Corpora lutea with a short life-span induced by rams in seasonally anovulatory ewes are prevented by progesterone delaying the preovulatory surge of LH

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Abstract
Seasonally anovulatory Merino ewes isolated from rams were allocated to three treatments before the re-introduction of rams. Ten ewes received a single injection of progesterone (20 mg), 18 ewes received the injection of progesterone but had the ram-induced preovulatory surge of LH replaced by a series of injections of GnRH 24 h after the introduction of the rams, and 20 control ewes had no hormone treatment. Of the 48 ewes, 44 ovulated within 5 days of the introduction of rams and the treatments had no significant effect on the incidence of ovulation. The frequency of corpora lutea with a short life span (the interval between successive preovulatory surges of LH being 5.1 .+-. 0.9 days) was 72% for control ewes and 58% for ewes treated with progesterone and GnRH, but such CL were prevented completely after the injection of progesterone alone (P < 0.001). The injection of progesterone also delayed the preovulatory surge of LH (P < 0.001). These results suggest that progesterone assures normality of corpora lutea by lengthening the period of gonadotrophin priming of follicles before ovulation.