Typically, pigs have been sorted in an attempt to optimize growth rate, make efficient use of available space, ensure pig health, and reduce pen weight variation.

Sorting By Weight

A Canadian study evaluated weight variation at market of weaning pigs which were sorted into either a high weight variation (20 lbs.) or a low weight variation (10 lbs.) treatment. The results indicate that the low weight variation in a pen was not maintained through finishing. The social strife in a pen appears to be the greatest between pigs of similar weight because the hierarchy structure of the pen is possibly pushed to increase variation until social order is established.

Sorting By Sex

Producers have begun sorting pigs by sex to reduce feed costs and better match nutrient supply with requirements. Barrows grow faster, eat more and have higher nutrient requirements than gilts. While research has shown split sex feeding does not impact market weight variation in a pen, it does decrease cost of gain.

Sorting By Litter

It is known that mixing pigs results in more aggression, which ultimately affects the productivity for at least the following 2 weeks. One way to maintain familiarity is to keep litters of pigs together at weaning. However, studies have shown no differences in performance due to the number of pigs coming from the same litter within the same pen.

The data suggests that the wean-to-finish response is a nursery phase response with no difference in growth performance between the housing (wean-to-finish) and mixing during the growing-finishing phase. Those pigs housed in a wean-to-finish building did have a lower coefficient of variation at market weight or when the first pig was removed from the pen, which could result in better market premiums or less sort loss applied by the packer.

Wean-to-Finish Technology

The objective behind the development of wean-to-finish facilities was to minimize the moving and resorting of the growing pigs between the nursery and finishing phase. Earlier wean. to. finish research has only looked at the impact during the nursery period on growth performance. The research concluded that pigs housed in wean-to-finish housing systems were slightly heavier in body weight at the end of the 8 week nursery period compared to pigs weaned into a conventional nursery. However additional research has shown little improvement in subsequent grow-finish performance.

An experiment was conducted evaluating the following four housing treatments:

1. Wean. to-Finish (WF) into 2.4 x 4.3 m pens (15 pigs/pen)
2. Double stock wean-to-finish (Same pen)
3. Double stock wean-to-finish (Move to new pen)
4. Nursery (N)

Conclusion

The research referenced indicates the best method to sort or mix a group of pigs at weaning is whatever minimizes labor requirements and maximizes the pig flow through facilities. There appears to be no lasting impact of mixing or sorting on nursery or grower pig performance. Sorting by sex can improve feed efficiency and reduce feed costs. Regrouping pigs for the finishing phase should be avoided.