Variation in pig weights has a major impact on pig flow and market price received, particularly in wean-to-finish production facilities. Weight variability has wean-to-finish managers routinely overstocking pens at weaning, then sorting off the lightest pigs and remixing them within the first three to five weeks after stocking. This is done in the belief that removing the lightest pigs from a pen and remixing with other lightweight pigs results in better overall performance of the group and, possibly, enhances facility utilization.

Recently, the NCR-89 Committee on Swine Management tested this sort-and-mix practice in both wean-to-finish and grow-finish production systems. Several universities in the north central region (NCR) participated. Diets were the same at all cooperating universities. Each station had at least two replications of each treatment. Facilities were both full and partial slats. Feeder space and drinkers were standardized.

In each of two experiments, the following treatments were applied:

  • Fifteen pigs/pen from initial weight to slaughter (15S),

  • Twenty pigs/pen from initial weight to three weeks postweaning (wean-to-finish) or the week the population weighed 150 lb. (grow-finish), then reduced to 15 pigs/pen to slaughter (20/15), and

  • Fifteen pigs/pen comprised of the five lightest pigs from each of three 20/15 pens (15M).

In each study, diets were based on the average weight of the whole group rather than individual pen averages. Thus, lightweight pigs did not receive any special diet or other management following remixing, a practice that is typical of many production facilities.

The populations that were sorted and mixed (20/15 and 15M) were compared to those that were never sorted (15S).

Sorting of the lightest pigs was effective in reducing the within-pen weight variation at the time of sorting (Tables 1 and 2), but had minimal effect on reducing within-pen weight variation at Day 158 postweaning for the wean-to-finish trial, or when the first pig in the pen weighed at least 250 lb. for the grow-finish trial.

Figure 1 displays the variation in pig weight of each population on Day 158, when the heaviest pigs in the wean-to-finish facility were removed for slaughter. The sorted and mixed population is represented in both ends of the population weight curve. The unsorted population is not represented in either the two lightest weight groups or the heaviest weight groups (281-290 lb., >300 lb.).

Figure 2 is a similar display of weight variation in each population, when the first pig was removed on the week it weighed at least 250 lb. for the grow-finish trial. This data has more spread since it represents the combined data of pigs at the University of Nebraska, University of Minnesota, University of Illinois and Iowa State University. However, the overall pattern is the same. That is, no pigs in the unsorted population were in the heaviest weight category and fewer landed in the lightest weight categories than did the sorted and mixed pigs. In the grow-finish study, 14% of the pigs in the sorted population weighed less than 191 lb. versus 10% of the unsorted population.

In the grow-finish study, pigs were removed for slaughter on the week they weighed 250 lb. or more. Beginning the week when 50% or more of the pigs had been removed from a pen, the remaining pigs were fed for up to three weeks or until the pen averaged 250 lb. Using this method to market pigs, Figure 3 shows the average days until the pen was empty for pigs at the University of Illinois, University of Minnesota, Iowa State University and Michigan State University.

While the pens that had the pigs removed averaged 108 days to empty, the pens that had the mixed pigs took 125 days. Pens with unsorted pigs took an average 118 days to empty.

In this study, it took seven days longer (125 vs. 118) to empty the last pen of pigs that had the lightweight pigs removed and remixed versus the population where no sorting occurred.

In these studies, removal and mixing of the lightweight pigs did not decrease the variation in population weight, nor did it improve facility utilization as measured by the days to empty pens in the grow-finish trial.

Pig Social Implications

There are several explanations why this common management practice is not effective.

First, when the lightweight pigs were removed and remixed, they had to become acquainted with new penmates, pen social structure and pen location. All of this probably contributed to a period in which feed intake and growth were minimal. When social stability was achieved, variation in the pen increased to that of other pens since some pigs became dominant, others submissive, and some settled somewhere in the middle.

