Mixing unfamiliar pigs causes acute aggression that causes injuries and stress, with management strategies significantly affecting the welfare cost of inevitable mixing events.
Pig aggression at mixing is an unavoidable feature of pig production systems that moves pigs between groups at weaning, stage completion, and movement to finishing. The welfare cost — injuries, stress, reduced feed intake, and immune suppression — is real and substantial. Management interventions that reduce the intensity and duration of mixing aggression have both welfare and production benefits. Sensory familiarisation before physical mixing, mixing at night when activity is lower, providing distraction materials, and ensuring adequate space all reduce aggression. Some producers have invested in systems that maintain stable social groups through production stages, largely eliminating mixing-related welfare problems.