The evidence base for welfare-positive group housing in commercial pig production
Individual gestation stalls — metal enclosures that prevent sows from turning around for 16-18 weeks of pregnancy — have been banned in the EU since 2013 (except first 4 weeks post-mating) and in multiple US states. The transition to group housing systems was contentious due to concerns about fighting injuries and management complexity, but accumulated research demonstrates that well-designed group systems provide substantially better welfare outcomes.
Static groups: Same sows kept together throughout pregnancy; low ongoing aggression after hierarchy established; requires careful initial mixing management.
Dynamic groups: Sows added and removed as pregnancy progresses; ongoing hierarchy challenges; more management-intensive but allows flexibility.
Electronic Sow Feeding (ESF): Individual feeding in group pen via RFID-controlled feeder; allows precise nutrition control and reduces feeding competition.
Straw-based group housing: Deep litter, ad libitum rooting material, low injury rates; high welfare outcomes; higher land and bedding requirements.
The welfare concern most cited about group housing is aggression, particularly at mixing. Research shows: mixing injuries peak in first 24-48 hours; subside to low levels once hierarchy established; can be minimized by mixing at weaning (before return to estrus), providing adequate space (>2.5 m² per sow), multiple feeding stations, and areas where subordinate animals can avoid dominant sows. Farms with good stockmanship routinely achieve group housing with injury rates comparable to stall systems.