Photoperiod and Sexual Maturation of the Female Lamb: Early Exposure to Short Days Perturbs Estradiol Feedback Inhibition of Luteinizing Hormone Secretion and Produces Abnormal Ovarian Cycles*

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Abstract
First ovulation normally occurs in the springborn lamb at about 30 weeks of age during the autumn, when daylength is decreasing. This study determined if early exposure to short days can advance the onset of reproductive cycles. Beginning at 4 weeks of age, spring-born lambs were transferred from natural environment to artificial long days (15 h light, 9 h dark), where they remained for variable periods. One group (control) was returned to natural conditions on the summer solstice at 15 weeks of age; first ovulation, inferred from patterns of circulating progesterone, occurred at the normal age (30 ± 0.3 weeks, n = 10). The other two groups were transferred from long days to short days (9 h light, 15 h dark) at 15 or 10 weeks of age. Females (n = 6) exposed to short days from 15 weeks of age began cycles 3 weeks earlier (27 ± 1 weeks) than those in natural environment. Exposure to short days from 10 weeks of age produced mixed results; cycles began early in 3 of 10 lambs (∼25 weeks), the remaining 7 being greatly delayed (beyond 1 yr of age). Both groups exposed to artificial short days exhibited aberrant cycles consisting of isolated normal luteal phases, multiple short luteal phases, and premature anestrus. Responsiveness to estradiol inhibition of LH secretion was examined in ovariectomized lambs that experienced the short days from 10 weeks of age, the photoschedule which produced mixed results in intact females. The initial decrease in response to negative feedback occurred early in some lambs and late in others; thereafter, periods of hyperresponsiveness were interspersed with periods of hyporesponsiveness. The results indicate that premature exposure of lambs to short days can either advance or delay puberty. In either case, abnormal reproductive cycles are produced that appear to be related to perturbations in the system governing sensitivity to estradiol inhibition of tonic LH secretion. The findings raise the possibility that in natural conditions, the early exposure to the long days of summer influences the timing, quantity, and quality of the first cycles under subsequent short days through alterations in sensitivity to estradiol negative feedback.