Effect of social group size and initial live weight on feeder space requirement of growing pigs given food ad libitum

Feeder space allowance should be sufficient to ensure adequate access to food for all group members, irrespective of competitive ability. However, the influence of social group size on minimum feeder space requirement of pigs given food ad libitum is poorly understood. Performance, aggression and feeding behaviour were assessed over a 6-week period from 29.3 (s.e. 0.19) kg live weight, using four replicates of a 2 X 2 factorial design with two group sizes (20 v. 80) (small and large) and two feeder space allowances (32.5 v. 42.5 mm per pig) (low and high). Food intake was significantly lower in the low feeder space allowance treatments (1.44 v. 1.56 (s.e.d. 0.050) kg per pig per day, P < 0.05) and group mean growth rate was reduced in the later phase between 41 and 56 kg live weight. There was no main effect of group size or interactive effect between group size and feeder space allowance on performance. Mean skin lesion score and the frequency of aggression given or received at the feeders was unaffected by treatment. Heavy weight pigs showed similar feeding behaviour in each treatment, but light pigs visited the feeder more frequently in groups of 20 (23.1 v. 16.0 (s.e.d. 2.04) visits per pig per day, P < 0.05) and spent the greatest time feeding when at a low feeder space allowance (5461 v. 4397 (s.e.d. 288.8) s per pig per day, P < 0.05). To avoid a depression in growth rate, pigs >40 kg should be allocated a minimum feeder space allowance of 42.5 mm per pig. There was little indication of a need to specify differential feeder space allowances according to group size.