According to Ohio State University Equine Extension, vitamin D is called the sunshine vitamin because the ultraviolet rays of the sun convert a compound in the skin into vitamin D. Horses exposed to four to six hours of outdoor light (even on cloudy days) will make sufficient vitamin D. Also, sun-cured forages contain vitamin D. If a horse is not exposed to the sun, or is fed poor-quality hay, a vitamin D deficiency could occur.
Antioxidants-Defenders of Health in Horses
Antioxidants have received a lot of attention. Everyone has heard the term, knows they are beneficial, but may not completely understand what they are or what they do-and, just as importantly to horse owners, why horses need them too. Many horse feeds now include antioxidant ingrediants in them.