In pens where pigs were removed, similar social disruptions occurred. While the pigs remaining in the pen didn't have to become acquainted with a new pen, the removal of the lightweight pigs most likely removed the lowest social ranking pigs in the group. With their removal, one or more of the remaining pigs acquired the low social rank. This most likely explains why sorting and removal was not effective in changing within-pen variations in weight at slaughter.

It is possible that results may have been different if the lightweight, remixed pigs had been offered a diet formulated to more closely match their nutritional needs versus a diet formulated to the average needs of the population. Results may also differ if the heaviest or middleweight pigs are removed and remixed.

Another common management practice is to sort pigs by weight upon entry into wean-to-finish, nursery and grow-finish facilities. This is done with the belief that pens of pigs begun at uniform weights will have less variation at slaughter weight and may have better daily gain, etc.

When this management practice was first utilized in confinement facilities, it made sense, considering the farrowing/weaning practices common to the industry. When farrowing and weaning were continuous flow, sorting by size also implied sorting by age. Thus, sorting by size was a management tool to minimize the age variation of pigs within a pen. However, in the majority of today's production systems, age variation within a facility is often minimal. It is common to put 1,000 pigs into a finishing facility with no more than three days in age variation.

Recent research has reexamined the practice of sorting by size at the time of barn placement. There are at least five controlled studies that examined the impact of sorting pigs by size (light, medium and heavyweight or light and heavy) at placement versus placing light and heavy pigs in the same pen on performance to slaughter.

In none of the studies did the practice of sorting by weight at arrival improve overall pig performance or facility utilization. In at least two studies, facility utilization was improved when pigs of varying weights were penned together (heavy, medium and light), versus when pigs of similar weight were penned together at placement.

Don't Sort

As a result of this data, the recommendation is to not sort pigs by size upon placement into a grow-finish facility, nursery or wean-to-finish facility. The exception to this recommendation is when it is possible to use management tools to treat a group of sorted pigs in a special manner. For nurseries and wean-to-finish facilities, this means that it remains accepted practice to pen the very lightweight pigs together, assuming they will remain on a starter diet sequence one to five days longer than the rest of the pigs in the facility.

For grow-finish facilities where the nutrition, temperature and other management decisions are made on the basis of the barn average, sorting by size is not recommended.

Both the sorting-and-mixing results and the sorting-by-size-at-placement results suggest that the idea of “peas-in-a-pod” pig flow from production facilities will not be a common occurrence, at least in the near future.

In confinement facilities with small pens (15 to 30 pigs/pen), the social interactions and other causes result in a natural variation in pig performance. Until more is known about effective methods to modify the pigs' social interactions with penmates, advances to decrease variation in healthy pigs will occur slowly.

Table 1. Effect of Sorting and Removal on Wean-to-Finish Pig Performance
20/15 15M 15S
Weaning wt., lb. 10.6 10.6
Day 21 - presort wt., lb.a 19.8 20.9
Day 21 - postsort wt., lb.b 21.3 15.4 20.9
Day 158 postweaning wt., lb.a 257.2 243.1 253.2
Coefficient of Variation (pig weight with pen), %
Day 21 - presort 19.5 17
Day 21 - postsortb 13.7 11.3 17
Day 158 postweaning 7.6 7.9 6.9
J. Anim. Sci. 2002. 80:1166-117
aTreatment effect, P<.05
b20/15+15M vs 15S, P<.01

Table 2. Effect of Sorting and Removal on Finishing Pig Performance
20/15 15M 15S
Placement wt., lb. 57.6 57.6
Presort and removal wt., lb. 160.4 155.8
Postsort and removal wt., lb.a 163 135.5 155.8
Coefficient of Variation (pig weight with pen), %
Placement 13.4 13.3
Presort and removal 10.2 11.4
Postsort and removalb 9.2 8.6 11.4
First pig removedb 8.2 8.7 10.2
J. Anim. Sci. 2002. 80:1166-117
aTreatment effect, P<.01
b20/15+15M vs 15S, P<.05