Effect of the rate of progesterone decline at luteolysis on the ovulatory follicles and subsequent estrous cycle length in ewes

Follicular and interestrous characteristics were examined in 34 ewes after experiencing either a rapid decline in plasma concentrations of progesterone at luteolysis [prostaglandin F 2α (PGF 2α)-induced] or a slow rate of decline, lasting over 72 h. All ewes were given PGF 2α on day 10 (day 0 = estrus). A slow rate of decline was established in 17 ewes by the intravenous infusion of progesterone initially at 72 ml h −1, delivering 4.5 mg progesterone h −1, then decreasing the infusion rate by 1 ml h −1 for the next three days. Seventeen additional ewes, predestined to experience a rapid decline in progesterone, were infused with vehicle. In Experiment 1, after infusion, ewes (6 ewes/group) were necropsied at the onset of estrus and follicle diameter was determined, follicular fluid was aspirated and the remaining follicular wall was microscopically examined to determine the number of granulosa cell layers. In Experiment 2, the interestrous interval, after infusion, was observed in both groups of ewes (11 ewes/group). Ewes experiencing a rapid rate of progesterone decline at luteolysis had no differences in follicle diameter nor follicular concentration of progesterone or estradiol but their ovulatory follicles contained fewer ( P < 0.01) granulosa cell layers and the resulting estrous cycle was longer ( P < 0.05) than ewes experiencing a slow rate of progesterone decline.