Seasonality of Reproduction in Sheep and its Control by Photoperiod

Seasonality of the reproductive cycle in sheep is a general phenomenon for mid-latitude breeds. The proximal part (breeding season) and also partially distal part (end of gestation and beginning of lactation) of this cycle is controlled by photoperiod, whatever the form of light regimens. Data are presented which indicate that male and female do not necessarily have the same photoperiodic sensitivity. Gonadal stimulation in the ram starts 1'5-2 months earlier than in the ewe under annual variations. Photoperiod controls the reproductive cycle by the intermediary of the hypothalamo-pituitary axis. There are both a steroidindependent and a steroid-dependent effect of light, depending on both decreasing and increasing daylength in mid-latitudes.