There is a lack of experimental information on the level of
vitamins required in well-balanced horse diets, as well as on
which vitamins need to be added (Cunha 1980). The vitamins
most likely deficient for all classes of horses are vitamins
A and E, with vitamin D also being deficient for horses
in confinement.
Inadequate vitamin D may be provided to
racehorses that are exercised only briefly in the early morning,
when sunlight provides less antirachitic protection.
Requirements for vitamins A, D, and E can be met with a
high-quality (e.g., green color) sun-cured hay.