Influence of regrouping strategy on performance, behaviour and carcass parameters in pigs

One thousand two hundred pigs were weaned at 4 weeks of age and mixed to form groups of ten animals that were balanced for gender. The groups consisted of uniform weight groups (i.e. separate groups of small, medium or large pigs), or mixed weight groups (i.e. groups containing small, medium and large pigs). Half of the groups were retained from weaning until slaughter at 21 weeks of age, and half were regrouped at the start of the finishing period at 10 weeks of age. In this regrouping, uniform weight groups were regrouped to form mixed weight groups, and mixed weight groups were regrouped to form uniform weight groups. In addition, some mixed weight groups were regrouped to form mixed weight groups in order to assess the effect of regrouping at 10 weeks of age on performance and aggressive behaviour. The formation of uniform weight groups at 4 weeks of age did not significantly reduce within-group coefficient of variation in 21-week weight or carcass weight relative to mixed weight groups formed at this stage. It is suggested that the lack of treatment effect was due to high variability in growth during the growing period. Forming uniform rather than mixed weight groups at 10 weeks of age led to significant reductions in within-group coefficient of variation in 21-week weight and carcass weight (P<0.05). However, regrouping at the start of the finishing period was associated with significant increases in aggressive behaviour during the 2-day post regrouping period (P<0.001), and thus has negative welfare implications. Mean feed intake, growth rate and food conversion ratio during the growing and finishing periods were not significantly affected by forming either uniform or mixed weight groups. In addition, aggression during the post regrouping period at 10 weeks of age was not significantly affected by forming either uniform or mixed weight groups. The results of the present study show that there are benefits in forming uniform weight groups at 10 weeks of age, in terms of reducing within-group variability in weight at slaughter. However, regrouping at the start of the finishing period has negative welfare implications in terms of leading to increased aggression. It is suggested that future research should concentrate on reducing variability in growth during the growing period so that forming uniform weight groups at weaning reduces variability in weight at slaughter.