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6 Things You Didn’t Know About Herbicide-resistant Weeds

They’re not new, they’re worldwide, and glyphosate resistance isn’t the worst of it.

Think you’ve heard it all about herbicide-resistant weeds? Here are six often-overlooked facts about them to consider as you ready your weed-management plan for the coming growing season in 2017.

1. They’re not new. “The first resistant weeds to atrazine were reported in 1970,” says Ian Heap, director of the International Survey of Herbicide-Resistant Weeds (weedscience.org). At latest count, 252 weed species worldwide have evolved resistance to herbicides

“The interesting thing is, we’ve known how to combat weed resistance for most of that time, too,” Heap says. Herbicide labels from the 1980s – before Roundup Ready was in anyone’s vocabulary – encouraged chemical rotation, full-use rates, and herbicide use only when necessary. 

2. More are en route every year. Heap says, on average, 11 new cases of herbicide-resistant weeds are reported annually by weed scientists somewhere in the world. That’s been a steady pace for about 30 years. Up through 2015, 461 unique cases were documented, involving 247 weed species. 

The reason there are two different numbers – 461 unique cases of 247 weed species – is that many weed species now show resistance to multiple chemicals. For instance, Heap says, wild oat shows resistance to five different herbicide classes, putting it on the list of unique cases five times. Meanwhile, rigid ryegrass is resistant to 11 classes. About 90 weed species are resistant to more than one herbicide.

Corn used to be the crop most impacted, but now it’s wheat, Heap says. About 130 weeds that are significant to wheat growers are resistant to at least one herbicide. Corn has about 100 resistant weeds, and soybeans and rice have about 80.  

3. They’re bigger than glyphosate. Weeds that resist glyphosate in herbicide-tolerant systems may get the most attention. That’s especially true in North America, where Roundup Ready soybeans burst onto the scene and dominated the weed-control market in just a few years, says Heap. The class of herbicides known as ALS inhibitors (such as Pursuit and Accent) actually has had the fastest worldwide growth in resistance in recent years, followed by PSII inhibitors (such as Buctril).

Worldwide, there are 56 registered ALS inhibitors on the market and 26 PSII inhibitors, compared with just one glyphosate. Hence, the former two classes lead the resistance race.

Part of the problem associated with the Roundup Ready era is that it’s been 30 years since we’ve discovered a new class of herbicides, Heap says. 

“Glyphosate came along, and people thought it was game over. They (agricultural chemical companies) shut down their discovery programs,” he says.

“What is concerning are the herbicide groups 14 and 27,” says Mike Owen, Iowa State University Extension weed specialist. Group 14 herbicides are the PPO inhibitors (like Cobra and Flexstar). Group 27 herbicides are the HPPD inhibitors (like Balance Flexx and Callisto). 

These have been popular alternatives where glyphosate resistance has surfaced. Yet, even in 2011, the estimated percentage of waterhemp in sampled fields estimated Group 14 resistance at 10% to 12%. With group 27, the resistance was estimated at 24% to 27%. Continued use will only raise these numbers. 

4. They’re everywhere. North America does have the most resistant weed cases (over 160 and counting). However, western Europe has over 100 cases now. China is growing fast with over 40 resistant weeds, compared with just 15 in 2010. 

“Countries with cheap labor sources have the fewest cases of resistant weeds,” says Heap. 

5. There are 7 top offenders. Of the 32 weed species resistant to glyphosate, only seven actually account for about 99% of the economic losses, says Heap. Here is the biggest offenders list. 

  • Palmer amaranth
  • Marestail (horseweed)
  • Tall waterhemp
  • Giant ragweed
  • Johnsongrass
  • Rigid ryegrass
  • Sourgrass

For good measure, Heap adds kochia. It dries out and becomes a tumbleweed, spreading seeds as it rolls. It may soon crack his worst-offenders list.

6. You can keep track of herbicide-resistant weeds.
A website (weedscience.org) is devoted exclusively to this topic and is managed by Heap. Public and private industry weed scientists in over 80 countries use the site to track new cases with updates and maps by state, region, crops, sites of action, and much more.

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