Pig Aggression at Regrouping and Mixing Management

Regrouping aggression is an unavoidable welfare cost of commercial pig production, occurring whenever unfamiliar pigs are mixed. Understanding pig social behaviour and applying evidence-based mixing protocols significantly reduces injury severity and duration.

Key Facts

Pigs establish social hierarchies through fighting when mixed with unfamiliar individuals — this initial aggression can last 24 to 72 hours
Injuries from regrouping include ear, flank, and tail biting wounds that become infected and cause chronic pain if not treated
Same-weight mixing reduces the physical advantage of larger animals and measurably lowers injury rates compared to mixed-weight groups
Introduction of unfamiliar pigs at feeding time increases competition and aggression; introduction at night under dim light reduces initial fight intensity
Olfactory camouflage using essential oils applied to all animals in a mixing group reduces agonistic behaviour in some trials

Welfare Considerations

The pain and stress of severe regrouping aggression accumulate over the first two to three days after mixing and cause cortisol elevations, feed intake reduction, and immune suppression. Severe biting wounds become chronically infected, causing ongoing pain that depresses productivity and welfare for weeks after the initial mixing event. Welfare-centred producers minimise regrouping frequency through stable group management, apply evidence-based mixing protocols when regrouping is unavoidable, provide additional space on the day of mixing to allow retreat, and monitor mixing groups intensively for the first 48 hours to intervene if aggression escalates to serious injury.

What You Can Do

Support pork certification schemes that require documented mixing protocols and aggression monitoring standards
Ask retailers about the regrouping frequency and mixing management protocols in their pork supply chain producer codes
Donate to pig welfare charities funding research into effective low-aggression mixing protocols for commercial conditions
Encourage pig farmers to adopt night mixing, same-weight grouping, and escape route provision as standard mixing practice
Choose pork from farms that maintain stable groups through production and minimise regrouping events