EFFECT OF THE BOOROOLA GENE ON TIME OF OVULATION AND OVULATORY DYNAMICS

A three-quarter Romney Marsh x one-quarter Merino Booroola flock, composed of 31 (FecB Fec+) and 53 (Fec+ Fec+) ewes, was used to investigate the ovulation time and ovulatory dynamics after the onset of spontaneous estrus during the reproductive season and after MAP (medroxy-progesterone acetate) treatment and gonadotrophin injection during seasonal anestrus. These studies involved laparoscopic examination of ovaries at pre-determined times, between 16 and 40 h after onset of estrus (Trial 1); and serial laparoscopies between 48 and 80 h after MAP + 500 IU PMSG injection (Trial 2). In Trial 1, the FecB Fec+ ewes had smaller pre-ovulatory follicles than the Fec+ Fec+ ewes (P < 0.05). A similar tendency was observed in Trial 2 after gonadotrophin injection. The ovulatory dynamics in the genotypes studied was similar in both trials (P > 0.05). A great variation in the time of ovulation was observed in both trials, ranging over a period of 16 h in Trial 1 and 24 h in Trial 2. Ovulation began at 24 h after the onset of estrus, and the mode was observed at 32 h, although 20% of ewes had not ovulated by 40 h. In Trial 2, the first ovulation was observed at 54 h after PMSG injection, and the mode at 72 h. However, at 80 h ovulation had not been observed in about 10% of ewes. Ovulation rate was greater in Booroola gene carriers than in non-carriers in either spontaneous or induced ovulation (P < 0.01). This demonstrates that the Fec gene acts on the ovulation rate without changing the aspects of time of ovulation and ovulatory dynamics studied in this experiment.