The effect of flight distance on aggression and skin damage of newly weaned sows at mixing

The mixing of unfamiliar sows at weaning results in the establishment of dominance hierarchies, frequently involving aggression. In commercial situations, with limited available flight distances, this can result in injuries and poor sow welfare. The objectives of this study were to measure flight and chase distances, and the incidence of aggressive interactions and levels of skin damage that occur when newly weaned sows were mixed in a large area where flight and chase distances were unlikely to be limiting. Eight replicate groups of six unfamiliar sows were mixed in an arena (18x10.5m) at weaning. The sows were mixed at 09.00 h on Day 1, removed on Day 2 at 08.00 h for feeding in individual stalls, and returned to the arena at 09.00 h. Sows were observed directly from 09.00 h to 16.00 h on Day 1, and from 09.00 h to 13.00 h on Day 2. Video tape records were taken continuously for 28 hours from 09.00 h on Day 1. All aggressive interactions were recorded and categorised into three classes: brief (single knock, snap or bite), one-sided fight (no retaliation by defender), or two-sided fight (defender retaliates).