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War on weeds

If you keep controlling weeds the same way year after year, don't sweat it. You'll always have a job, along with jobs for your friends.

“Glyphosate-resistant Palmer amaranth is the species that has brought new employment opportunities to mid-South cotton fields,” jests Aaron Hager, University of Illinois Extension weeds specialist. “It has brought back chopping crews.”

In the South, chopping Palmer amaranth is the only way to rid fields of it. Resistance has rendered herbicides useless in many cases.


Learn more about the history of herbicides and herbicide-resistant weeds

These may be a predictor of things to come in the Midwest.

“When I look at fields in the Midwest and see waterhemp and giant ragweed problems, it looks like Arkansas in 2005 and 2006,” says Bob Scott, University of Arkansas Extension weeds specialist “We knew we had a problem, but it wasn't bad enough to take action. Using glyphosate-tolerant crops and applying glyphosate after glyphosate have led us to the problems we have.”


Download the War on Weeds game!

Weed resistance to herbicides isn't new, says Mike Owen, Iowa State University (ISU) Extension weed specialist. Resistance to atrazine, a popular corn herbicide, first surfaced in the U.S. in 1970. ALS inhibitors, a widely used herbicide mode of action, suffered rampant weed resistance in the early 1990s following repeated applications of the then-popular herbicide Pursuit.

“Today in Iowa, you can't swing a dead cat by the tail without hitting an ALS-resistant weed,” jests Owen. “With glyphosate, the same thing is happening again.”

Waterhemp way

Waterhemp is the weed that changed the rosy Roundup Ready weed-control scenario of the 1990s.

When Pat Tranel took an ISU under-graduate weed science class over 20 years ago, waterhemp wasn't even on the list of weeds to learn. “Twenty years ago, it was also unknown in Illinois,” says the University of Illinois weed scientist. “Now it's the worst weed in Illinois.”

Historically, this pigweed family member grew in wet habitats like pond margins. Waterhemp became common in crops in the late 1980s and early 1990s. Herbicide-use patterns helped prompt its proliferation. ALS-inhibitor herbicide use approached 90% in soybeans and 65% in corn. This heavy usage helped prompt ALS-resistant waterhemp that first was identified in 1991. In 1998, a waterhemp biotype was identified that resisted triazines (atrazine) and ALS inhibitors.

“It is now hard to find waterhemp (in Illinois) that is sensitive to ALS-inhibitor herbicides,” says Tranel.

More woes surfaced in 2000, when weed scientists identified a waterhemp biotype that resisted both PPO inhibitors and ALS inhibitors. Then came 2001, when a weed-resistance triple stack of PPO inhibitors, triazines, and ALS inhibitors was found in a waterhemp biotype.

Some promising new crop technology is also coming, including soybeans that resist 2,4-D, dicamba (Banvel), or HPPD inhibitors (Balance). Still, it needs to be used with other tools. “We already have resistance to those modes of action,” says Tranel. “They in themselves will not solve waterhemp problems.”

So what do you do?

To manage resistance, you'll have to tailor treatments for individual fields. Don't just think of the coming year, either.

“Make a five-year plan,” says Owen. “Think about what you will do next year, the year after that, the year after that, and the year after that. Get a spreadsheet to map it out. You need to have as much diversity as possible. Convenient and simple solutions will inevitably fail.”

To gather ideas on how to manage your weeds, play the War On Weeds game on the next four pages. You'll traverse the history of herbicides and herbicide-resistant weeds, and pick up tips you can use to forestall herbicide resistance.

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