Mixing unfamiliar pigs causes aggressive fights as they establish social hierarchies, causing injuries and significant welfare harm that better management can substantially reduce.
Social mixing aggression is one of the most predictable and preventable acute welfare harms in commercial pig production. The fighting that follows introduction of unfamiliar pigs is not random aggression but biologically driven establishment of dominance — an inherent response to social disruption that causes real injury and stress. Modern production systems routinely mix pigs at weaning, regrouping, and movement between production phases, multiplying these welfare costs across the production cycle. Maintaining stable social groups throughout production, or implementing evidence-based mixing protocols when regrouping is unavoidable, dramatically reduces injury rates and stress without requiring major infrastructure changes.