Sensitivity of sheep to exogenous prostaglandin F_2α early in the estrous cycle

One hundred ewes were monitored twice-a-day for estrus and only those ewes displaying an estrous cycle of 16.5-18.0 days in length were utilized in the experiment. Ewes were subsequently treated with a single injection of 5 or 10 mg of prostaglandin F_2α (PGF_2α) on days 3.0, 3.5, 4.0, 4.5 or 5.0 (n=10 ewes per day) of the estrous cycle (day 0.0=the a.m. or p.m. of first detected estrus) to determine their sensitivity to induction of estrus with prostaglandin F_2α. Ewes that were detected in estrus within 24-72 h after treatment with PGF_2α were considered sensitive to PGF_2α treatment. This definition of sensitivity to PGF was inclusive as all ewes either responded within 24-72 h after PGF_2α treatment or they came into estrus at the expected time based on their previous, untreated, estrous cycle. Results indicated that a significant (P<0.05) proportion of ewes became sensitive to the induction of estrus with PGF_2α between days 3.5 and 4.0, as 10 and 50% of the ewes, respectively, responded to treatment. PGF_2α treatment on days 4.5 and 5.0 caused a non-significant increase in responsiveness as 70 and 75% of the ewes, respectively, came into estrus within 1-3 days. The 10 mg dose, being more effective than the 5 mg dose, shifted the response curve by roughly 20 percentage points beginning on day 3.5 and continued through the later exposure times. In conclusion, the CL early in the estrous cycle of ewes was variable in its responsive to a single exposure to PGF_2α and some of this variability was attributed to physiological events occurring between days 3.5 and 4.0.