Breeding may simultaneously reduce pig aggressiveness at regrouping and in stable social groups but management may not

Abstract Text:

Aggression between pigs compromises welfare and productivity. Individual sow stalls are now banned in the EU after week 4 of gestation and are being phased out in some parts of North America. Better control of both acute regrouping aggression and chronic aggression in stable social groups is required at all ages. This paper examines whether efforts to control these two forms of aggression may be complementary or antagonistic and whether optimum aggression phenotypes can be identified to target in management or breeding. Acute regrouping and subsequent chronic aggressiveness are stable traits of the individual but highly variable between animals and groups. Delivery of aggressive behaviour at regrouping is heritable (h²>0.31 s.e. 0.04) as is the number of skin lesions from regrouping or chronic aggression (h²=0.19 s.e. 0.02 to 0.43 s.e. 0.04). The lesion count is genetically correlated to reciprocal fighting or receipt of non-reciprocal bullying and is therefore an indicator of aggressive propensity (e.g. r_g between reciprocal fighting duration at regrouping and lesions to the anterior of the body = 0.67 s.e. 0.04). The lesion count shows a low but positive genetic correlation between 24h and 3wks post-regrouping (r_g0.28 s.e. 0.07 to 0.50 s.e. 0.09) suggesting that breeding to reduce regrouping lesions would also reduce lesions from chronic aggression.  However, at the phenotypic level, individual pigs or entire social groups that fight greatly at regrouping, even if this is often unsuccessful, show few injuries from chronic aggression (group level, r=-0.28 to -0.38, p<0.05). Furthermore, pigs with a beneficial effect on the growth of penmates can be more aggressive at regrouping but receive fewer lesions 3wks later. Acute aggression may therefore reduce the costs of chronic aggression at the phenotypic level. Fight quantity appears to primarily drive this association as fight outcome at regrouping had less impact on later lesions. However some pigs show few injuries from both acute and chronic aggression. The behaviour of these pigs will be discussed to highlight whether optimum phenotypes can be targeted in management or breeding. Currently, breeding for reduced regrouping aggressiveness is likely to simultaneously reduce subsequent chronic aggression, but phenotypically reducing regrouping aggression through management change may lead to long-term increases in aggression unless controlled.

Keywords: pig; aggression; breeding