Timing of multiple ovulations in the ewe after treatment with FSH or PMSG with and without GnRH.

Ovulation rate, median time to first ovulation, median time of all ovulations and median time from first to last ovulation were studied by repeated laparoscopy in Merino ewes. Treatments with FSH or PMSG significantly affected ovulation rate (8.4 +/- 0.81 and 7.3 +/- 1.21 respectively, P less than 0.05) and in median time of all ovulations (60 and 54 h respectively after progestagen sponge removal, P less than 0.05). Differences in the median time to first ovulation (60 and 48 h) and median time from first to last ovulation (6 and 6 h) for the respective treatments were not significant. The synchrony of ovulation after both treatments was adversely affected by (1) the occurrence of premature ovulations before the onset of superovulation, (2) variability in the time of commencement of superovulation, and (3) variability in the time from first to last ovulation. Administration of GnRH synchronized the timing of ovulation with both gonadotrophin treatments. This synchrony was due to a reduction in the period during which superovulation began and in the interval from first to last ovulation. The median time of all ovulations was significantly less with FSH + GnRH than with PMSG + GnRH (45 and 48 h after progestagen sponge removal, respectively, P less than 0.05). Administration of GnRH at 16, 20 or 24 h after progestagen sponge removal significantly affected all traits examined except ovulation rate. Administration at 20 and 24 h produced an equally good synchrony of ovulation which was better than that obtained at 16 h. We suggest that the use of GnRH in embryo collection programmes appears justified and is likely to improve embryo yields due to improved rates of fertilization.