INFLUENCE OF PREVIOUS PHOTOPERIODIC EXPOSURE ON THE REPRODUCTIVE RESPONSE TO A SPECIFIC PHOTOPERIOD SIGNAL IN EWES

Two experiments were carried out to determine whether the reproductive response of ewes to a specific photoperiodic signal depends on the ti me of year that the signal is given, and, if so, whether this dependen ce can be attributed to the photoperiodic history of the ewes. The aim of experiment 1 was to expand upon previous findings that the reprodu ctive response to a specific photoperiodic challenge in ewes previousl y maintained on natural photoperiod varies with time of year. Ewes wer e transferred at one of three times of year from natural photoperiod t o photochambers and were immediately exposed to 35 long days (18L:6D) followed by continuous exposure to short days (8.5L:15.5D); this treat ment is referred to as LD-->SD. The three times of year when long days started corresponded to the beginning of the breeding season, the mid -breeding season, and early anestrus (September 21, December 21, March 21, respectively). In ewes exposed to LD-->SD beginning in September, the breeding season and subsequent anestrous season was not altered. In ewes exposed to LD-->SD beginning in December, the transition to an estrus was advanced (p < 0.05) relative to that in controls maintained in simulated natural photoperiod. Subsequently, half of these ewes re sumed reproductive activity within 180 days; this occurred 131 +/- 8 d ays after transfer to short days. In contrast, all ewes exposed to LD- ->SD beginning in March resumed reproductive activity; this began 100 +/- 3 days after transfer to short days (p < 0.05 versus December grou p). The purpose of experiment 2 was to assess the extent to which the difference in response to a photoperiodic challenge can be attributed to photoperiodic history. Ewes were maintained on short days from the winter solstice interrupted with 35 long days from March 21, June 21, September 21, or December 21. The majority of ewes exhibited an onset of reproductive activity after exposure to LD-->SD at the different ti mes of year, and there was no group difference in latency to onset of reproductive activity. The duration of reproductive activity, however, was longer (p < 0.05) in ewes exposed to LD-->SD beginning in lune th an in the other groups. Thus we conclude that the seasonal difference in the ability of the photoperiodic challenge of long followed by shor t days to induce reproductive activity in ewes previously maintained o utdoors can be attributed, in large measure, to photoperiodic history. Other factors, such as phase of the endogenous rhythm, however, may i nfluence the duration of reproductive activity resulting from this pho toperiodic challenge.