Pigs establish social hierarchies through agonistic behaviour. Commercial management practices that mix unfamiliar pigs cause severe fighting with welfare consequences that can be substantially reduced.
Pig social welfare is significantly affected by the common commercial practice of regrouping unfamiliar animals at multiple points in production. Each regrouping event triggers a period of intense fighting to re-establish hierarchy — fighting that is more severe, prolonged, and injurious than the rare disputes in stable groups. Wounds from these fights cause acute pain and can become infected, causing chronic welfare problems. Ears, flanks, and faces bear the most injury. The welfare cost of regrouping is well-documented and manageable: maintaining stable groups from birth through slaughter eliminates regrouping aggression, a practice used in some higher-welfare production systems. Where regrouping is necessary, providing adequate space for escape, using familiar olfactory cues, and providing distraction through enrichment all reduce fighting severity and welfare harm.