Selenium and Vitamin E function jointly in protecting body tissues from oxidation induced damage. Selenium has a sparing effect on Vitamin E by making it more efficient. Almost two thirds of the soils in the United States, and the resulting hay produced from those areas are deficient in selenium. Calcium and phosphorus are the two most important macro minerals in the horses ration. These need to be supplied at a ratio of approximately two parts calcium to one part phosphorus.
Most grass hay based rations are very low in calcium and out of balance with calcium.
Inversely, alfalfa based rations do not contain enough phosphorus. Copper and zinc are almost as important because they are related to the optimal development of the foal. Copper and Zinc are common deficiencies in most base rations. There is no way to determine from the appearance of a product its vitamin potency. Equine vitamin and minerals are not different from their human counterparts, when you mix the two together you rapidly deplete the vitamin potency.