The size of pig social groups in commercial production significantly affects aggression, stress and welfare, with very large groups and very small groups each presenting different welfare challenges.
Pigs are highly social but aggressive toward unfamiliar individuals. Mixing causes intense fighting that can last 24-72 hours with serious injuries. Large stable groups that are never mixed have better welfare than smaller groups subject to repeated mixing events. Individual monitoring and disease detection becomes progressively more difficult as group size increases, leading to delayed treatment of illness and injury.