Sociable swine : indirect genetic effects on growth rate and their effect on behaviour and production of pigs in different environments

Social interactions between pigs can influence their health, welfare, and productivity. The effects of social interactions on individuals are partly genetic, and this genetic effect is known as an Indirect Genetic Effect. IGEs are thus the heritable effects of an individual on the trait values of its social partners, e.g. group mates. Previous research has identified IGE for production traits, which suggests that selection for IGE may contribute to selection response. However, validation through selection experiments is required. The objectives of this thesis were a) to determine the consequences of selection for ‘IGE on growth rate’ (IGEg) for production traits and behaviour of pigs, and b) to study possible mechanisms underlying IGEg in pigs. First, the relationship between pig behaviour and growth rate was studied in several trials. This showed that oral manipulative behaviours directed at pen mates, such as tail- and ear biting and chewing, can reduce growth rate of the victims, whereas receiving social nosing may enhance growth rate. Second, a one-generation selection experiment was conducted in pigs. Sires (n= 24) and dams (n= 64) were selected to create a high vs. low contrast for IGEg in the offspring (n= 480). The contrast was 14 g average daily gain (ADG). Offspring were studied in a 2×2 arrangement with IGEg (high vs. low) and housing conditions (conventional vs. enriched with straw bedding) to examine genotype × environment (GxE) interactions. Selection did not alter production traits, including ADG. Behaviour showed consistent changes, whereby high IGEg pigs showed less biting behaviour towards group mates and objects. High and low IGEg pigs did not differ in aggression or body lesions during 24-h regrouping with unfamiliar pigs. They did, however, differ in aggression towards their own group members when they were reunited after the temporary regrouping test. In combination with other tests and observations, this might indicate that high IGEg pigs are less fearful or less stress sensitive than low IGEg pigs. There were no G×E interactions, but enrichment had a positive effect on behaviour which was additive to that of selection. Despite the lack of response in ADG, genetic selection for IGEg and enriched housing conditions improved the behaviour and welfare of pigs.