Mechanism for Delay of First Ovulation in Lambs Born in the Wrong Season (Fall)1

Abstract

The influence of season of birth and photoperiod on the mechanism governing initiation of ovulation in lambs was studied. The ages at the decrease in responsiveness to estradiol-feedback inhibition of tonic LH secretion and first ovulation in lambs born out of their normal birth season (fall) was compared with those in lambs born during the normal season of birth (spring). The change in feedback responsiveness was determined from the pattern of circulating LH in ovariectomized lambs treated chronically with estradiol (Silastic implant) from 20 weeks of age. First ovulation was determined from the pattern of serum progesterone in intact lambs. In control lambs born in spring (March) and raised in natural photoperiod, first ovulation occurred at 31 ± 1 (mean± SEM) weeks of age, about 4-8 weeks after the onset of the breeding season of adults. Lambs born in the fall (October) and reared in natural environment exhibited no evidence of ovulation at 26-35 weeks, ages which in fall-born females are attained during the anestrous season; rather, ovulations were delayed until shortly after the onset of the breeding season when they were 49 ± 1 weeks old. Rearing fall-born lambs in an annual photoperiod similar to that which they would have experienced had they been born in the spring, markedly reduced the delay in onset of cycles. In these females, ovulations were initiated at 35 ± 0.5 weeks. In each of the three groups, the decrease in responsiveness to negative feedback, as reflected by the increase in serum LH in estradiol-treated lambs, occurred at the time ovulations began in intact females. Relative to spring-born lambs, it was delayed in fall-born lambs under natural environment and was restored to near normal in fall-born lambs under reversed photoperiod.