Managing Aggression in Group-Housed Pigs: Welfare at the Mixing Point

When unfamiliar pigs are mixed together, a burst of aggression occurs as animals establish social hierarchy. Managing this critical period is one of the most important welfare challenges in the transition from gestation crates to group housing.

Key Facts

Pigs establish dominance hierarchy through fighting when unfamiliar individuals are mixed
Mixing aggression peaks within 24–72 hours and typically subsides as hierarchy stabilises
Injuries from mixing include skin lacerations, ear bites, and occasionally serious wounds requiring veterinary attention
Sow weight and parity influence mixing outcomes — younger, lighter sows are most vulnerable to aggression
Providing hiding barriers and visual obstacles in group pens reduces serious injury during mixing
Mixing in larger groups or in novel environments can reduce aggression compared to mixing in established spaces
Olfactory familiarity — allowing pigs to smell each other before physical contact — reduces initial aggression intensity
Electronic sow feeding (ESF) systems reduce feed competition that exacerbates post-mixing aggression

Welfare Considerations

Mixing aggression is the primary welfare challenge of group housing systems for sows — and managing it effectively is what separates high-welfare group housing from a simple cage replacement. The welfare harm of severe mixing aggression includes acute pain from wounds, chronic stress for subordinate animals, and risk of serious injury. Good management practice addresses the problem through multiple complementary interventions: scent familiarisation before mixing, providing retreat spaces in the pen, ensuring sufficient feeding space so competition is reduced, and monitoring animals closely in the first 72 hours to separate animals that are being severely bullied. Mixing strategy matters too — introducing animals in larger groups at novel locations tends to reduce the intensity of individual targeting. Producers who invest in the stockmanship skills and pen design features needed for good mixing management achieve group housing welfare outcomes that substantially exceed those of crate systems.

What You Can Do

Support training programmes for pig farmers that develop the stockmanship skills needed for safe group housing management
Advocate for welfare assessment schemes that include mixing aggression monitoring as a performance indicator
Choose pork from farms where trained stockpeople conduct close monitoring during and after pig mixing events
Support research into practical pen design features that reduce mixing aggression in commercial sow housing
Understand that group housing is better than crates when well managed — and advocate for the support farmers need to do it well