Regrouping unfamiliar pigs triggers intense aggression as they re-establish dominance hierarchies, causing injury and chronic stress that better management and system design can substantially reduce.
Regrouping is one of the most predictable sources of welfare harm in pig farming and is also one of the most manageable. Every time unfamiliar pigs are placed together, a period of often-intense fighting is near inevitable as animals establish a new hierarchy. The welfare cost — injuries, chronic stress, suppressed feed intake — is predictable and substantial. The primary driver is management convenience: pigs are mixed because production systems require sorting by size, sex, or stage, not because mixing is biologically necessary. Systems designed to maintain stable social groups throughout the production cycle demonstrate that most regrouping events are avoidable with appropriate infrastructure investment.