Mixing unfamiliar pigs in group housing systems causes aggression and injury that can be minimized through evidence-based management strategies.
The transition to group housing for sows — required in EU from first farrowing, progressively adopted globally — brings welfare benefits from freedom of movement but welfare risks from social mixing aggression. The aggression at mixing is predictable and manageable rather than inevitable and uncontrollable. Management strategies that reduce the welfare cost include pre-mixing familiarization, providing adequate space and retreats during the initial hierarchy establishment period, timing mixing strategically, and maintaining stable groups that are not routinely disrupted. Electronic sow feeding systems eliminate competitive feeding aggression by feeding individuals in sequence. The welfare outcome of group housing depends on whether the management investment in transition protocols is made — poorly managed group housing may cause more acute welfare harm than well-managed individual housing, despite the latter's chronic confinement costs.