Group housing for pigs is widely recognised as welfare-beneficial, but the transition to and management of groups requires careful attention to aggression, which can cause significant injury and chronic stress.
Group housing delivers major welfare benefits — pigs can move freely, socialise, explore, and express natural rooting and play behaviour — but poorly managed group formation causes acute suffering through fighting and chronic stress through sustained low-level aggression toward subordinates. The welfare challenge is in the transition: the moment of mixing is when the highest acute welfare risk occurs, and good management protocols are designed to minimise this risk. Space allowance, substrate provision, and mixing strategy all have evidence-based effects on aggression severity. The welfare of subordinate pigs in stable groups must also be monitored — individuals that are consistently displaced from feeders, rest areas, or enrichment objects suffer ongoing deprivation that can be harder to detect than acute fight injuries. Welfare outcome monitoring should assess not just injury prevalence but also individual resource access.