The size and composition of pig social groups significantly affects aggression, stress, and welfare outcomes, with research consistently showing that large, unstable groups create welfare problems that smaller, stable groups avoid.
Pig social organisation is finely calibrated to stable, familiar groups where hierarchies are established and respected without constant physical challenge. When pigs are repeatedly mixed with unfamiliar individuals — as commonly happens at weaning, regrouping for growth stages, and transport — the hierarchy must be re-established through fighting, causing acute welfare harm and injuries. Keeping groups stable from weaning to slaughter is one of the most effective and economical welfare improvements available to pig producers, requiring no physical infrastructure change. Where mixing is unavoidable, timing it at off-peak activity periods, providing additional space during mixing, and supplying rooting substrate to redirect aggression are evidence-based mitigation strategies.