Two experiments were performed to examine the effects of immunoneutralizing PMSG at the onset of oestrus on the reproductive performance of sows. In experiment 1, 12 multiparous sows received 750 i.u. PMSG at weaning and six sozus acted as controls and were injected with saline. At 84 h after PMSG injection, all sows received an injection of 500 i.u. human chorionic gonadotropin (hCG) and six of the PMSG-treated sows also received an injection of 750 i.u. of a monoclonal anti-PMSG serum. The ovaries of all sows were examined by real time ultrasonography at 60-min intervals from 36 to 48 h after hCG injection for evidence of ovulation. In sows observed to ovulate, there was no significant effect of treatment on the interval from hCG injection to the start of ovulation, the duration of ovulation or subsequent litter size. In experiment 2, 253 primiparous sows received injections at weaning of 750 i.u. PMSG (no. = 134), 400 i.u. PMSG plus 200 i.u. hCG (no. = 69) or served as controls (no. = 50). At breeding, 67 of the PMSG-treated sows received an injection of 750 i.u. of monoclonal anti-PMSG serum.

More gonadotropin-treated than control sows were bred by 7 days after weaning (P < 0·05). Farrowing rates were higher in sows receiving the PMSG followed by anti-PMSG and PMSG plus hCG than in controls (P < 0·05), with those receiving PMSG alone being intermediate. There was no effect of treatment on subsequent litter sizes. These data indicate that the reproductive performance of primiparous sows may be improved by gonadotropin administration at the time of weaning. However, there was no advantage to using any particular hormone combination or anti-PMSG.