Effects of duration and timing of progesterone priming on the incidence of corpora lutea with a normal life-span in Merino ewes induced to ovulate by the introduction of rams

Three experiments investigated the effects of progesterone on the incidence of corpora lutea with normal life-span in seasonally anovulatory Merino ewes induced to ovulate by the introduction of rams.Ewes in Experiments 1 and 2 received a single i.m. injection of 20 mg progesterone in oil at the time of, or up to 5 days before introduction of rams. Control ewes received oil alone. Progesterone priming had no effect on the proportion of ewes induced to ovulate, and the beneficial effect of progesterone on the proportion of corpora lutea with a normal life-span (P < 0.001, Experiment 1; P < 0.01, Experiment 2) was independent of its time of administration. Injection of progesterone delayed the time of the surge of LH compared to control ewes (P < 0.001), but only when given at the time of introduction of rams.Administration of progesterone by intravaginal devices for 6 h, 12 h or 24 h before introduction of rams (Experiment 3) increased the incidence of corpora lutea with normal luteal life-span compared to control ewes, but had no effect on the proportion of ewes ovulating. The incidence of corpora lutea with a normal life-span was significantly and positively correlated (R=0.977; P < 0.05) with duration of progesterone priming before introduction of rams.These results indicate that duration but not timing of progesterone priming is critical to its ability to eliminate short life-span corpora lutea. Such findings support the concept of a mode of action of progesterone at the ovarian level.