The effect of mixing levels on aggression at weaning in piglets

•Higher aggressiveness at mixing resulted in fewer lesions.•Higher mixing level was associated with higher fighting activity and more skin lesions.•Animals from group of higher mixing level had a shorter latency to be involved in the first fight.•Higher mixing level induced more weight loss.

This study was undertaken to investigate the effects of mixing levels on aggressive behaviors at weaning in piglets. In this study, 144 28-day-old piglets, which were from 24 litters, were weaned and relocated to four experimental tests with different mixing proportion (A: 6:0; B: 5:1; C: 4:2; D: 3:3), and mixed into 24 pens comprising three males and three females from two different litters with the balance of sex and weaning weight. Latency involving in the first fight, number, duration and initiator of fights, fight outcomes and bullying events were scored for 5 h and 1 day after mixing and skin lesions were counted at 5 h, day 1 and day 2 after mixing. Data were analyzed using a general linear model (GLM-ANOVA) to test the effects of post-weaning day, mixing level, sex, weight and their interactions. Overall, with the increase of mixing level, the number of fights (P < 0.001), time spent fighting (P < 0.001), mean duration of a fight (P = 0.019), time spent bullying (P = 0.009) and lesion (P < 0.001) on the day of mixing increased. Males won more fights (P < 0.001), initiated more fights (P < 0.001), bullied more frequently (P < 0.001) and have been inflicted more skin lesions (not significant). Meanwhile, heavier pigs won more fights (P = 0.03), initiated more fights (P = 0.02) and bullied more frequently (P < 0.001). In conclusion, growing mixing level increased aggression, lesion and weight loss after mixing. Sex and weight were related to some aspects of fighting behaviors.