Large variations in weight of pigs within groups at slaughter can lead to either:

1. inefficient use of resources, as some pigs have to be retained longer than others
2. large variation in carcass weight, which is a problem for the processor

Research

A total of 1200 pigs were used in a study which involved sorting pigs into even-weight groups, i.e. separate groups of light, medium or heavy pigs, either at 4 weeks of age, or at the start of the finishing period at 10 weeks of age. Mixed-weight groups, i.e. groups that contained light, medium and heavy pigs, were also formed at these stages. All pigs were housed in mixed gender groups of 10 animals, and were kept in the same group from regrouping until slaughter at 21 weeks of age.

Are there benefits in sorting for weight at weaning?

Sorting pigs into even-weight groups at weaning did not have any benefits in terms of reducing variability in weight at slaughter (range in weight was 35 kg in even-weight groups and 33 kg in mixed-weight groups). This was due to high variability in growth rate during the growing period. For example, the range in weight within groups increased 6-fold over the growing period (from 4 to 10 weeks) and only 3-fold over the finishing period (from 10 to 21 weeks).

Are there benefits in sorting for weight at the start of the finishing period?

Within-group range in slaughter weight was reduced by 7 kg through forming even-weight groups at the start of the finishing period (range in weight was 26 kg in even-weight groups and 33 kg in mixed-weight groups). If pigs are growing at a rate of 1 kg/day, this suggests that the time taken for all pigs to reach a target slaughter weight (or the ‘pen clearing time’) will be reduced by 1 week through forming even-weight groups at the start of the finishing period.

As expected, even-weight groups reached the target slaughter weight of 100 kg at different ages. Groups of heavy pigs reached the target slaughter weight at 20 weeks of age, groups of medium pigs at 21 weeks of age, and groups of small pigs at 22 pigs of age. On average, mixed-weight groups reached the target slaughter weight at 21 weeks of age.

Conclusions from the study

• Forming even-weight groups at the start of the finishing period makes more efficient use of finishing accommodation.
• Future research should concentrate on reducing variability in growth during the growing period, so that forming even-weight groups at weaning leads to reduced within-group variation in slaughter weight.

Sorting for weight at the start of the finishing period can produce more uniform groups of pigs at slaughter