Sorting hogs according to size to reduce competition-based stress has long been the rule in pork production. However, new research shows that sorting hogs may not be all it’s cracked up to be.

Mike Brumm, University of Nebraska Extension swine specialist, coordinated a regional study that shows sorting groups of pigs while in the nursery and grow/finish stages may not improve pig performance after all.

“Sorting off lightweight pigs actually slightly increased the overall weight variation,” notes Brumm, “and the primary reason for sorting is to decrease weight variation.”

Researchers used three treatments, the first of which involved 15 pigs per pen formed when pigs reached about 58 pounds. They remained as an undisturbed group through market weight. The second treatment involved a group of 20 pigs per pen, beginning at 58 pounds. When the pigs reached about 154 pounds, the five lightest pigs were removed to reduce the group to 15 pigs per pen. Those 15 pigs remained together until they reached market weight. The third treatment involved 15 pigs per pen – made up of the five lightest pigs from three pens taken from group No. 2 – starting at 154 pounds. The study took place on ag experiment stations in Illinois, Minnesota, Iowa, Michigan and Nebraska.

Using these treatments, Brumm could focus on pig performance after the groups reached 154 pounds to determine differences between the management strategies.

Treatment No. 1 reflects the result when no sorting and remixing took place, while treatments No. 2 and No. 3 reflect what happens to the population of pigs when sorting and mixing occur.

The result was a group of pigs that was heavier than the average (treatment No. 2) and a group that was lighter than the average (treatment No. 3.) Pigs in the lightweight group grew slower and consumed less feed compared to the other groups.

To determine the effects of sorting and remixing pigs, the combined performance results in treatments No. 2 and No. 3 need to be compared to the performance in treatment No. 1.

“If your decision-making process is based on average weights of the pig groups, you should not sort,” says Brumm.

He adds that management options, like being able or committed to feeding different diets to different groups of pigs influence whether sorting is a worthwhile option for you.

“If you sort off lightweight pigs and do something different, like feeding them a first-phase diet for a longer time, or provide extra warmth in the winter, then sorting may be effective,” says Brumm.

Ken Stalder, University of Tennessee Extension swine specialist, says determining why pigs are slower growing is key to fixing the problem, whether or not it involves sorting. He points to some recent technology as adding benefit to sorting off the fastest growing pigs. “Some producers sort out the really fast-growing pigs, then feed the rest Paylean to try to get consistent weights.”

Harold Gonyou, Prairie Swine Centre, also conducted a series of studies to examine the effects of sorting pigs by weight into pens of large, medium or small pigs versus keeping a mix of pigs in the same pen. As in Brumm’s study, no differences were found in the performance of similar-sized pigs. However, in this study, pens that contained a wide range of body weights emptied earlier.

Gonyou’s data did suggest that pigs should be sorted into uniform groups whenever feed is restricted, to avoid competition. However, grow/finish pigs are rarely fed with a restricted-feed system, so they don’t generally benefit from sorting by weight. Gonyou adds that sorting on the basis of nutritional needs can be effective, and should include sorting by sex. Newly-weaned pigs should be sorted by size in order to provide the best diet to each group, he says.

There will always be a need to do some sorting, Brumm acknowledges, such as to minimize effects of animal injuries and disease. But the merits of sorting as a management tool are increasingly debatable.

However, Stalder believes in some cases, sorting pigs can still let you use your space more efficiently. For example, sorting allows you to crowd the pens early and take advantage of differences in animals’ rate of gain, by “topping off” the pens, which gives the slower growing pigs more room later.

Today, the No. 1 reason not to sort your hogs is the demand it places on labor. However, overstocking and leaving some spaces sitting empty is a somewhat inefficient use of space, says Brumm.

Similar to sorting at heavier weights, topping off pigs in a wean-to-finish facility at about three weeks increased group No. 2’s average weight, after sorting. It also resulted in the lower initial weights in group No. 3. The lightweight pigs had poorer growth and feed intake compared to the other groups. However, when management strategies were compared (No. 1 versus No. 2 and No. 3 combined) no differences in pig performance were observed. Even though sorting reduced initial variation within the pen, there was no significant difference at the experiment’s conclusion.

“If you have a herd with extremely consistent growth, sorting might not be worth the labor it requires,” says Stalder, “but that is not common.”

Sorting pigs became more necessary when continuous farrowing was the norm, and large variations in age were common to groups of pigs. But with the adaptation of all-in/all-out management systems, the need for sorting may have passed.

“With all-in/all-out, the industry removed the single biggest reason for past size variations among animals – age differences going into a room or building,” says Brumm. “Now most variation is due either to health problems or differences in weaning weights.” 

A pig group’s social order is central to the debate on sorting, because pigs low in the pecking order tend to have more limited access to feed and consequently gain less. Brumm says if you start with a pen of 25 pigs and sort out slow-growing pigs, which tend to be low in the social order, a new group of pigs will simply end up filling the spot. You will end up with two sets of poor-doing pigs, he contends.

“I am convinced that in small pens during the grow/finish phase, pigs create variation and if you reduce that variation through sorting, the pigs will create new variation,” says Brumm.

Stalder is less convinced. “I’ve conducted studies that market 25 percent to 50 percent of the pigs in an effort to mess up the group’s social structure,” he says. “I saw nothing to show the pigs were remaking the social order.”

Factors such as operation size – and the size of the pig group – may be factors in whether social order is re-established and whether or not sorting is advantageous. At it’s best, sorting can be a tool that lets you manage pigs to reduce sort loss. At it’s worst, sorting involves a lot of legwork for little impact, unless you commit to managing the pig groups differently. 

A New Age of Sorting
When you think of sorting, the image likely conjures up someone directing hogs with a wooden, hand-held panel. But sorting is changing with the times.

Today’s automatic sorters offer reduced labor and increased precision. Products like Farmweld’s FAST and Osborne’s Weight Watcher offer new possibilities, but the technology comes at a cost, and with some concerns.

Automated sorters can be used with pigs housed in conventional-sized barns with large pens holding up to 500 to 600 pigs. The sorters feature a scale that weighs pigs as they eat, then routes heavy pigs to one area and lightweight pigs into another based on a preset weight cutoff. Other features vary depending upon the system, but automatic systems take the guesswork and legwork out of sorting pigs.

“The new technologies offer more questions than answers,” says Mike Brumm, University of Nebraska Extension swine specialist. “They are good at weighing, and they will help you hit the packer matrix windows more easily.”

“Automatic sorters look like they could be a good new technology,” says Ken Stalder, University of Tennessee. “I don’t know whether the savings you can achieve from reduced sort loss will counter the extra equipment costs or not. More research is needed.”

Management changes may be necessary to accommodate such technologies. Brumm points to one producer who had previously marketed hogs weekly, but now sends hogs to market twice a week, because of  the automatic sorters’ precision. The producer now sells half of the pigs early in the week, and the other half later in the week, to hit a more narrow weight window. The producer needed to address that pig-flow change throughout his production system, including how it affected labor demands.

With these types of automatic sorting systems, there’s little doubt that if everything functions properly, you will rarely if ever miss your packer matrix’s weight window, says Stalder. However, the value of that varies greatly, depending on your packer’s matrix.

“The largest users of automatic sorters tend to sell to a Hatfield or Hormel matrix. Returns won’t be there as fast if you sell to an IBP matrix,” says Brumm.

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