Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: Synchronization of an endogenous rhythm of reproduction

Abstract

A study was conducted to test the hypothesis that different portions of the annual photoperiodic cycle play different roles in timing the breeding season of the ewe, Ovis aries, an animal in which an endogenous rhythm generates the seasonal reproductive transitions. Adult female sheep were pinealectomized to disrupt transduction of photoperiodic cues at 4 times of the year (summer and winter solstices, vernal and autumnal equinoxes), and the effects on seasonal reproductive neuroendocrine activity were evaluated. Time of pinealectomy greatly influenced the subsequent seasonal reproductive cycle such that the following inferences are possible. Lengthening days between the winter and summer solstices synchronize reproductive onset to the appropriate time of year. The relatively long days around the summer solstice act to suppress reproductive activity and forestall the start of the breeding season until late summer/early autumn. The shortening days between the summer solstice and autumnal equinox maintain a normal intensity and duration of reproductive neuroendocrine induction during the impending breeding season. However, the shortening days between the autumnal equinox and winter solstice (i.e., after breeding season onset) do not appear to play a critical role in maintaining the breeding season of that year, but may provide important cues for timing the breeding season of the following year.

Effect of the presence of male and female flockmates on reproductive activity in ewes

“…G(n) values for peak determination were G(l) = 3.8, G(2) = 2.6, G(3) = 1.9, G(4) = 1.5 and G (5) (Restall, 1992). These effects of the ram on the end of the breeding season were evident at a time of year when normal changes in the photoperiod are hypothesized to be relatively unimportant in determining the time of change from the breeding season to the anoestrous state (Wayne et al, 1990 Karsch, 1983) or pinealectomy (Oldham el al, 1979 …”

“…The photoperiod that occurs after the winter solstice has little effect on the duration of the ongoing breeding season (Wayne et al, 1990). This (1990).…”

The effect of nutrition on the seasonality of reproduction in ewes

“…Animals in these conditions continue to display cyclical variations in reproductive activity, although not synchronised among animals, and have periods differing synchronisation from 1 year [11]. Particular segments of the annual photoperiodic cycle seem to be more relevant in the synchronisation of endogenous rhythms [12,13]. Thus, the increasing daylength in the spring might be responsible for the onset of the breeding season at the end of the summer, whereas the long, but decreasing daylengths from the summer solstice to the autumnal equinox seem to insure the normal duration of the subsequent reproductive season.…”

The effect of melatonin replacement and month of treatment after the winter solstice on the reproductive activity of two Mediterranean breeds of sheep

“…42, No. 1, February 2011, 5-14 which play a critical role in synchronizing the entire rhythm, particularly in relation to the onset of the breeding season in mid-late summer (Malpaux et al 1989;Wayne et al 1990;Woodfill et al 1994). The still long but shortening days experienced after the summer solstice provide the photoperiodic information needed to induce a breeding season of full duration (Malpaux et al 1988;Wayne et al 1990;Woodfill et al 1991) and contribute to maintaining the animal in an active reproductive state (O'Callaghan et al 1991a).…”

“…1, February 2011, 5-14 which play a critical role in synchronizing the entire rhythm, particularly in relation to the onset of the breeding season in mid-late summer (Malpaux et al 1989;Wayne et al 1990;Woodfill et al 1994). The still long but shortening days experienced after the summer solstice provide the photoperiodic information needed to induce a breeding season of full duration (Malpaux et al 1988;Wayne et al 1990;Woodfill et al 1991) and contribute to maintaining the animal in an active reproductive state (O'Callaghan et al 1991a). Little, however, is known about the photoperiodic signal provided by the short days around the winter solstice, although Jackson et al (1988) reported that shorter days from the autumnal equinox to the winter solstice serve to break long-day photorefractoriness, which ensures that ewes can respond to the subsequent lengthening days that will synchronize reproductive activity in the next breeding season.…”