Hierarchy formation in newly mixed, group housed sows and management strategies aimed at reducing its impact

•Aggression is highest when sows are first introduced and hierarchies are formed.•Aggression negatively impacts sow welfare and production.•Published recommendations for optimal mixing conditions are inconsistent.•Research should identify practical improvements to mixing conditions.

Aggression is at its highest when sows are first introduced to new animals and hierarchies are being established. Thus, methods to reduce aggression should focus on this period. The aggression that occurs during mixing results in physiological stress responses, which can have detrimental effects on sow welfare and reproductive parameters. In pigs, the short-term nature of this response means that these effects can be avoided with careful management. Many attempted aggression management techniques, such as sedation and boar presence, only lead to short term reduction of aggression at mixing. Other methods, such as high space allocation at mixing, same parity mixes and small group sizes, highlight that simple factors can reduce aggression, but optimum management has not yet been determined. The objectives of future studies should be to isolate the techniques which can be practically managed on commercial farms and aim to provide further and conclusive information on optimum mixing management. In the interim, management techniques such as providing as much space as practical at mixing and reducing the number of limiting resources can be implemented, until optimum management techniques can be defined.