Calcium Metabolism and Skeletal Dynamics of Laying Pullets

Twenty-four pullets were fed a constant specific activity ration for 26 days, after which an identical but Ca45-free ration was fed for 29 days. The specific activity of the egg shell calcium was used to determine calcium balance and parameters of skeletal metabolism. Of the calcium intake 78% was absorbed, 8% was excreted as endogenous calcium and 70% was retained. Shell formation caused a significant increase in calcium retention and a shift of calcium from apposition in bone and endogenous excretion to mobilization from bone and shell deposition. From 4.3 to 4.9 g of the skeletal calcium participated in egg shell formation, of which 1 g was turned over daily. The size of the exchangeable bone calcium pool was related to the quantity of shell produced and was larger in pullets with a negative calcium balance than in pullets with a positive balance. The quantity of calcium removed from the skeleton per day was positively correlated with the size of the exchangeable calcium pool, the quantity of calcium absorbed per day and shell mass. The correlation coefficient between the specific activity of medullary bone and egg shell calcium was 0.845, confirming the important role of medullary bone in shell formation.