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.
