Role of short days in timing the onset and duration of reproductive activity in ewes under artificial photoperiods

Abstract

The objective of this work was to evaluate the role of short photoperiod in timing the onset and duration of reproductive activity in ewes. The perception of photoperiod was disrupted by pinealectomy following transfer from long (17L:7D) to short (8.5L:15.5D) photoperiod and the subsequent reproductive response was monitored. Ovariectomized ewes given Silastic implants containing estradiol-17β were exposed to long days until Day 0 (May 24) and then were allocated to the following groups (n = 5-6/group): Group 1) short-day control-moved to short days; Groups 2 to 5) pinealectomy after 0, 30, 60, or 90 short days, respectively; Group 6) long-day hold-kept on long days; Group 7) long days after 60 short days-moved to short days on Day 0 and returned to long days on Day 60. Six ewes kept outdoors served as additional controls. Reproductive neuroendocrine activity was assessed from plasma LH concentrations, high values being indicative of the breeding season and low values indicative of anestrus. Time of reproductive neuroendocrine activity onset (LH rise) did not differ among animals in the 7 groups kept indoors, but was advanced (p < 0.05) relative to that of ewes outdoors. In contrast, duration of the LH elevation differed among ewes in groups kept indoors. Specifically, the duration was longer (p < 0.05) in the short-day control and long-day hold groups (90 ± 16 and 94 ± 16 days, respectively; mean ± SEM) than in ewes pinealectomized on Day 0 or Day 30 of short days (48 ± 12 and 50 ± 8 days, respectively) and in ewes moved to short days and back to long days on Day 60 (43 ± 15 days). Pinealectomy after 60 or 90 short days did not affect the duration of reproductive activity relative to the short-day controls. It is concluded that, in this photoperiodic model, reproductive onset is not initiated by short days. After a shift from long to short days, however, maintenance of a normal duration of breeding activity requires continued exposure to short days, but only for some 30-60 days. These findings support the hypothesis that the shortening days leading up to the transition into the breeding season do not time the onset of reproductive activity of the ewe, but they do contribute to maintaining its full duration.