Agonistic Interactions between Littermates Reappear after Mixing Multiple Litters at Weaning in Pigs

Weaning involves mixing of unfamiliar litters and is accompanied by an outbreak of aggression, which is usually attributed to between-litter (BL) interactions. In the present study, we thus focused on post-weaning agonism (fighting and mounting) between littermates (within-litter, WL). Two litters were weaned into two pens separated by an empty pen and connected by narrow passages that were opened 24 h after weaning. WL interactions accounted for 38% and 68% of all fights and mounts, respectively, during a 7-day experimental period. After the passages were opened, not only BL, but also WL interactions increased significantly (by 6- to 7-fold on the first day of mixing). WL fights then gradually decreased, while WL mounts continued to increase. During the experimental period, the proportion of both WL fights and mounts decreased. The majority of WL fights (≈80%) and mounts (≈65%) occurred in home pens. A significantly higher percentage of draws was found in WL fights (50% of initiator wins and 41% of draws) compared to BL fights (80% of initiator wins and 11% of draws). Results also showed less asymmetry in the body weight of piglets involved in WL interactions. Mixing of litters at weaning was shown to involve not only intense interactions between unfamiliar piglets but also the recurrence of agonistic interactions between littermates, which is something we should keep in mind when pursuing the highest standards of weaner welfare.