New outlook to vitamin D functions in dairy cows: non- classical roles

In addition to the well-studied effects in regulating calcium and phosphorus balance, vitamin D has many non-calcemic effects that include acting as an immune modulator or an antioxidant. Cows acquire vitamin D either from photosynthesis in the skin or through swallowing fungi in the forage or vitamin D supplements. Although vitamin D deficiency is rare, today we are facing an increasing number of vitamin D deficiencies in cows due to the indoor housing away from sunlight exposure. According to the NRC recommendation, to maintain the vitamin D serum concentration in the range of 20 to 50 ng/ ml, it is necessary to administer 21,000 IU/ d of vitamin D in cattle. In addition, considering the involvement of vitamin D in various calcemic and non-calcemic effects, it seems that previously recommend levels of vitamin D supplementation have not been enough for preventing many diseases and disorders in cattle. Vitamin D toxicity may also occur due to over-supplementation of vitamin D or overgrazing in plants with high amounts of vitamin D metabolites. This review article will discuss various roles of vitamin D in dairy cattle health, normal physiology, and disease prevention.