Vitamin D
Many compounds possess vitamin D activity, but only vitamin D2 (plant derived) and D3 (animal derived) are important dietary sources. Sun-cured forages tend to be good sources of vitamin D; however, there are large variations in vitamin D content of forages and vitamin D deteriorates over time in storage. Sunlight or ultraviolet light is needed for the animal to convert both vitamin D2 and D3 to their active form. Vitamin D3 is used more efficiently by dairy cattle. Prior to performing its function, vitamin D must be converted to a much more active form (four to five times more active) by the liver and kidneys.