Q: What is the vitamin content of grass hay? Specifically A, D, and E?
A: We are not aware of any published values for vitamins in grass hays or other forges, mostly because vitamins are generally only seen in low amounts or not at all in dried forages (hays). Plants do not actually contain vitamin A, but they do contain beta-carotene which is converted into vitamin A within the horse's gastrointestinal tract. However, beta-carotene isn't stable once the forage has been harvested, so levels decrease quickly (within weeks) until the amount left is insufficient to meet a horse's nutritional needs. Consequently, vitamin A should be supplemented when horses are fed harvested forages. Vitamin A is known for its role in night vision.
Vitamin D plays an important role in calcium absorption and similarly to vitamin A, the active form of vitamin D is not found in plants; however sun-dried forages tend to be good sources of the vitamin D precursor, which is later converted in the skin (after ultraviolet irradiation) and in the liver and kidney to the bioactive form. Hays tend to be good sources of the vitamin D precursor although there can be variations in the content of forages and the precursor will deteriorate over time in storage. Horses with normal sun exposure do not usually need additional Vitamin D supplementation.
Vitamin E is a best known for its antioxidant properties and is readily found in fresh forage, but levels quickly decline upon hay processing and storage. While extra vitamin E would have been stored in the liver of the horse that was consuming fresh forage during the summer, the levels are depleted in the fall when the horse is fed hay that has insufficient levels. Consequently, vitamin E should be supplemented if horses are fed a diet consisting mostly of dried hay. Generally, naturally derived sources of vitamin E have higher biological activity than do chemically synthesized sources.
By Krishona Martinson, PhD, and Marcia Hathaway, PhD, University of Minnesota