Calcium metabolism and requirements of chickens are affected by growth.

The interaction between growth and calcium homeostasis was studied by comparing the responses of (a) fast-growing broiler chickens (Cobb) and slow-growing Leghorns, and (b) fast-growing chickens (Cobb) fed either high energy (12.13 kJ/g) or low energy (9.2 kJ/g) diets, to dietary calcium concentration ranging between 4 and 20 g/kg). Plasma calcium increased as dietary calcium increased, reaching an apparent plateau between 0.8 and 1.5% dietary calcium, regardless of basal growth rate. Dietary calcium levels of 1.5% and higher induced hypercalcemia and hypophosphatemia in fast- but not in slow-growing chickens. Weight gain was unaffected by dietary calcium in the slow-growing Leghorns, but followed a bell-shaped response pattern in the fast-growing Cobb chickens. Growth inhibition by feeding of low energy diets changed the response pattern from a quadratic form to that of an increase towards a plateau. The response of bone ash to dietary calcium was characterized as quadratic in fast-growing chicks, changing to a pattern of increase towards a plateau in slow-growing chicks. Intestinal calbindin was suppressed by dietary calcium and was higher in the fast-growing than in the slow-growing chicks. An increase in dietary phosphorus resulted in a shift in the response curves of weight gain and bone ash and an increase in the calcium requirements. The results indicate that the response of chicks to dietary calcium and calcium requirements is markedly modified by growth rate.