Mixing unfamiliar pigs at weaning, regrouping, and entry to finishing causes aggressive fights; stable social groups with minimal mixing dramatically reduce injury and stress.
Pig aggression from social mixing represents a predictable, frequent, and largely preventable welfare problem. Pigs establish social hierarchies through fighting, which is biologically normal, but the severity and duration of fighting when unfamiliar groups are mixed is exaggerated compared to natural group formation. Commercial practices including batch weaning and weight-based sorting create repeated mixing events that cause repeated cycles of intense aggression and injury. Stable group systems that keep pigs together from birth through slaughter prevent this by allowing stable hierarchies to form once and be maintained throughout production.