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The effect of age on egg production was investigated in the domestic fowl. The rate of egg production was reduced with increasing age and the incidence of thin-shelled and cracked eggs was markedly increased. Older hens which remained in lay produced fewer but larger eggs than the younger birds. The ovaries of these two groups of hens were of similar appearance but the largest preovulatory follicle and the oviduct were significantly heavier in the older birds. No differences in the circulating levels of progesterone or estradiol were apparent between the young and old birds but the activity of the renal 25-hydroxycholecalciferol 1 alpha-hydroxylase was found to be significantly reduced with age. Circulating levels of total and ionized calcium were very similar in the young and old laying birds, plasma ionized calcium levels being markedly depressed during egg shell calcification in both groups. The decreased rate of ovulation in the older birds and the increase with age in the size of the follicles ovulated is thought to be associated with a reduced rate of recruitment of follicles for rapid growth followed by a prolonged period of follicular growth and development. The poor calcification of egg shell in old birds would appear to be due to some dysfunction of the shell gland, possibly associated with reduced synthesis of 1,25-dihydroxycholecalciferol by the kidney. The maintenance of plasma ionized calcium concentrations does not appear to be of primary importance in this respect.

The concentrations of biologically active parathyroid hormone (PTH) and ionic calcium (Ca2+) were determined in the plasma of control and calcium-deficient laying hens during the egg cycle. The same measurements were made in hens that had ceased laying because of a deficiency of vitamin D and in normal hens during pauses in egg production. Changes in the concentration of Ca2+ during the egg cycle followed the same pattern and were of the same magnitude in the control and calcium-deficient birds: mean levels were high when egg shell calcification was not occurring and they were at least 0.10 mmol/litre lower during active shell formation. There was a reciprocal relationship between the concentrations of PTH and Ca2+; thus, PTH values were high during shell calcification and low before shell formation began and after oviposition. Mean concentrations of PTH in the birds given the low-calcium diet were more than twice as high as in the control birds both during shell formation at 0700 hr (12.85 and 5.95 pg/ml, respectively) and after oviposition at 1600 hr (4.25 and 0.55 pg/ml, respectively). The results of these experiments provide direct evidence that the mobilization of skeletal calcium for egg shell formation is under parathyroid control. In the non-shell-forming control and vitamin D-deficient birds there were no significant changes over the 27-hr sampling period in either PTH or ionic calcium.(ABSTRACT TRUNCATED AT 250 WORDS)