Because Beta-carotene is gradually destroyed by light and heat most equine rations do not contain enough of this important nutrient. Hay is the primary source of beta-carotene for horses. Baled hay stored for as little as 160 days has little or no detectable beta-carotene. Unfortunately, grains are naturally low in beta-carotene as well. 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.