Mixing unfamiliar pigs in group housing triggers intense fighting that causes injuries and chronic social stress — managing this welfare challenge is critical for group housing systems.
Pig mixing aggression represents a welfare challenge inherent to the shift toward group housing — a positive development overall, but one that introduces new problems if poorly managed. Every mixing event triggers fighting that causes physical injury and psychological stress. The most welfare-positive approach avoids mixing by keeping stable groups from establishment through to slaughter without regrouping. Where mixing is unavoidable (at weaning, or when animals are moved between stages), providing hiding areas, increasing space, and using dimmed lighting during the post-mixing period reduces injury rates. The welfare costs of mixing must be weighed against the substantial welfare benefits of group housing — the balance remains strongly in favor of group housing when managed properly.