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Diversify Weed Management

Last fall, some enterprising southern Minnesota farmers discovered Palmer amaranth in their soybean field. 

“They found it because they were hand-weeding stray weeds prior to harvest and discovered the Palmer amaranth nestled in waterhemp,” says Jeff Gunsolus, University of Minnesota Extension weed specialist. 

Fortunately, they destroyed the stray Palmer amaranth plants prior to harvest, saving the farm from a future infestation of this pugnacious pigweed. 

For Gunsolus, discovering a weed infestation via walking beans represented a full circle from when he started studying weed science back in the 1970s. Back then, soybean herbicides were a hit-and-miss control tool that required supplemental tools like
field rouging.  

For soybeans, this changed with the advent of herbicides like Pursuit in the early 1990s and even in corn with herbicides like Accent, says Gunsolus. Eventually, though, weed biotypes that resisted these herbicides developed. Fortunately, the Roundup Ready system waited in the wings to give excellent control via glyphosate applications. Predictably, weed resistance soon developed, just as it has with numerous classes of herbicides. 

Herbicides still remain a major weed control. Still, herbicide resistance means farmers must think differently about weed control. 

“None of the strategies, from Pursuit and Accent in the 1990s to glyphosate and dicamba now, is the end-all solution,” says Gunsolus. “We have to have some diversification in weed management.”

One area he and other weed scientists are stressing is weed seed bank management. Left intact at harvest, weed seeds will pass right through the back of a combine – an effective way to seed herbicide-resistant weeds, he says. 

“Thirty years ago, every farmer’s goal was to minimize the size of the weed seed bank,” says Bob Hartzler, Iowa State University Extension weed specialist. “Herbicides were not as effective as the ones we have today. Now that we have highly effective herbicides, farmers will see escapes, and say, ‘Well, I can deal with those next year.’ ”

Those escapes, though, can contain herbicide-resistant weed biotypes that can come back in full force over several years to create a major weed headache. 

So what do you do? 

Herbicides still remain a major component in weed management. This includes stacks of herbicide-tolerant traits, such as those that contain tolerance to glyphosate and glufosinate.

“Stacked resistant traits have value,” says Gunsolus. “They will be more durable if you do not have stacked resistance in weeds.”

Instead, forestalling resistance requires a multi-pronged approach. 

“Focus on early-season weed control from April through June,” says Gunsolus. “Start with a strong soil residual preemergence program.”

This is equally important in both corn and soybeans, he says. 

“There are excellent products for both crops,” he says. 

They can buy farmers time prior to use of a post-emergence herbicide.  

“Timing of post weed control is challenging,” he says. “Dicamba still works best on early emerging weeds like lambsquarters and common and giant ragweed. It is effective on waterhemp, but it has to be when it is small.” 

Later dicamba applications on soybeans also heighten the risk of off-target movement, says Gunsolus. 

“My parents used to say nothing good happens after midnight,” he says. “It’s kind of the same thing with weed management. What I have learned through 33 years is that few positive things happen in July in weed control.”

Postemergence Contingency Plan  

Even the best-laid plans for preemergence and early weed season control can go awry, though. Dry weather may cause preemergence residual herbicides not to activate. Meanwhile, June rainfall can nix early postemergence herbicide applications. In those cases, Group 14 herbicides Flexstar or Cobra have been effective in addressing late-emerging waterhemp that doesn’t resist Group 14 herbicides. 

“Flexstar is more of a go-to herbicide than Cobra,” he says. Cobra is a hotter herbicide that tends to burn soybeans. Injury is particularly worse on fields plagued by iron chlorosis. 

However, Cobra also has the advantage of fewer crop rotation restrictions. Flexstar cannot be applied 10 months in advance of corn.

Cultural practices

Besides rouging soybeans, the following other cultural practices advised by Gunsolus, Hartzler, and others can take some of the pressure off herbicides for weed control.

  • Spot-cultivate. “This isn’t advised across every field,” says Mike Owen, retired Extension weed specialist. However, targeted cultivation in areas plagued by herbicide-resistant weeds can slow spread, he says. 
  • Periodically clean your combine. This may be limited in removing small-seeded weeds like waterhemp, but it can reduce weed seed numbers. 
  • Harvest the most weed-infested field last. 
  • Plant locally sourced CRP and pollinator seed. Palmer amaranth outbreaks occurred in Iowa and Minnesota in 2016 due to contaminated out-of-state seed. 
  • Consider source of manure. Manure sourced from weedy fields can spread weed seeds.
  • Consider narrowing your rows. A caveat in soybeans exists, as narrow rows that key limited air circulation can worsen white mold infestations. If not, though, they can serve as a weed-control tool. “Anything that can be done to increase early canopy closure is free weed control,” says Gunsolus. 
  • Address issues like soybean cyst nematode and iron chlorosis. “Soybean cyst nematode and iron chlorosis can shift the competitive advantage to weeds if they cause the soybean canopy not to close as early as it normally would,” says Gunsolus. 

Nixing Weed Seeds 

Long term, weed seed destructor technology like the Harrington Seed Destructor holds promise. 

“It is 60% to 99% effective in destroying weed seeds,” says Gunsolus. “You are harvesting anyway, so why not get some extra value from destroying weed seeds?”


Soybean varieties that resist soybean cyst nematode (SCN) are a good management tool, but they don’t perform as well in some cases as in the past. 

Over 95% of SCN-resistant soybeans share the same source of resistance—PI88788. As with weeds, repeatedly using the same source of resistance leads to SCN races resisting these varieties. That means farmers need to use integrated approaches that couple SCN-resistant varieties while considering use of seed treatments and rotation to nonhost crops, says Greg Tylka, an Iowa State University (ISU) Extension nematologist.

“I liken it to monitoring blood pressure,” he says. “We have had high blood pressure in the form of SCN,  but have not had to check it because of resistant varieties.”

No more. Now, soybean farmers need to sample their fields for SCN and use an integrated approach, he says.  

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