasing the conception rate of mares and lack of vitamin E can and has caused many mares to abort. The potency of a stallion can be greatly increased by sufficient amounts of the vitamin. I know from my own experience with my stallion, especially if you are breeding quite heavy, that feeding extra vitamin E three months before breeding season eliminates several mares for return breedings.