Vitamin D is required for the normal calcification of the growing bone and promotes the calcification of the eggshells of birds as well as maintaining blood plasma calcium and phosphorus levels and aiding calcium absorbtion from the intestine.

Many factors, the length of time an animal spends in the sunshine; its living habits and intake of calcium and phosphorus, all affect its dietary requirements.

Over a long period of time a deficiency of vitamin D in the young will lead to retarded growth and rickets, and to osteomalacia (lack of mineralised bone) in the adult animal, especially during reproduction and lactation.