Aggression when pigs are mixed into new groups is a major welfare issue causing injury, stress, and death — evidence-based management strategies significantly reduce the welfare costs of inevitable group changes.
Mixed pigs engage in escalating agonistic encounters including head-to-head pushing, shoulder barging, and biting — subordinate individuals flee repeatedly, exhausting themselves and accumulating injuries. Fresh wounds are vulnerable to secondary infection, and severe biting can cause life-threatening blood loss. Large groups (over 40 individuals) paradoxically show less severe individual aggression than medium-sized groups because dominance cannot be established clearly. Enrichment with straw or rooting substrate redirects aggression. Anti-aggression partitions in pens allow losers to escape persistent attackers.