Changing the animal or the environment: changes in breeding strategy and housing conditions to improve the welfare of pigs

Interactions between pigs may, apart from the physical environment in which they are kept, profoundly affect their welfare, and are influenced by their genetic background. Behaviour of pigs is, however, difficult to address through breeding. Indirect (or associative, or social) genetic effects (IGE), i.e. heritable effects that individuals have on traits of their group members, might be used to indirectly obtain pigs that behave well in groups. We studied pigs (n=480) diverging in IGE on their pen mates’ growth (IGEg) in 80 barren (B) or straw-enriched (E) pens. We recently found pigs with a favourable IGEg (IGEg +) to inflict less tail damage, show less ear biting and chewing on toys than IGEg - pigs. Moreover, after 24-h temporary mixing with unfamiliar pigs, IGEg - pigs showed more aggression upon reunion with their own pen mates. These behavioural differences may be associated with or reflect physiological differences. Here, we aimed to study effects of the new breeding strategy and housing on concentrations of leukocytes, lymphocytes and haptoglobin. Blood was drawn at 8, 9 (3 days after mixing) and 22 weeks. Housing, IGEg class, week and interaction effects were analysed with mixed models. Leukocyte levels decreased over weeks (P