Social organisation and resource requirements of pigs housed in large groups
The resource requirements and social behaviour of pigs housed in large groups were examined in a series of experiments. (i) The nipple drinker requirement was addressed by using four treatments (60 pigs, 3 drinkers; 20 pigs, 1 drinker; 60 pigs, 6 drinkers and 20 pigs, 2 drinkers). Drinker provision had no effect on water use, but in a larger group more water was used in less time (p<0.001). The diurnal pattern of drinking, overt aggression and lesion score indicated no difference between treatments. (ii) Pigs, housed on deep straw in groups of 20 or 80, were provided with a low (50 kg/m2) or high (32 kg/m2) floor space allowance. Large groups had a lower growth rate. Skin lesions were elevated and immune response was lowered by a low space allowance. (iii) Two feeder space allowances (32.5 and 42.5 mm/pig) for pigs housed in groups of 20 or 80 were investigated. Food intake was lower in the low feeder allowance treatments and pigs in large groups tended to have a reduced growth rate. (iv) Pigs from the same pen in an unfamiliar arena maintained a similar degree of proximity regardless of origin group size. (v) Pigs from groups of 80 demonstrated reduced aggressiveness (increased latency to fight, decreased rate of aggression) towards unacquainted pigs in an arena, but showed even less aggression towards pen mates. (vi) No evidence of spatial sub-division of the large group into smaller units capable of maintaining a dominance hierarchy was found. Resource provision and group size largely did not interact, but may independently compromise productivity and behaviour. A large group was associated with a reduction in performance. The dominance hierarchy was of less importance in large groups, despite recognition being intact, and sub-grouping behaviour did not provide an alternative strategy for group social organisation.