The effect of group housing on feeding patterns and social behaviour of previously individually housed growing pigs

Group housed pigs make less frequent feeder visits of longer duration, and eat at a faster rate than pigs housed individually. They also have lower growth rates which may be due to elevated stress levels resulting from changes in the concentrations of hormones, such as cortisol and adrenaline associated with aggression and social stress. The aim of this experiment was to investigate the effects of grouping on feeding pattern, time budgeting and the social behaviour of pigs kept as individuals from weaning until grouping. In total, 12 Large WhitexLandrace male pigs (four pigs per block) mean (+/-S.E.) start weight 22.5+/-0.7kg were housed individually for 3 weeks (Period 1) after which in two replicates (Blocks 1 and 3), pigs were combined into a group of four (Period 2) before being returned to individual housing for a further 3 weeks (Period 3). In Block 2, the four pigs remained as individuals across periods but were moved between pens at the end of Periods 1 and 2 to account for any pen effects. Feeding pattern and food intake were recorded throughout and pigs were weighed three times a week. Video recordings and live behavioural observations were made to record time budgets and social behaviour. Grouped pigs made less visits to the feeder in Period 2 than when they were housed individually in Periods 1 (P<0.001) and 3 (P<0.01). Visit duration was longer in Period 2 than in Periods 1 (P<0.01) and 3 (P<0.05). Food intake and weight gain were greater in Period 3 than in Periods 1 and 2 (P<0.001 and P<0.01, respectively). There were no significant effects of moving pigs between pens in Block 2 on feeding behaviour and timebudgets. In Period 2, grouped pigs slept more (P<0.01) and spent less time feeding (P<0.01) and rooting (P<0.01) than in Periods 1 and 3. The frequency of aggression decreased over time from mixing (P<0.001). Possible explanations for the changes in feeding behaviour when pigs are moved from individual to group housing are competition, group cohesion, or that the high frequency of feeder visits when the pigs are housed individually is a consequence of a lack of social stimulation. Of these different possibilities, the results suggest that group cohesion is most likely to have been causal in the observed changes in feeding behaviour.