Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: April 5, 2011
Publication Date: July 11, 2011
Citation: Marchant Forde, J.N., Garner, J.P., Johnson, A.K., Marchant-Forde, R.M., Lay Jr, D.C. 2011. The effects of group size on aggression when mixing unacquainted sows in outdoor paddocks. Journal of Animal Science. Proceedings. Technical Abstract: Aggression is a challenge when pigs are kept in groups. Sows fight at mixing when space is limited but this project sought to determine the amount and type of aggression observed when unacquainted Berkshire sows were mixed in pairs or in two established sub-groups of three in outdoor paddocks. Treatment 1 (PR) used 16 pairs of sows mixed into a 5000 m**2 paddock. Treatment 2 (GP) used 28 unacquainted groups of 3 sows, with 2 groups mixed into a 5000 m**2 pen. Behavior was recorded continuously for 60 min post-mixing and all-occurrences sampling was used to extract social interactions. The data were analyzed to determine the number of social interactions that did or did not contain aggressive components (i.e. pushing, knocking or biting) and fighting defined as interactions that contained 10 or more reciprocated, aggressive component actions. Within each interaction, data were analyzed to determine the number of component actions and the number of aggressive component actions. The data were compared using a GLM, with treatment as a fixed effect. The number of social interactions was similar in GP (15.7 ± 2.7) and in PR (14.4 ± 2.5, P<0.001). The number of interactions that contained aggression was also similar (6.9 ± 1.4 v. 5.6 ± 1.2, P>0.05), but GP interactions contained more aggressive components (26.8 ± 2.5) than PR interactions (16.6 ± 1.4, P<0.05). Twelve of the 16 PR pairs fought and aggression occurred quickly, beginning with biting after 2.1 ± 0.4 interactions and 17.3 ± 6.9 components. In GP mixing, 8.5 ± 0.3 of the 15 possible pair combinations per group interacted, of which 5.6 ± 0.4 interacted aggressively and 1.5 ± 0.4 fought. A higher proportion of unacquainted pairs interacted aggressively (0.57 ± 0.07) and fought (0.17 ± 0.05) than acquainted pairs (0.10 ± 0.04 and 0 respectively, P<0.01). Bites were delivered quickly after 2.2 ± 1.7 interactions and 9.2 ± 2.6 components. Mixing pairs or groups of sows in paddocks did not prevent aggression. Aggression occurred quickly but reduced rapidly, with sows using space for avoidance. The results further our understanding of aggression at mixing and will help to identify best practice for producers.