Effects of Low Dietary Calcium on the Dynamic State of Bone Mobilization along with Shell Formation

To elucidate the dynamic state of bone mobilization along with the egg shell formation cycle, the effects of low calcium (Ca) diets on the fluctuations of the Ca and phosphorus (P) excretion were studied. Laying hens were fed a control diet containing 2.9% Ca for 7 days and then divided into two groups and fed low Ca diets (1.38% or 0.26% Ca) for 2 days. These experimental diets contained chromic oxide (Cr2O3) as an indicator. The excreta was collected every 3h on the 7th day after feeding the control diet and for 2 days following switching to the low Ca dietsThe continuous excretion of Cr2O3 was observed even during the dark period. This indicates that the food consumed before dark was maintained in the crop and gradually moved into the lower digestive tract during dark period. When hens were fed the control diet, Ca retention did not decrease and P retention decreased during later shell formation. This indicates that the excretion of endogenous P increased, because P consumption was constant and P excretion increased during this period.After switching to the low Ca diets, the eggshell weights and Ca contents decreased and Ca and P balances were negative. P retention decreased and the time when it began to decrease was hastened by feeding the lower Ca diet. This indicates that dietary Ca deficiency increase the amount of bone mobilization during shell formation and that of the excretion of endogenous P.These results suggest that the contribution of bone Ca for shell Ca changed along with the change of the balance between dietary Ca intake and the demand for shell Ca.