Blood ionized calcium cycles in the chicken

Changes in blood ionized calcium concentration (Ca++) appear to be dependent on the physiological state of the chicken. In the laying hen, blood Ca++ levels were affected by the ovulatory cycle; Ca++ was elevated when the shell gland was empty, declined following entry of an egg into that organ, and reached a minimum level approximately 16 hr prior to the subsequent oviposition. Pullets not in laying condition had blood Ca++ that varied with the photoperiod and/or rate of feed intake. In those pullets, blood Ca++ levels were depressed during the 9.5 hr of light, then rose in plateaued during the dark period. These studies indicate that blood Ca++ in laying hens is affected by the reproductive state first and the photoperiod and/or pattern of feed intake second.