Effect of gonadotrophin-releasing hormone on the fertility of sows kept outdoors

A total of 1,231 mixed parity sows from five outdoor units in East Anglia were randomly assigned to one of three treatment groups. Sows in the control group were given no treatment, those in group 1 were injected intramuscularly with 8 microg of the analogue of gonadotrophin-releasing hormone (GnRH), buserelin, within 24 hours after first service and those in group 2 received the same treatment on day 11 or 12 after first service. There was no significant effect of treatment on farrowing rate to first service (85.0 v 85.4 v 84.7 per cent, respectively) or on overall mean (se) total litter size (11.6 [0.2] v 11.9 [0.2] v 11.9 [0.21]), although litter size was significantly increased by both treatments on one unit (11.6 [0.3] v 12.4 [0.3] v 12.8 [0.3]; P<0.05). However, there was a significant increase overall in the numbers of pigs born alive in both treatment groups (10.9 [0.2] v 11.3 [0.1] v 11.4 [0.3]; P<0.05) to sows of parities 0 to 2 only (P<0.05). This increase in numbers born alive is difficult to reconcile with the lack of overall effect on total litter size.