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Effects of social rank on agonistic behaviors and associated injuries of gestating sows at mixing
Group housing systems present challenges to low ranking sows because they suffer more injuries associated with aggression than higher ranking pen-mates. It is not clear how sows of differing rank are involved in fights and get injured. This study was conducted to investigate the relationship between the social rank of sows and their injuries associated with agonistic behaviors. Sows (n = 150, parity 1 to 9) were mixed in pens of 15 sows after weaning, with the frequency composition by parity remaining consistent across pens. All sows were video-recorded for 48 h immediately after mixing. Aggressive interactions were classified as parallel pressing, knocking, and threatening. The number and outcomes (won, lost, and unsolved) of each fight, and the sows involved in were registered during the first 4 h after mixing. A rank index (RI) was calculated for each sow according to outcomes of aggressive interactions. Based on RI, sows in each pen were categorized as high (ranked 1 to 5), middle (ranked 6 to 10), and low rank (ranked 11 to 15). Injuries caused by fighting were assessed for each sow 48 h after mixing. Data were analyzed using the Glimmix procedure of SAS with sow ranking, parity and their interaction as fixed effects. Compared to middle and low ranking sows, high ranking sows fought more frequently (15.3 vs. 9.8 and 8.6 fights/h/sow, SE=1.33; P < 0.01), and won more fights (13.0 vs. 3.2 and 0.6 fights/h/sow, SE = 1.32; P < 0.01). Of the total fighting events, 77% were knocking, 21% were threatening, and 2% were parallel pressing. Compared to low and middle ranking sows, high ranking sows were involved in more knocking fights (12.2 vs. 6.8 and 7.3 fights/h/sow, SE=0.77; P < 0.01) and threatening encounters (2.7 vs. 1.7 and 2.1 events/h/sow; SE = 0.63; P = 0.04). High-ranking sows tended to have lower injury scores (6.47 vs. 7.63 and 7.66; SE =0.51; P = 0.07), with fewer injuries on the rear parts of body than middle and low ranking sows (1.50 vs. 1.89 and 2.04; SE =0.18; P < 0.01), respectively. These results indicate that high ranking sows fought more frequently and injured less than low ranking sows. The involvement and outcomes of knocking fights contributed to the difference in fighting frequency and aggression-induced injuries between high and low ranking sows.
Keywords: injuries, social ranking, sows