Seasonal Changes in the Œstrous Response by the Ovariectomized Ewe to Progesterone and Œstrogen

Abstract

ROBINSON et al. ¹ have presented results of the quantitative requirements of progesterone and œstrogen for œstrous behaviour in the spayed Merino crossbred ewe. These results were derived mainly from experiments conducted over short periods (2–3 months) and within the normal œstrous season of that ewe. We have noted² that the œstrous response of the spayed Romney ewe following progesterone-œstrogen treatment during November and December (months within the anœstrous season) was less than during the œstrous season. This difference could have resulted from the vasectomized rams exhibiting reduced libido during the summer and so failing to mark all ewes in œstrous³. However, our observations suggest that rams in this district will detect ewes in œstrous as we have seen following treatment of ewes with progesterone and pregnant mares' serum during the anœstrous season. Also, ewes have been marked at this time following administration of greater quantities of œstrogen, than previously². Therefore we have investigated further the response to progesterone and œstrogen in the spayed ewe.