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Tackling weed resistance

Tackling weed resistance

Weeds are the ultimate foe for corn and soybean growers. They easily adapt to new herbicides, changes in cultivation and diverse weather. Growers need a solid playbook to help them fight their adept foes well into the future.

As the standing-room-only crowd gathers one hot September afternoon, it’s not just the heat they are trying to avoid, it’s weeds — more specifically, herbicide-resistant weeds.

“If I had given this talk 12 months ago, the room would be half empty,” says Aaron Hager, University of Illinois Extension weed science specialist. “But producers are starting to experience weeds that they can’t control, and they are concerned.”

Michael Owen, extension specialist at Iowa State University, who shares the stage with Hager at the seminar, tells the crowd bluntly, “Weeds are the single most economically important pest complex producers have to manage. And weed management isn’t what it used to be.”

Hager and Owen show growers images from areas where resistant weeds have taken over. Palmer amaranth and waterhemp, along with marestail, are resistant to several classes of herbicides and render entire fields unfarmable. Crews of workers are using hoes to chop down resistant Palmer amaranth before it can produce seed. And fields that two years ago only had a “few” weeds are suddenly choked with resistant waterhemp.

“The issue of herbicide-resistant weeds is a crisis to the producers who are living it,” says Arlene Cotie, product manager, communications and trait integrity for Bayer CropScience. “One grower in Arkansas lost $230 an acre and had to hire a chopping crew to cut down Palmer amaranth with stalks so large they couldn’t get their hands around [them].”

And while weeds that have developed resistance to glyphosate have been put under a microscope, experts are quick to point out that glyphosate-resistant weeds are by far not the only concern. Lurking in the shadows are weeds that are resistant to other chemistries, and in some cases multiple chemistries.

“The most important thing to remember is that this is not a problem with the herbicide,” Owen says. “How the herbicide has been used, and the management programs used, have caused this problem.”

In fact, the issue of weeds adapting and changing predates herbicides, and in some cases weeds have changed due to cultural practices. “More than 100 years ago, Charles Darwin described how organisms respond to their environment. They either adapt or die,” Owen says. “Any single thing you do, if it is successful and puts selection pressure on that organism, ultimately that organism will change to survive.”

Producers using a single class of chemistry, year after year, to control weeds turned their fields into petri dishes where the continual use of one mode of action meant that ultimately a few weeds would survive, passing along this resistance to the next generation.

For amaranth species, such as Palmer amaranth and waterhemp, a single plant can produce more than a half million seeds. If these weeds are left to mature, a combine becomes a very large seed spreader during harvest.


A playbook for fighting resistance

While the list of herbicide-resistant weeds continues to grow, weed scientists are focused on the “driver weeds” that can potentially cause the most economic damage — Palmer amaranth, waterhemp and marestail (also known as horseweed).

“While we have concerns regarding the evolution of resistance in several weed species, these are the weeds of major concern moving forward over the next five years as the infestation levels intensify and expand,” says Bryan Young, weed scientist at Southern Illinois University.

“We need to improve the control of these weeds, learn how to recognize failure due to glyphosate resistance, and take a proactive approach to deter the shift towards resistance in areas that would still be feasible,” he adds. “If you wait, and react to weed resistance to glyphosate on your farm, you are too late.”

Growers need a playbook of strategies to fight the encroachment of weed resistance. Here are some key plays growers can use to start the offense.


Review the opponent

Young says early resistance problems can be difficult to spot, or are reasoned away by producers. “If a producer sees a slight failure rate, he may blame it on a faulty nozzle, poor herbicide coverage, or bad weather — anything but resistance. If those few plants go to seed, the next year you will have an even greater problem. The weeds will still be there, and as long as you keep using the same chemistry, you will continue to select for that weed.”

Weed resistance is a numbers game, says Les Glasgow, technical asset lead and head of weed-management strategies for Syngenta. “Weed resistance is all about how many weeds are being exposed to a single mode of action,” he says. “That sets you up for resistance.

“It doesn’t matter if the active ingredient is glyphosate, atrazine, or any of the other chemistries,” Glasgow continues. “If you overuse it, the plant will develop resistance.”


Clean the field

Another rule in the playbook is start with a clean field and keep that field free of weeds.

“When you spray a field and 5% of the weeds remain and don’t die, you want to deal with those weeds immediately,” says Steve Muhlenbruch, agronomy manager for Dows Farmers Cooperative, Dows, Iowa. “We still have options available to us, but we don’t want those weeds to get out of control.”


Multiple attacks on offense

The message to producers is: diversify, diversify, diversify. And control options can, and should, include several modes of action, preplant and post application, and in some cases even cultivation and rotations to different crops and different herbicide-tolerant packages.

“The reality now is that we cannot rely on a one-size-fits-all weed management strategy,” Glasgow says. And for producers who would once tank mix one herbicide for grasses and one for broadleaf weeds, it now means putting more than one active ingredient in the tank to target the same weed species and using a preemergence herbicide to keep weeds from emerging in the first place.

An example of a diverse program approach for soybeans, according to Glasgow, includes Boundary pre-applied followed by Flexstar GT 3.5 early post application. It uses four modes of action and provides residual control.

Diversifying is striking a chord with growers, reports Rajan Gajaria, marketing director, Dow AgroSciences. He’s heard more growers ask about the company’s future 2,4-D choline and glyphosate product called Enlist Duo to use with SureStart as a preplant in corn. “We are seeing more producers ask about this program in an effort to diversify their weed management system,” he says.


Future defense

Glyphosate remains effective on a wide array of weeds, but reinforcements are coming. Here are just a few products in the pipeline.

Zidua herbicide, developed in a partnership between BASF and Kumiai Chemical Industry, has shown strong residual control of amaranthus species in corn and soybeans and Italian ryegrass in wheat. BASF is expecting EPA approval of the herbicide by the end of the year. It will join the company’s Kixor herbicide launched last year.

Enlist Duo, Dow AgroSciences’ new herbicide, includes a new formulation of 2,4-D mixed with glyphosate to be used over the top of Enlist corn and soybeans.

An HPPD trait for soybeans is in the works through a partnership between Bayer CropScience and MS Tech. Bayer launched its LibertyLink soybeans and Ignite herbicide in 2009. The two companies expect to have a GlyTol- and HPPD-stacked trait in 2015, and with a LibertyLink trait on top the following year. In addition, Bayer recently announced collaborations with Syngenta on HPPD technologies coming the latter half of the decade. HPPD represents the first residual herbicide-tolerant in soybeans.

Dicamba-resistant traits also are in the pipeline. These are being developed by Monsanto and BASF.


Potential superstars?

Glyphosate has been called a once-in-100-years herbicide. But if you are looking for the next new glyphosate, think again. While there is active research and development to find the next great active ingredient, any search is still in its infancy. No superstar product like glyphosate is being talked about openly by any company.

“We have some exciting leads, but we are still a year or more away from getting anything into the early development stage,” says Steve Bowe, team lead, biology, BASF. “And then it would still be several more years before anything would reach the market.”

Instead, growers need to take a long-term, multi-pronged approach to weed management. “We don’t need a short-term fix,” Hager says. “Even if a new active ingredient were introduced tomorrow, if we simply relied on that one AI, in a few years we’d be right back to where we are today.”

And as the crowd filters out from Hager and Owen’s seminar into the Illinois heat, that message is resonating.

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