The aim of the present study was to assess whether within-group variability in slaughter weight and carcass weight could be reduced by the regrouping strategy used at either weaning at 4 weeks of age, or at the start of the finishing period at 10 weeks of age. Groups were formed to be either uniform in body weight (i.e. separate groups of small, medium and large pigs), or mixed in body weight (i.e. each group containing small, medium and large pigs). Pigs either remained in the same group from 4 to 21 weeks of age, or were regrouped at 10 weeks of age.
Forming uniform weight groups at 4 weeks of age had no effect on within-group range in slaughter or carcass weight. It is suggested that this was due to high variability in growth during the growing period. Forming uniform weight groups at 10 weeks of age led to significant reductions in within-group range in slaughter weight. This meant that the time taken for all pigs in a group to reach slaughter weight was reduced by 1 week when uniform weight groups were formed at 10 weeks of age, compared to when mixed weight groups were formed at this stage, or when groups were formed at weaning.
Regrouping pigs at the start of the finishing period led to significant increases in aggressive behaviour during the post mixing period, but did not significantly affect production performance during the finishing period.
The results show that forming uniform weight groups at the start of the finishing period results in more efficient use of finishing accommodation. However, regrouping is associated with increased aggression, which has negative welfare implications. 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 leads to reduced within-group variation in slaughter weight.