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22 Sobering Need-to-Know Facts About Herbicide-Resistant Weeds

Gil Gullickson Updated: 05/12/2015 @ 11:29am Crops Technology Editor for Successful Farming magazine/Agriculture.com

1. Nothing new is coming

Ever notice the “new” herbicide products that come on the market? They really aren’t new.

Most of the time, they’re a mix of existing herbicide active ingredients. That’s a strategy the late Marshall McGlamery, University of Illinois Extension weed specialist, called “Can ’em and confuse ’em.” 

Other times, the compound actually contains a new active ingredient. Still, it belongs to an existing herbicide mode of action. The last new mode of action launched in the corn and soybean market was the HPPD inhibitor (Callisto, Balance Flexx) in the late 1990s. 

“The streak continues,” says Mike Owen, Iowa State University (ISU) Extension weed specialist. “There are no new herbicide modes of action available for 2014 or the foreseeable future.”

That’s not good news as far as resistant weeds are concerned. For now, you’ll have to make do with the existing chemistries to manage weeds.

2. You can’t diversify enough 

The good news is farmers are diversifying herbicide use. In Iowa, for example, use of nonglyphosate herbicides in Roundup Ready crops is 46% in soybeans and around 78% in corn. That’s according to 2011 Monsanto research. 

There’s still a ways to go, though. “In Iowa, there are around 8 million acres where just glyphosate is being used,” says ISU’s Owen. “That’s still too much. I do not think over last the last five to 10 years that we have pushed the bar very far in regard to adopting more diverse weed-management systems.” 

3. Pre's don’t always get applied 

Use of multiple and effective herbicide modes of action is a cornerstone of diversification. 

Part of this is applying residual preemergence herbicides that nix early-emerging weeds.

“I know that last winter and going into spring, a lot of people had a lot of good intentions to do the right thing,” says ISU’s Owen. “As the season developed, a lot of those good intentions went by the wayside.” 

Weather makes timely applications a struggle, adds Owen. Still, stick with it. 

“Year in and year out, preemergence herbicides will give you better control,” says Kevin Bradley, University of Missouri (MU) Extension weed specialist.

One option may be to consider early preplant residual applications, adds Owen. This could enable you to spread workload and to get more preemergence herbicides on fields for early applications.

4. Preemergence benefits are limited 

Numerous preemergence residual herbicides squelch weeds well. Still, they nix weeds for only 30 to 45 days. That’s a problem with season-long germinators like waterhemp.

“On average, waterhemp has five germination events, with the last one coming in late June or early July. No herbicide you apply in April or early May will last that long,” says ISU’s Owen. 

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