Relationship between growth rate and oral manipulation, social nosing, and aggression in finishing pigs

Pigs may affect each other's health, welfare and productivity through their behaviour. The effect of a pig on the growth rate of its pen mates is partly heritable and is referred to as its social genetic effect. Social genetic effects, also known as indirect genetic effects, have been found in a number of livestock breeds, in natural and laboratory populations, and in plant breeding and forestry, and have become an important research topic in recent years. In pigs, social genetic effects are hypothesized to be related to behaviour. The mechanism behind social genetic effects for growth, as well as the relationship between behaviours and growth itself, is largely unknown. To gain insight in the mechanism behind social genetic effects, we investigated the relationship between behaviours and growth rate in pigs. On a commercial pig farm, 398 finishing pigs in 50 pens (eight pigs/pen) were observed at 12 weeks of age using 2-min instantaneous scan sampling for 6 h during daytime. For 324 observed pigs, growth rate during the finishing period was known. The relationship between behaviours and growth rate during the finishing period was analysed with behaviour as explanatory variable in a mixed model. Results show that time spent giving behaviours, like oral manipulation, social nosing, aggression and belly nosing, was not related to own growth rate. Receiving behaviours, however, did relate to growth. Pigs that received more oral manipulation, observed as tail biting, ear biting and paw biting, grew less well (P < 0.05). Growth rate was 43(+/- 17) g/d lower in pigs that received oral manipulation during more than 2% of the observations as compared to pigs that did not receive oral manipulation. Pigs that received social nosing, a gentle touch or sniff at any part of the body, had a higher growth rate (P < 0.05): growth rate differed 29(+/- 17)0 between pigs that received social nosing during more than 2% of observations as compared to not receiving social nosing at all. Receiving aggression and belly nosing, a forceful rubbing of the belly, did not influence growth rate. In conclusion, receiving oral manipulation and social nosing related to growth rate. This suggests that pigs selected for positive social genetic effects for growth may potentially show behavioural changes. Effects of selection for social genetic effects on behaviour and growth will be studied in future research. (C) 2012 Elsevier B.V. All rights reserved.