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Industry weed resistance report: Mechanisms, suggestions and an empty pipeline

Annually, some 150 to 200 suspected resistant weed samples are studied at Syngenta's Vero Beach, Fla., research center.

“Since 2000, we've had an active program monitoring glyphosate resistance,” said Nick Polge, Syngenta research and development scientist at the center. “When we do our testing in the greenhouses, we report the results to several people. (Any findings of resistance) must be reported to EPA, mandated under the Federal Insecticide and Fungicide Rodenticide Act (FIFRA). Basically, once a product is labeled any new, adverse affects — including weed resistance — surrounding that product must be reported to EPA.”

Once positive confirmatory testing in greenhouses under controlled conditions is complete, Polge and colleagues have 30 days to inform the government agency.


In addition to confirmatory testing, Polge also studies the mechanisms of resistance. Two mechanisms have been reported. One is alteration of target sites — confirmed in goosegrass (in Malaysia) and rigid ryegrass (in Australia). The second is reduced translocation of glyphosate within plants. This has been reported in both horseweed and rigid ryegrass.

“What's important…is both methods can occur in (the same species). We've already seen that in rigid ryegrass…We must also consider other types of (resistance) mechanisms may develop or are already out there.”

While his concern with research is paramount, Polge says, “It's key as we do this work to keep in close contact with local, Extension and university scientists to develop management solutions. Part of that work involves field visits with local cooperators.”

The entire agricultural enterprise needs to get out of “a reactionary mode. We need to gather information and communicate it. We need to report a resistant population as soon as we can.

“We need to follow label directions. Growers are always pinched for time and money. But it's important they not cut rates. When they do that, it just allows more of the genes that can resist the herbicides to survive and enrich the next population.”


Since 2001, Syngenta's message hasn't changed with regard to managing weed resistance, said Chuck Foresman, Syngenta's head of weed resistance strategies. To combat resistance, Foreman offered the following suggestions:

  • In glyphosate-tolerant corn and soybean systems, don't use more than two applications of glyphosate over a two-year period.

  • Diversify with alternate herbicides and cultural practices. “We aren't telling people not to incorporate different cultural practices. We ask them to seriously consider it.”

  • In cotton, up to three applications of glyphosate can be used. “But you've also got to be using a residual herbicide or be cultivating. That helps break up the resistance cycle.”

  • Use alternative burndowns. “Gramoxone is a great burndown material. The new formulation is more user-friendly. Syngenta has worked for 20 years on this…It's a very good tank-mix herbicide.”

Luck and faith

Many growers seem inoculated against fears of weed resistance, said both men. Resistant weed discoveries don't seem to be resonating as they should.

“(Producers) have lived through ALS-resistance, waterhemp resistance, resistance to Scepter and Pursuit,” said Foresman. “One of the reasons there's little reaction now is they've been saved before. Roundup Ready saved them. But if they think we're going to have a powerful option once (more) weeds become resistant to glyphosate, they're wrong. What we have for growers is what's already in the toolbox. Glyphosate-resistance is not going to be an easy fix.”

In a recent survey, Indiana producers were asked if they were concerned with glyphosate resistance. “Something like 66 percent said they were,” said Foresman. “But 87 percent said even if there was, industry would come up with a solution, a new mode of action.”

That just isn't going to happen, the researchers insist. And such views by producers are “dangerous and that may be one of the reasons why people aren't being as pro-active as they should be.”

Consider what waterhemp did to ALS chemistry.

“(Pursuit) was hugely popular and broadly used. Then (resistant weeds surfaced and) were rough on Pursuit. It was just lucky that we had the Roundup Ready technology ready to come on in 1996. Pursuit resistance was building from the early 1990s onward. By 1996, there was a problem.”

In 1995, Syngenta introduced a herbicide, Flexstar, which helped with the resistance. “We had outstanding sales of it because Pursuit wasn't doing a very good job of controlling waterhemp,” said Foresman. “Sales built until Roundup Ready came on the scene. Roundup Ready is an outstanding system — it allowed growers to manage this looming issue of Pursuit-resistant waterhemp.

“I think growers think about that and may believe, ‘Well, we had a waterhemp resistance issue and Roundup Ready saved us.’

“But if glyphosate-resistant waterhemp surfaces — along with Palmer pigweed — the remedies aren't going to be so simple.”

Some questions

Could EPA mandate a certain spraying programs to prevent weed resistance?

“Well, never say never,” said Foresman. “But I don't have any first-hand information that suggests they will. When you embark down that road there are a lot of additional costs with respect to monitoring, tracking and reporting. Someone would have to shoulder the burden of that cost. We'd like to see the industry step up and properly steward and manage this challenge of weed resistance. I think we can do it.

“We want people to put only one application in-crop of glyphosate in corn and soybeans. But to get there, producers will need to put out a residual herbicide.”

At least at first, Foresman believes the adoption of resistance-delaying tactics will be regional.

“I think it will be in areas where people are more concerned about glyphosate-resistant weeds. I also think it will be in areas where growers understand how much money is put into an acre of a row-crop — the seed costs, the diesel fuel, the anhydrous ammonia and the herbicide costs. If you put out all that on an acre and then don't protect that investment with a broad-spectrum herbicide, it doesn't make sense. I think people will consider a broad spectrum herbicide and cultural practices for those reasons…I like to see more than three modes of action used per acre.

“Growers are sharp and they understand Mother Nature can be formidable. They know if we keep doing the same thing, she'll figure out a way to win.”

Could the resistance problem be worse worldwide than is currently known?

“I think we're under-reporting where it's happening,” said Polge. “A farmer isn't going to (immediately recognize) a resistant weed until (it's a bigger problem).”

How do Syngenta's stewardship recommendations differ from Monsanto's?

“Theirs change all the time,” said Foresman. “I'm not prepared to say, ‘They do this and we do that.’ We've been consistent for five years. I don't know where they're at. I've heard they're coming around to the prospect that a residual herbicide needs to be applied on Roundup Ready acres. The message seems to be getting through to them, as well.”

“Certainly, when Roundup Ready crops were first introduced, there was some discussion about the risk of glyphosate resistance developing,” said Polge. “Papers were published on that from Monsanto that said the risk was very low. Well, within the last 10 years, we've seen rapid development of glyphosate resistance worldwide.”

Foresman understands why, with no set recommendations for preventing resistance, farmers are confused. “You have one company saying one thing and another saying something else. I'd like to see more participation by universities. They're viewed as the unbiased voice…But when you get into the area of applied agriculture, universities are starving for funds. They aren't getting the funding they should get to address these issues.”

An empty pipeline

The easy answers have already been found. There are no new modes of action in late development. The new herbicide pipeline is empty. Farmers shouldn't expect a bail-out miracle cure for glyphosate resistance anytime soon.

“If our chemists found a new mode of action today, it would take 10 years to get it registered,” said Polge. “The tools we have today, we need to properly steward. If we're to continue controlling weeds with high-tech tools and herbicides where growers are able to spray a 90-foot swath through the field and sustain it, we need to adopt some new management practices.”

“If we don't,” warned Foresman, “you'll see growers jump back on the tractor to cultivate.”

(For more on Syngenta's resistance work, visit


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