A dose-response curve for ovulation rate in Merino ewes homozygous for Booroola FecB gene was generated by injecting PMSG at dose rates increasing from 3300-1200iu in 100iu increments (n=8-13 per group). PMSG was administered, at a concentration of 1000iu/ml in a saline carrier, as a single injection 15 days after the withdrawal of intravaginal progestagen sponges. An additional 20 control ewes were injected with 1ml of saline. The mean lifetime ovulation rate (LO) was calculated for each ewe from data collected for up to three years prior to the study. In individual ewes, a "response" to PMSG was defined as an ovulation rate at least 4 standard deviations (flock mean SD 0.87) greater than LO. Using this arbitrary value, no animals responded at doses below 500iu and less than 50% responded at 500 and 600iu. Above 800iu 75-100% of ewes in each treatment group responded. When log ovulation rate was regressed against dose of PMSG there was a highly significant dose-response (P<0.001, RSD 0.512). Using a subjective assessment of changes in ovarian morphology it was observed that >/=70% of ewes given 1000iu or more of PMSG showed over- stimulation of one or both ovaries. There was no apparent carry-over effect of PMSG at the oestrous cycle following treatment.

PL, Nuthall

Proceedings of the New Zealand Society of Animal Production, Volume 52, , 115-118, 1992
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