The effect of cycle stage and plasma progesterone level on the induction of multiple ovulations in heifers

Abstract

The relationship between the endogenous progesterone level and ovulation rate in PMSG [Intervet] treated cows was examined and the effect of increasing the endogenous progesterone level during the early and mid-luteal phases of the estrous cycle on the ovarian response to PMSG was determined. There was no evidence for a significant correlation between plasma progesterone level and ovulation rate at any of the days examined. Increasing the endogenous progesterone level early (Day 3) or in the middle (Day 10) of the cycle did not affect the ovulation rate, but the day of the cycle on which PMSG was administered was important. The follicular response (number of follicles .gtoreq. 10 mm in diameter) and the total ovarian response (follicles .gtoreq. 10 mm + [corpus lutea]) were not affected by the day of the cycle on which PMSG was administered. It is probable therefore that it is the type of follicle population in the ovary at any particular time rather than the number that affects the ovulation response to gonadotropin.