Impact of Aggression and Social Rank at Mixing on Stall Utilization and Subsequent Farrowing Performance in Free Access Stall Systems

Abstract Mixing pregnant sows in gestational group-housing leads to heightened aggression as social hierarchies form, raising concerns about potential impacts on farrowing performance. Free access stalls (FAS) may reduce these interactions by providing individual, protected stalls for each sow. The objective of this study was to evaluate the effect of aggression and social rank at mixing on stall utilization and subsequent farrowing performance of pigs housed in a FAS system. Nulliparous gilts (n = 32) and primiparous sows (n = 32) were housed separately in FAS pens (n = 8/pen). At 25 days of gestation (dg), pigs were moved into the FAS and locked in stalls for acclimation to the new environment. At 30 dg, the stalls were unlocked and 72 h of continuous overhead video was recorded. To determine stall utilization, videos were manually decoded for time of stall entry and exit. Aggression was assessed by total skin lesion counts 1 d post-mixing and video was manually decoded for agonistic interactions including fight duration, agonistic behaviors (e.g., head-to-head, biting), initiator/receiver, and outcome (win, loss, stand-off). A dominance index was calculated and social rank within pen was assigned. Common farrowing metrics (e.g. gestation length, number born alive, litter birth weight) were recorded. Farrowing performance measures were analyzed using a Gaussian linear mixed model including fixed effects of aggression and parity, and random effect of pen. Total stall utilization was negatively associated with number of fights (P 0.001), but was not associated with dominance index, social rank, or total lesions (P 0.05). Therefore, it is unclear whether pigs utilized the FAS for protection against aggression or if pigs with lower stall utilization had greater opportunity to engage in fighting. No differences were detected in gestation length, number born alive, or litter wean weights based on aggression measures (P 0.05). Sows that farrowed stillborns had a significantly higher social rank than sows that did not (P 0.05). Litter birth weights tended to be higher for sows that engaged in more fighting during the 3 d post-mixing period (P = 0.08) and was significantly higher in sows that initiated and won more fights (P 0.05). In conclusion, while aggression events around mixing did not negatively influence farrowing performance, the number of fights won early at mixing may be associated with higher litter birth weights.