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) an d slow-growing Leghorns, and (b) fast-growing chickens (Cobb) fed eith er 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 b etween 0.8 and 1.5% dietary calcium, regardless of basal growth rate. Dietary calcium levels of 1.5% and higher induced hypercalcemia and hy pophosphatemia in fast- but not in slow-growing chickens. Weight gain was unaffected by dietary calcium in the slow-growing Leghorns, but fo llowed a bell-shaped response pattern in the fast-growing Cobb chicken s. Growth inhibition by feeding of low energy diets changed the respon se pattern from a quadratic form to that of an increase towards a plat eau. The response of bone ash to dietary calcium was characterized as quadratic in fast-growing chicks, changing to a pattern of increase to wards a plateau in slow-growing chicks. Intestinal calbindin was suppr essed by dietary calcium and was higher in the fast-growing than in th e slow-growing chicks. An increase in dietary phosphorus resulted in a shift in the response curves of weight gain and bone ash and an incre ase in the calcium requirements. The results indicate that the respons e of chicks to dietary calcium and calcium requirements is markedly mo dified by growth rate.