Induction of Ovulation in Seasonally Anovular Ewes by the Introduction of Rams: Effects of Progesterone and Active Immunization Against Androstenedione

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The effects of progesterone priming and active immunization against androstenedione on ovulation, oestrus ad premature luteal regression were observed in seasonally anovular Merino ewes which had been induced to ovulate by the introduction of rams. Ovulation was induced in 27 out of 36 ewes, and the response was not affected by either immunization or progesterone priming. The ovulation rate (mean number of ovulations per ewe ovulating) was higher in ewes immune to androstenedione, but there was no significant effect of this treatment on oestrus or luteal maintenance. Priming with progesterone prior to the introduction of rams prevented premature regression of the corpora lutea but had no effect on the other measures of reproductive function. If active immunization were to be applied on a commercial scale, it would not interfere with the practice of mating ewes during the anoestrous season. The increase in ovulation rate in these ewes, with no increase in the proportion of ewes ovulating, indicates that the mechanism for the control of ovulation is separate from that controlling ovulation rate. The mechanism by which progesterone pretreatment prevents premature luteal regression awaits further investigation.