PHYSIOLOGY OF SEASONAL REPRODUCTIVE TRANSITIONS IN THE EWE - REGULATION BY PHOTOPERIOD AND OTHER ENVIRONMENTAL CUES

This article reviews different factors influencing seasonal reproducti ve activity in sheep. Under natural photoperiod, ewes are reproductive ly active during the autumn and winter. Ewes kept under constant envir onmental factors still express cycles of changing reproductive activit y, providing evidence that seasonal reproductive activity is generated by an endogenous rhythm. In ewes exposed to natural daylength, reprod uctive transitions are timed to occur at specific times of the year by photoperiod. Neural signals from the retina are transmitted into mela tonin signals which influence GnRH secretion. Long days entrain the en dogenous rhythm such that the breeding season commences at a later tim e and during summer prevent an onset of the breeding season. Exposure of ewes to long days after the winter solstice followed by short days initiates an earlier onset of reproductive activity, wheras long days before the winter solstice do not advance onset of the breeding season . Finally some practical methods of manipulating the timing of the bre eding season using artificial light, melatonin and social cues from fl ock mates are discussed.