The Role of Environmental Enrichment and Back Fat Depth in the Intensity of Aggressive Behavior Performed by Sows during the Establishment of the Dominance Hierarchy

SIMPLE SUMMARY: The need to establish a social hierarchy represents a period of severe stress for sows when they are introduced into new groups because of the associated aggressive behavior. The aim of this study was to determine the effect of providing access to straw in racks and manila ropes (IMPROVED) on aggressive behavior after mixing, compared to a typical minimal enrichment gestation system (CONTROL), and to understand the effect of parity order and backfat on the level of the aggression. The overall average number of fights and the initiated aggressive behavior in the three observation days were greater in the CONTROL pens, although within time, we found a significant effect of treatment only on the number of fights performed 3 weeks post-mixing. The effect of backfat thickness revealed an effect only on the observations of initiated aggressive behavior, which was most frequently performed by sow with a low backfat thickness. Parity order did not have a significant effect on the display of any of the aggressive behaviors. These results suggest that aggression at mixing is unavoidable and, indeed, is essential to ensuring the establishment of the dominance hierarchy and thereby achieving group stability. Nevertheless, optimal enrichment materials could have a beneficial effect on at least reducing its frequency. ABSTRACT: For sows introduced into new groups, the aggressive behavior associated with establishing a social hierarchy represents a period of severe stress. The aim of this study was to investigate the effect of providing sows with an improved pen environment (straw in racks and ropes) on aggressive behavior after mixing and to understand the role played by sow back fat thickness and parity order. At 29 d post-service, sows were mixed into IMPROVED or CONTROL pens with individual feeding stalls (6 groups/treatment, 20 sows/group). Aggressive behavior was recorded for 2 h at mixing (T0) and 24 h (T1) and 3 weeks post-mixing (T21). Overall, the sows in the CONTROL pens performed more fighting behavior compared to the IMPROVED sows (p < 0.001). This difference was significant only at T21 (p < 0.001). Additionally, the sows in the CONTROL pens generally initiated more aggressive behaviors than the sows in the IMPROVED pens (p = 0.02). The sows with a low back fat thickness initiated more aggressive behaviors, but parity had no significant effect on any of the aggressive behaviors. These results indicate a beneficial effect of improvements to the pen environment on the aggression performed by group-housed sows between the time of mixing and three weeks later. The effect was reduced on the day of mixing, which is in accordance with the necessity for sows to employ aggression to establish the dominance hierarchy.