The Changing Face Of Weed Control
March 15, 2011 
“What’s good for the goose is good for the gander,” goes an old saying. Besides waterfowl, this adage could also be applied to the world of agriculture, in particular when talking about crops and weeds.
In truth, resistant varieties of weeds have been quite common across many fields of the U.S. for several years now. But most market watchers agree that 2010 was a banner year for herbicide-resistant weeds. The favorable crop growing conditions in 2010 — an early planting window, sunny days and decent rainfall throughout the summer — also appealed to an entire range of weed varieties. In fact, Darrin Dodds, Mississippi Extension cotton specialist, described to CropLife’s® sister magazine Cotton Grower how glyphosate-resistant Palmer amaranth (also known as Palmer pigweed) spread in 2010. “Prior to this year, I had not gotten a call about pigweed control failures with glyphosate south of Highway 82,” said Dodds. “This year, I’ve gotten calls out of Holmes County, out of Humphreys County, and I’ve even gotten some calls off the Mississippi Gulf Coast where we grow cotton. It seems as if it’s just steadily spreading.”
Larry Steckel with the University of Tennessee Extension Service echoed this trend for herbicide-resistant weeds. “It really showed up in middle Tennessee this year, and north Alabama,” Steckel told Cotton Grower. “Palmer pigweed had never been there before — no Palmer pigweed at all, let alone resistant Palmer pigweed. So it’s basically got all of our cotton acres now. Wherever we grow cotton, it’s there to some degree.”
Other Contenders
In addition to Palmer amaranth, several other weed varieties with known resistance are troubling growers. In a recent survey of 800 growers conducted for BASF, common lambsquarters and marestail were chosen as the top weed threats by 45% apiece of respondents. Finishing a close third and fourth on the list were giant ragweed at 43% and waterhemp at 41% (see chart). Among Midwestern-based respondents, waterhemp topped the list of hard-to-control weeds at 60%, followed by lambsquarters at 57% and ragweed at 56%.
Given recent news, these results are not surprising. In July, Kevin Bradley, Extension weed scientist for the University of Missouri, did a survey of waterhemp in the state at 144 locations. He found that 58% of this population were resistant to glyphosate. And a survey of giant ragweed from 27 farms within Missouri discovered 12 glyphosate-resistant varieties.
“My concern is not just herbicide resistance, but multiple resistances,” said Bradley of his findings. “What started as resistance to glyphosate, the most widely used herbicide, has become resistance to other herbicides.” In fact, he added, the newly resistant giant ragweed was also found to have at least moderate resistance to ALS herbicides.
More recently, researchers at the University of Illinois confirmed they had discovered a species of waterhemp that had developed resistance to HPPD-inhibiting herbicides. “A fifth example of resistance in one weed species is overwhelming evidence that resistance to virtually any herbicide used extensively on this species is possible,” said Aaron Hager, weed specialist at the University of Illinois. “We are running out of options. This mulitple-herbicide resistance in waterhemp has the potential to become an unmanageable problem with currently available postemergence herbicides used in conventional or glyphosate-resistant soybeans.”
For some growers in the South, unmanageable has already become a reality. According to Dr. Dan Westberg, technical marketing manager for BASF, there were many cotton growers in 2010 that were forced to hire gangs of workers using hoes to control the herbicide-resistant Palmer pigweed in their fields. “Based upon what I heard from these growers, it cost them between $50 and $100 per acre to employ this strategy,” says Westberg. “In addition, no till cotton farming, which has been very beneficial for soil conservation in these areas might end. For many growers, the only way to control herbicide-resistant weeds is to dust off the plow.”
Developing New Strategies
In the end, says Westberg, growers will have to change their approach to weed control. “It was always amazing to me that some growers didn’t think twice about spending $100 per acre on seed and fertilizer, but tried to get by on weed control for less than $20 per acre,” he says. “Those days are probably going away very rapidly.”
University experts recommend that growers follow these steps to manage tough-to-control weeds in their fields:
• Begin with clean fields and control weeds early with a burndown and/or preemergence application of residual herbicide.
• Scout your fields regularly, identify weeds and their location in the field and respond quickly to changes in weed populations.
• Institute an integrated weed management program that employs multiple modes of action.
• Follow label directions closely to achieve optimal performance and pay close attention to rate and timing information.
• Use other agronomic practices such as crop rotation, cultivation and cleaning equipment between fields that diversify your weed control options.
In particular, says Westberg, using a herbicide with good residual characteristics such as Kixor will become even more important moving forward. “This can help provide broadspectrum control of broadleaf weeds including ALS-, triazine- and glyphosate-resistant biotypes,” he says. “This provides burndown and soil activity across a wide range of crops.”
Jeff Carpenter, corn portfolio manager for DuPont Crop Protection, agrees that different modes of action and good residual characteristics are now needed to provide effective weed control. “Agriculture needs sound science-based approaches and new ways of controlling resistance weeds,” says Carpenter. “This is what virtually every crop protection supplier in the market is working on right now. We all lived through ALS resistance when it first appeared. This is no different.”
A More Complicated Future
In this new reality for weed control, industry insiders agree that the practice of applying a single mode of action herbicide such as glyphosate is quickly being replaced by one of applying multiple herbicides to achieve control and reduce the chance for future resistance evolution. “That’s not to say that glyphosate will completely disappear as an important weed control tool for agriculture to use,” says Susan Macy, soybean portfolio manager for DuPont Crop Protection. “But I doubt it will even again be the only herbicide growers use to control weeds.”
And the prospects for any new single product being the “magic bullet” for all weed varieties also appear dim. As DuPont’s Carpenter points out, the last new mode of action herbicides (HPPD-inhibitors) were introduced back in the early 2000s. Even with plenty of activity in the development pipeline, he doesn’t foresee another “magic bullet” in the industry’s gun barrel.
Going into 2011, BASF’s Westberg reports that several weed varieties bear watching. In particular, marestail — which was first confirmed showing glyphosate-resistance in the early 2000s in Delaware — is continuing to spread, now found in some 20 states. The latest to report a glyphosate-resistant marestail is Oklahoma, earlier this year.
Two other weeds that are showing some signs of resistance include morningglory and burcucumber. “Morningglory always has shown some hints of resistance to certain herbicides,” says Westberg. “As for burcucumber, it tends to germinate over a long period of time and is an aggressive grower. These are the same kind of characteristics other resistant weeds tend to possess.”
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