Time of Ovulation in Merino Ewes Superovulated with PMSG and FSH-P

An experiment examined the effects of treatment with gonadotrophin releasing hormone (100 micrograms GnRH injected 24 h after progestagen sponge removal), season of treatment (autumn v. spring), the effect of supplementary feeding with lupin grain (in autumn only, from 12 days before until 8 days after sponge removal) on the time of ovulation in 182 mature Merino ewes superovulated with a combination of 400 I.U. pregnant mare serum gonadotrophin (PMSG) and 12 mg follicle stimulating hormone (FSH-P). Time of ovulation was influenced by both season and the administration of GnRH. Two peaks of ovulation were observed in spring, the first between 24 and 30 h after sponge removal, and the second commencing 42 h after sponge removal. In autumn, there was one peak of ovulation between 42 and 54 h after sponge removal. Treatment with GnRH advanced the time of ovulation in autumn (P less than 0.05), but had no apparent effect in spring. In contrast to the effect of GnRH, time of ovulation for ewes treated in autumn was delayed (P less than 0.001) by supplementary feeding with lupins.