Vitamins are very tiny organic molecules, but play a huge role in livestock’s normal body functions. Vitamin A comes from beta-carotene, a pigment in green plants that animals convert into vitamin A. Cows need 30,000 to 50,000 IU of vitamin A per head per day. Vitamin D is formed from exposure to sunlight or other ultraviolet light rays that animals convert into vitamin D through their skin. Cows need 20,000 IU of vitamin D per head per day.