Feeding too much Vitamin A can cause severe health problems as well. The pelleted feed, however, has twice as much as our ground feed and is within safe limits. When green forages are provided, supplementation of Vitamin A is most likely not needed. Vitamin D results from ultraviolet irradiation of ergosterol, which is synthesized in plants. Living plants are unaffected by the irradiation, presumably because chlorophyll screens out the wave length of light.
Vitamin D is found in plants after they have been cut and exposed to sunlight.
Horses synthesize Vitamin D through exposure to sunlight. A deficiency of Vitamin D is unlikely when animals have exposure to sunlight. Vitamin E and selenium requirements are interrelated. Storage of forages can affect the concentration of Vitamin E. Grinding and storage reduces the concentration of Vitamin E in feeds. We would not expect deficiencies if we feed hay along with a complete feed normally supplemented with Vitamin E. Summary Feeding good quality hay is an important nutritional practice for horse owners.