Individual aggressiveness of pigs can be measured and used to reduce aggression after mixing

Many studies have been carried out with the aim of reducing aggression after mixing unfamiliar pigs. A major problem in these studies has been the individual variation in aggressiveness between pigs. This study examined whether aggressiveness, as measured in tests on individual animals in a resident-intruder situation, is predictive of the level of aggression shown after mixing unfamiliar pigs, and whether information on this individual aggressiveness can be used to reduce aggression after mixing. One hundred eighty-nine pigs were tested for individual aggressiveness in their home pens and categorised as high-or low-aggressive (H or L), according to their attack latency. Eighty-eight of these pigs were then mixed in groups of eight, with four pigs from each of two litters. The combinations used were H/H (4H + 4H), H/L (4H + 4L) and L/L (4L + 4L). In a follow-up study, a further 32 pigs were mixed into the combinations HL/HL (HHLL + HHLL). The pigs were observed for 3 h on the day of mixing, and for 2 h on days 1, 2, 6 and 7 after mixing. During observations, aggressive interactions, and whether the pigs avoided lying down next to a pig from the unfamiliar litter were recorded. Fresh skin lesions were counted on each pig 2 h following mixing, and again 2 days later. In the majority of the groups, there was a clear distinction between a winner-and a loser-litter within the first 2 h after mixing. The number of pairs fighting in the 2 h immediately following mixing was lowest in the H/L groups. The number of skin lesions on the pigs from the winner-litter both immediately after mixing and 2 days later was highest in H/H groups. Thus, the relative level of aggressiveness seemed to determine the number of pairs that fought and the absolute level determined the intensity of fighting, with L pigs fighting less vigorously than H pigs. Speed of group integration was again affected by the absolute level of aggressiveness. The presence of PI pigs in a group slowed down group integration. These data, particularly those relating to group integration, suggest that if mixing is unavoidable, it is preferable for pigs to be mixed into groups containing low-aggressive pigs only. (C) 1997 Elsevier Science B.V.