Regrouping pigs from different litters or farms triggers intense aggression as individuals establish new social hierarchies, representing one of the most acute welfare challenges in commercial pig production.
Regrouping aggression causes acute pain from biting and physical injury, with some pigs sustaining wounds requiring veterinary treatment. The stress of social instability — the loss of familiar companions and the uncertainty of establishing new social status — likely causes psychological distress in addition to physical injury. Pigs that lose fights may be chased relentlessly, unable to access food or water, and suffer continuously until a new hierarchy stabilizes. The management systems that require frequent regrouping — weaning at 3-4 weeks, multiple moves between production stages — create repeated welfare insults throughout pig lives. Design changes that maintain stable social groups throughout production, combined with mixing strategies that reduce fight duration when regrouping is necessary, represent meaningful welfare improvements.