Grouping piglets by sex at weaning reduces aggressive behaviour

The aim of this work was to reduce aggressive behaviour when piglets are grouped at weaning.The experiment assessed whether it is better to group piglets by sex at weaning or to mix them in male–female groups. Large White piglets were weaned at 28 days and assigned to 4 conditions, with 6 groups of animals/condition: (L-MF) 4 males and 4 females reared together from birth, (A-M) 8 unfamiliar male piglets, (A-F) 8 unfamiliar female piglets, and (A-MF) 4 male and 4 female unfamiliar piglets. All groups were videotaped on days 28, 29, 31, 39 and 46 for 2 h. Aggressive interactions were quantified and scratches on each piglet were counted on days 27, 29 and 33. Feed consumption and piglets’ weights were quantified from birth to day 63. On days 27, 29 and 33, urine was collected between 7:00 and 8:00 for measurements of catecholamines and glucocorticoids.Fighting durations were longer in A-MF than in L-MF and both single-sex groups on day 29 (p < 0.05), and than in L-MF and A-F on day 28 (p < 0.05). Durations did not differ between L-MF and single-sex conditions. Fights were more severe and there were more scratches in the A-MF condition compared to the other three conditions. Conditions A-M and A-F did not differ. In the mixed-sex group, fights between two males tended to be more severe than fights between two females (p < 0.1) or than mixed-sex fights (p < 0.1); males initiated more aggression than females (p < 0.05). The presence of females seemed to increase males’ aggressive behaviour in mixed-sex groups. Growth and food intake were slightly affected by treatments but were similar between conditions on day 63. A rise of noradrenaline levels was observed after weaning for the unfamiliar piglets (p < 0.001). However, the grouping criterion by sex did not lead to changes in neuroendocrine activity.Grouping piglets by sex reduces aggressive behaviour at weaning and this should be taken into account in pig farms in order to improve animal welfare.