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 be en the individual variation in aggressiveness between pigs. This study examined whether aggressiveness, as measured in tests on individual a nimals in a resident-intruder situation, is predictive of the level of aggression shown after mixing unfamiliar pigs, and whether informatio n on this individual aggressiveness can be used to reduce aggression a fter mixing. One hundred eighty-nine pigs were tested for individual a ggressiveness in their home pens and categorised as high-or low-aggres sive (H or L), according to their attack latency. Eighty-eight of thes e 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 mixi ng. During observations, aggressive interactions, and whether the pigs avoided lying down next to a pig from the unfamiliar litter were reco rded. 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 c lear 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 lesi ons on the pigs from the winner-litter both immediately after mixing a nd 2 days later was highest in H/H groups. Thus, the relative level of aggressiveness seemed to determine the number of pairs that fought an d 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 o f PI pigs in a group slowed down group integration. These data, partic ularly those relating to group integration, suggest that if mixing is unavoidable, it is preferable for pigs to be mixed into groups contain ing low-aggressive pigs only. (C) 1997 Elsevier Science B.V.