Vitamin D₃ requirement of young chicks receiving diets varying in calcium and available phosphorus

1. Three battery experiments were conducted with broiler chicks during the 2nd and 3rd week of life. Graded amounts of cholecalciferol (D₃) were added to maize-soyabean meal diets that were designed to be (a) severely deficient in available phosphorus (P), (b) marginally deficient in calcium (Ca) or (c) adequate in both available P and Ca. 2. With diets containing 1·0 g available P and 6-3 g Ca/kg (assay 1), graded doses of D₃ between 0 and 37·5 μ/kg produced linear (P<0·05) positive responses in both weight gain and tibia ash. With a D₃ concentration of 1250 μg/kg, 250 times the requirement recommended by the NRC, bone ash was increased (P<0·05) over that of birds fed 37·5 μg/kg, and neither weight gain nor food intake were reduced. 3. With a P- adequate diet (4·5 g available P/kg) containing 8·5 g Ca/kg (assay 2), weight gain and bone ash increased linearly (P<0·05) upon supplementing the basal diet with , 2·5 and 5·0 μg D₃/kg. Higher doses of D₃ did not elicit further responses, and chicks fed on a diet containing 1250 μg D₃/kg gained as fast and had bone ash values that did not differ from those of chicks receiving 5, 10, 20 or 40 μg D3/kg. 4. When the maize-soyabean meal basal diet was fortified with Ca and P to achieve adequate amounts of Ca (10-1 g/kg) and P (4·5 g available P/kg) in assay 3, dietary additions produced results similar to those obtained in assay 2 where P was adequate and Ca was slightly deficient. Again, chicks receiving a surfeit of D₃ (1250 μg/kg) exhibited weight gains and bone ash values that were as great as those of chicks receiving 5, 10, 15 or 30 μg D₃/kg. 5. It is apparent that young chicks have a high tolerance for excess D₃, and chicks fed on diets that are severely deficient in available P continue to respond to D₃ in excess of 37·5 μg/kg.