Calcium logistics in the laying hen.

The hourly changes in the Ca2+ content of various physiological compartments in relation to shell formation has been calculated for the domestic hen throughout a 4-d ovulatory sequence. These calculations consider the circadian rhythm in Ca2+ intake when the hen is fed a complete feed or a diet containing oyster shell, the effect of the stage of the ovulatory sequence and the stage of shell formation. The calculations indicate that medullary bone accrues Ca2+ during the day and that Ca2+ is drawn from the bone reserve during the scotophase if shell calcification is active. When fed Ca2+ as an identifiable feedstuff, the hen will ingest large amounts of Ca2+ at the end of the photophase. Some of this Ca2+ is temporarily stored in medullary bone and the remainder is metered into the lower digestive tract throughout the early part of the scotophase. It is suggested that the preovulatory surge of estrogens sensitizes medullary bone to the osteoclastic effect of parathyroid hormone, the production of which is stimulated by a decline in the total and ionized concentrations of Ca2+ during shell formation.