Summary. This study tested the hypothesis that short days can prolong the breeding season of the ewe when reproductive activity is initiated by an endogenous process, as opposed to when it is driven by exposure to short days. Suffolk ewes were ovariectomized and treated with Silastic capsules containing oestradiol; reproductive activity was monitored from serum concentrations of LH. In this model, a rise in LH is indicative of onset of the breeding season and the duration of the elevation in LH is an indicator of length of reproductive activity. The ewes were subjected to 6-month alternations between long and short photoperiods such that the LH rise began during exposure to the inhibitory long photoperiod (i.e. it resulted from an endogenous process). When short days were provided soon after such a spontaneous onset of the LH rise, the duration of reproductive activity was greater than that observed when ewes were maintained in long days throughout the LH elevation (194 ± 10 vs 155 ± 15 days; P < 0·02). Since the transition from anoestrus to the breeding season in Suffolk ewes maintained outdoors does not require a decrease in daylength and appears to be generated by an endogenous process, our results support the hypothesis that shortening photoperiod sustains the natural breeding season which begins in early autumn.
Keywords: seasonal reproduction; photoperiod; photorefractoriness; LH; breeding season; sheep