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More weed resistance this year

Weed resistance to herbicides continues to become a more widespread problem in Iowa and elsewhere, as evidenced by the number of weedy fields late in the 2011 growing season.

“This was a breakthrough year for weeds evolving resistance to glyphosate herbicide,” observes Iowa State University Extension weed scientist Mike Owen.

He and colleague Bob Hartzler at ISU have been warning farmers and the crop chemical industry about this developing problem for a number of years, as the continual use of a single herbicide with a single mode of action intensifies the selection pressure for resistant weeds.

Key Points

Reports increase about problems in 2011 with glyphosate-resistant weeds.

Other herbicides can also run into weed resistance issues if not managed correctly.

Use a diverse set of management tactics to effectively control weeds.


“While glyphosate failures are growing increasingly evident, farmers and all of us involved with weed management need to go beyond just focusing on glyphosate,” says Owen. “We’ve had weed resistance problems with herbicides for a long time.

Advertisements for herbicides may say a certain product has two or three modes of action that will control herbicide-resistant weeds. Well, it’s not that simple. For example, all of the waterhemp in Iowa is already ALS-resistant. So, some of the herbicide modes of action being talked about aren’t going to be effective.”

Glyphosate resistance is fairly widespread across the Midwest, and it’s increasing at an increasing rate. But there’s also a new herbicide resistance issue developing. That is, weed resistance to the HPPD family of herbicides.

There are currently a number of HPPD products on the market, including Callisto, Laudis and Impact. These are all effective herbicides and can be good weed management tools, just as glyphosate was, if managed correctly, notes Owen.

If any herbicide is used continually in the same field, problems will eventually result. “Management of weed resistance to herbicides is becoming more complex,” he says, “and unfortunately, more costly to manage.”

Survival of the fittest

While weed resistance to glyphosate is a hot topic these days, the scientific theory isn’t new. “What we’re talking about is a natural function, one that Charles Darwin predicted over 100 years ago,” says Owen.

So far in Iowa, weed resistance has been confirmed in just three weeds — common waterhemp, giant ragweed and horseweed, or marestail, as horseweed is often called. But resistance is potentially present in other weeds, too.

Farmers in the southern U.S. are already learning the hard way. Due to continued use of glyphosate in cotton and soybeans, severe weed resistance problems have developed. Palmer amaranth, a type of pigweed similar to common waterhemp, has taken over many fields the past few years in some Southern states.

Farmers there now have to hire crews to walk beans with hoes to chop out glyphosate-resistant weeds that escape glyphosate treatments.

Whatever herbicide technology is used, Owen says growers need to improve the diversity of weed management tactics they use and recognize the risk of overusing any one treatment, regardless of its effectiveness in a given year. “If all you’re doing is spraying one herbicide and you’re going to do that for consecutive years, ultimately and inevitably, it will fail,” he says.

Farmers have to address this issue using the current products available that are effective on weeds. That basically gets away from the principles that have made Roundup Ready technology and glyphosate so popular. That is, the glyphosate system is simple, convenient and cheaper than most alternative herbicides.

Rotate herbicides in 2012

“We’re going to have to not just rotate herbicides but also think about practicing what I call ‘redundant’ weed control,” says Owen. For example, if you apply a post-emergence treatment, you need to do more than rotate modes of action, you need a couple of modes of action in your herbicide program. Otherwise, whichever mode of action you choose to use, if you use it continuously, it’s going to be the one that selects for the weed resistance.

In the 1980s weed resistance to ALS herbicides evolved, and in more recent years, the widespread and continuous use of glyphosate has resulted in serious problems, and now the HPPD resistance is developing. “This has happened because we didn’t use appropriate management,” says Owen.

“We basically thought, ‘Hey, these products are effective, economical, simple to use, and life will be great.’ But Mother Nature finds a way around the simple type of management approach, and problems with weed resistance can evolve faster than you think they will.”

What about new herbicides or new technology coming along? Aren’t there new products in the pipeline that farmers will be able to use to control resistant weeds? Many farmers are hoping the crop protection industry is going to ride in on a big white horse and save everything by coming up with new chemistry. But Owen doesn’t see such products or technology in the development pipeline.

“We have to use what products we have in the marketplace to cope with weed resistance,” he says. “There will be new products introduced in the future, but they will need to be managed correctly, too.”

For more information on weed management, go to www.weeds.iastate.edu.

Establish a longer-term plan

Don’t just think about rotating herbicides year to year. ISU’s Mike Owen suggests you establish a five-year weed management plan for your fields. “Understand which herbicides you are using, how they control weeds,” he says. “You need to know which weeds you have in each field, and which ones are hardest to control. That gives you an idea of resistant populations you may already have or which ones could evolve.”

Make a spreadsheet and write in you’ll use herbicide A and B this year, but not next year because they have the same kind of modes of action. From there you set up a plan for the coming years, a plan that is as diverse as you can make it, says Owen. “The way to get around this weed resistance issue is to diversify the weed management you use,” he adds. “Mother Nature is smart. You have to keep her off balance, and that is done by using multiple ways of controlling weeds. And that may even include cultivation in some cases.”

Increasingly, weeds are not being effectively controlled by glyphosate alone. “Farmers need to diversify their weed management program by using a preemergence herbicide, either an early preplant application, which I would recommend, or a preemergence application,” says Owen. “Then follow up with a postemergence herbicide where needed.”


This article published in the November, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.