Utilization of phytate and nonphytate phosphorus in chicks as affected by source and amount of vitamin D3.

Abstract

Commercial and laboratory-strain crossbred chicks responded (P < .01) markedly to 1α-hydroxycholecalciferol (1α-OH D₃) during the 2nd and 3rd wk of life. Bone-ash responses exceeded 50% when this compound was added at 20 μg/kg to phosphorus (P)-deficient corn-soybean meal diets containing surfeit levels (25 μg/kg) of cholecalciferol (D₃). Phosphorus excretion was decreased (P < .01) and, thus, retention was increased (P < .01) when 1α-OH D₃ was supplemented. A P-deficient (.10% P) casein-amino acid purified diet, devoid of D₃, was used to determine whether 15 μg/kg of D₃ was sufficient to facilitate optimal absorption of the nonphytate P contained in this diet. Bone ash responded to .075% P addition (KH₂PO₄), and chicks fed diets with .175% nonphytate P exhibited further bone-ash responses to 15 μg/kg of D₃ or 10 μg/kg 1α-OH D₃. Higher levels of either of these D₃ compounds did not produce additional responses. This suggested that 15 to 25 μg/kg of D₃ in a P-deficient corn-soybean meal diet (.28% phytate P and .14% nonphytate P) is more than adequate to facilitate optimal absorption of the nonphytate P present in the diet. A P-deficient casein-dextrose diet (.13% nonphytate P and 15 μg/kg D₃) was fed in the final chick assay, and chicks fed this diet did not show bone ash responses to 1α-OH D₃ or to microbial-derived phytase (1,470 units/kg). Thus, with P-deficient corn-soybean meal diets containing at least 15 μg D₃/kg, 1α-OH D₃ supplementation markedly increased weight gain and bone ash because it increased the utilization of phytate P.