Regrouping of pigs at weaning, transfer between production stages and mixing of unfamiliar groups causes intense aggression with significant welfare and production consequences.
Regrouping aggression is an acute welfare event that is inherent to commercial pig production as practised in most systems. The welfare harm can be substantially reduced through thoughtful management including timing, environment and resource provision but cannot be eliminated entirely in systems requiring frequent group changes. Stable group management that minimises regrouping is the most welfare-protective approach.