Growth performance and carcass traits in pigs selected for indirect genetic effects on growth rate in two environments

Production traits such as growth rate may depend on the social interactions between group members. These social interactions might be partly heritable and are referred to as indirect genetic effects (IGE), social-, associative-, or competitive genetic effects. IGE may contribute to heritable variation in traits, and can thus be used to increase the response to selection. This, however, has hardly been tested by selection experiments. Our objective was to determine the effects of one generation of selection on IGE for growth (IGEg) in pigs on ADG, BW, ADFI, feed efficiency, and post-mortem measurements. Sires (n=24) and dams (n=64) were selected to create a high vs. low contrast for IGEg in the offspring (n=480). The IGE difference was 2.8 g ADG per pen mate, corresponding to 14 g higher ADG in high IGEg offspring compared to low IGEg offspring when housed in groups of 6 (i.e. (6-1)×2.8 = 14). Male (barrows) and female (gilts) offspring were housed in groups of 6 of the same IGEg classification, in either barren concrete pens or pen enriched with straw and wood shavings (n=80 pens). Pigs were followed from birth to slaughter. Data were analyzed in a mixed model with pen as random factor. There was no difference in ADG between high and low IGEg pigs during the finishing period (wk 10 to 23). Opposite to expectations, high IGEg tended to have a 17 g lower ADG from weaning to slaughter (P=0.08), which was caused by a higher BW of low IGEg pigs in wk 5 (P=0.008). This led to a 2.3 kg lower carcass weight (P=0.02) and 2.2 mm less muscle depth for high IGEg pigs (P=0.03). High IGEg pigs had a higher stomach wall damage score (P=0.01). Pigs on straw had a 25 g lower ADG during finishing (P=0.03), and less stomach wall damage (P