evidence: Vitamin D3 is more stable and more active. As with vitamin A, vitamin D are not common in plant tissues, sun dried hay and fish oils are the main naturally occurring sources. As with Vitamin A it is good practice to supplement the diet with artificial, protected sources. Both Vitamins D2 and D3 are derived from their respective pro-vitamins in the presence of ultra- violate light. Vitamin D production therefore takes place in the skin. Animals like sheep with very thick fleeces are naturally poor at converting vitamin D so they absolutely must be supplemented wherever possible.