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 a fter experiencing either a rapid decline in plasma concentrations of p rogesterone at luteolysis [prostaglandin F-2 alpha (PGF(2) alpha)-indu ced] or a slow rate of decline, lasting over 72 h. All ewes were given PGF(2) alpha on day 10 (day 0 = estrus). A slow rate of decline was e stablished in 17 ewes by the intravenous infusion of progesterone init ially at 72 ml h(-1), delivering 4.5 mg progesterone h(-1), then decre asing the infusion rate by 1 ml h(-1) for the next three days. Sevente en additional ewes, predestined to experience a rapid decline in proge sterone, were infused with vehicle. In Experiment 1, after infusion, e wes (6 ewes/group) were necropsied at the onset of estrus and follicle diameter was determined, follicular fluid was aspirated and the remai ning follicular wall was microscopically examined to determine the num ber of granulosa cell layers. In Experiment 2, the interestrous interv al, after infusion, was observed in both groups of ewes (11 ewes/group ). Ewes experiencing a rapid rate of progesterone decline at luteolysi s 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 progestero ne decline.