It also functions in the absorption of several minerals for bone deposition. Vitamin D is converted from precursors through a series of reactions in the skin stimulated by sunlight. Rickets in young horses and osteomalacia in older horses are the two most common symptoms of Vitamin D deficiency. Giving large doses of Vitamin D should be avoided as toxicity may occur resulting in calcification of soft tissue. Natural sources of Vitamin D occur in sun-cured hay and cod liver oil.
Vitamin E is found in ample quantities in most natural feedstuffs to meet the horse's requirement.
Roughages, cereal grains and especially cereal germ oils are high in Vitamin E, particularly wheat germ oil. Vitamin E has been implicated in many physiological functions in the horse body. It maintains membrane stability and red blood cell integrity. Selenium and Vitamin E interactions may play a role in treating and preventing tying up, and possibly in assuring normal reproduction. It is believed that the microflora in the cecum will synthesize adequate amounts of B vitamins for absorption to meet the horse's requirement.