Intervals between multiple ovulations in PMSG-treated and untreated ewes and the relationship between ovulation and oestrus.

The timing of ovulations in 42 PMSG-treated ewes was determined by repeated endoscopy. The first ovulation occurred at a median time of 23 . 6 +/- 0 . 5 (s.e.m.) h after the onset of oestrus. The median interval between first and second ovulations was less than 1 h, and that between first and last ovulations was approximately 6 h. In 59 untreated ewes, probit regression analysis was applied to the number of ovulations which were found by endoscopy to have occurred by 23, 25 and 27 h after the onset of oestrus. The median time of first ovulation was 25 . 5 +/- 0 . 5 h after the onset of oestrus, this interval being similar in single- and twin-ovulating ewes. The median interval between twin ovulations was 1 . 2 +/- 0 . 6 h. Ovulation occurred after the end of oestrus in at least 75% of ewes.