The main precursor to vitamin A found in plants and consumed by ruminants is beta-carotene. Conversion of beta-carotene. To vitamin A occurs in the mucosal cells of the small intestine. High-quality forages contain large amounts of vitamin A. Precursors; however low-quality forages, or forages that have been stored for several months have low levels of vitamin. A activity. Whole milk is also a good source of vitamin A.