Pig social hierarchies cause significant welfare compromise when poorly managed, with mixing, regrouping, and inadequate space driving chronic aggression.
Social hierarchy aggression is a primary welfare challenge in commercial pig systems, compounded by management practices like routine mixing and regrouping that destabilise social structures. Stable groups maintained from weaning to slaughter without regrouping show the lowest aggression levels. However, production logic including batch management, treatment isolation, and space optimisation drives frequent mixing that imposes welfare costs. Enrichment reduces but does not eliminate mixing aggression. Welfare-conscious systems minimise regrouping events, provide adequate space, and use space partitioning to allow subordinate animals to avoid dominant individuals. The welfare impact of chronic aggression exposure is cumulative and severe in terms of cortisol, immune function, and physical injury.