Regrouping unfamiliar pigs causes sustained aggression that injures animals and creates chronic social stress. Management strategies can significantly reduce regrouping welfare costs.
The welfare cost of regrouping pigs is well documented and represents a significant, largely preventable harm in commercial pig production. Each mixing event — at weaning, between units, before slaughter — triggers a period of sustained fighting as a new hierarchy is established. The injuries sustained are genuinely painful, and the subordinate animals experiencing sustained attack suffer fear and stress beyond the physical wounds. Management strategies including odour masking, regrouping at night, providing ample resources to reduce competition, and maintaining stable groups throughout production all reduce these welfare costs. The optimal welfare approach avoids regrouping entirely where possible.