Braw-Tal, R., Institute of Animal Science, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel, Institute of Animal Science, ARO, Volcani Center, POB 6, Bet Dagan 50250, Israel
Yossefi, S., Institute of Animal Science, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Pen, S., Institute of Animal Science, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Shinder, D., Institute of Animal Science, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Bar, A., Institute of Animal Science, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel

1. The effect of age on ovarian function was studied in 245-, 350-, 500-, 700- and 800-d-old Lohmann hens. The effect of three different methods for moult induction on ovarian function and corticosterone concentration was studied in 500-d-old hens. 2. No significant reductions in ovarian weight or in number of follicles before the age of 700 d were found. The ability to produce progesterone and oestradiol-17β was unchanged up to the age of 700 d and the circadian secretion of these two steroids was identical in young (225 d) and old hens (600 d). 3. The effects of induced moulting by feed withdrawal (FW) and a high Zn (HZn) diet on body weight and ovarian function were very similar; those of a moderate Zn with low Ca (MZn/LCa) diet were smaller. 4. The first significant effect of moulting was a decrease in oestradiol-17β plasma concentration (d 2). Plasma progesterone decreased more gradually than oestradiol-17β, and reached a nadir on d 6 in FW- and HZn-treated hens and on d 9 in MZn/LCa-treated ones. 5. Hens treated with either FW or the MZn/LCa, but not those with the HZn diet, showed a very sharp rise in corticosterone concentration on d 2 of treatment. Thus the MZn/LCa diet was less efficient than the other treatments in induction of ovarian involution, but had a similar effect on stress induction, as indicated by increases in plasma corticosterone.