SEASONAL CHANGES IN THE OCCURRENCE OF OESTRUS FOLLOWING PROGESTERONE SUPPRESSION OF OVARIAN FUNCTION IN THE MERINO EWE

Abstract

Three groups of Merino ewes (a total of 150) kept in yards were studied for a period of 1 year. Vasectomized rams were with one group and occurrence of estrus was noted. The presence and number of corpora lutea were observed by laparotomy at 6-week intervals. The annual rhythms in these characteristics were determined. A second group of ewes received progesterone treatments at intervals of 6 weeks. Half the ewes were with vasectomized rams continuously but rams were with the others for only 1 week after the final progesterone injection. The period from the final injection to the onset of estrus was determined. Introduction of rams increased the number of ewes showing estrus at the beginning (late summer) and end (late winter) of the breeding season. The period to onset of estrus was related to dose and frequency of progesterone injections, the time of the year, and introduction of rams. Periods to onset of estrus became progressively longer after mid-winter (June) and a proportion of progesterone-treated ewes did not show estrus in the 1 week period after cessation of progesterone injections in July, at a time when all untreated ewes were experiencing estrus cycles. The implications of the seasonal change in the.effects of progesterone on the ewes are discussed in relation to the sexual season. A third group of ovariectomized ewes received progesterone followed by estrogen, also at intervals of 6 weeks. The number of ewes showing estrus during the period January to June was dependent on the dose of progesterone independently of the effect of dose of estrogen. The relationship did not hold at other times. In general, there was considerable heterogeneity in mean responses and slopes of the dose-response lines.