Time course for the formation and disruption of social organisation in group-housed sows

Seventy-eight Large-White×Landrace mixed parity sows were weaned from farrowing crates into groups of six and mixed into straw-bedded service pens with individual feeding stalls. Groups were transferred to similar gestation pens after 4 weeks where they remained until farrowing. In experiment 1 (n=24) lesion scores on days 1 (prior to mixing) 3, 7, 28, 56 and 84 were 0; 9.5; 6.5; 4.0; 5.0 and 6.0, respectively. Scores on day 3 were significantly higher than all other days (p<0.05). Aggressive interactions during floor feeding tests were significantly higher on day 28 (5.5) than on day 56 (2.5) and day 84 (2.0) (p<0.05). In experiment 2 (n=54), nine groups were divided into three pairs after the 4-week service period, housed separately for either 2, 4 or 6 weeks and then re-grouped into original groups. At mixing, 93% of fights were between sows which has not been housed together during the previous gestation period. There were, on average, three fights per group lasting 70 s at mixing. There were no fights when sows were re-grouped. Lesion scores and aggression during floor feeding tests did not increase after re-grouping and were unaffected by the duration of separation. Separation had no affect on feed displacement times which were significantly less at 4 weeks after weaning (41 s) than they were at 1 week after mixing (60 s) (p<0.05). Social rank positively correlated with sow weight and parity (rs: 0.63). Social rank had no affect on any production parameters measured. In 13 groups from both experiments, six formed non-linear hierarchies and in six groups, positions within the hierarchy changed over the gestation period. It was concluded that formation of social hierarchy in sows is a complex process. Aggression declined rapidly after mixing and became stable after 7 days but remained high for over 28 days when sows had to compete for food. Sows may be removed and returned into groups of six after a 6-week period without any major disruption to social organisation.