Aggression when pigs are mixed into new groups is a major acute welfare concern in intensive systems, causing injuries, stress, and lasting social disruption.
Mixing aggression in pigs represents intense acute suffering — biting, mounting, and sustained fighting cause wounds that become infected, particularly on ears, flanks, and tails. Subordinate pigs receive repeated attacks and may be unable to access food and water. Stress hormones remain elevated for days after regrouping. The welfare problem is structural: intensive systems rely on regrouping at every production stage, repeatedly disrupting social bonds. High-welfare systems aim to keep stable social groups from birth to slaughter — a simple but powerful intervention that dramatically reduces aggression-related suffering. Where regrouping is unavoidable, environmental modifications and management protocols significantly reduce harm.