Vitamin D is a group of related compounds that are obtained directly from the diet or from irradiation of the body. The two major natural vitamin D sources are vitamin D2 (ergocalciferol) which is obtained from plants, and vitamin D3 (cholecalciferol) which comes from animal sources. The dietary requirement for vitamin D3 in ducks is 200 IU/kg.
Although Vitamin D is normally produced by ultraviolet (UV) light acting on 7-dehydrocholesterol in the skin, it can also be absorbed from feed. Season, geographic latitude, time of day, cloud clover, smog, and skin melanin content are among some of the factors that affect UVB radiation exposure and vitamin D synthesis. Complete cloud cover reduces UVB energy by 50%; shade reduces it by 60%.
Vitamin D is essential for regulation of calcium and phosphorus, bone physiology, cell proliferation and epithelial integrity. Without adequate amounts of vitamin D in a duck's diet, calcium doesn't get absorbed well, resulting in calcium deficiency. This can cause a drop in egg production or abnormal egg shells even if the bird is fed sufficient calcium in their diet.
Growing ducklings with inadequate levels of vitamin D develop hypocalcemia or rickets, resulting in stunted growth. Vitamin D deficiency has also been linked to cardiovascular disease, skin conditions, cancer, and autoimmune disorders.