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Weeds rebound

If Growers don't protect current weed-control chemistries, they may someday find their fields infested with resistant weeds — with virtually no way to control them.

Weed scientists say the extensive use of glyphosate on cropping systems (Roundup Ready soybeans followed by Roundup Ready corn, for example) is creating a monoculture environment, and the same mode of action herbicide sprayed year after year significantly increases the intensity of selection for herbicide resistance.

Eight weed species in the United States and 13 worldwide have glyphosate-resistant biotypes. Comparatively, the International Survey of Herbicide Resistant Weeds has logged (worldwide) 314 resistant weed biotypes and 183 weed species resistant to herbicides.

“At the moment, if you look at glyphosate as a herbicide and the number of weeds resistant to it, it's not a major problem,” says Ian Heap, director of the survey group. “But our fear is that glyphosate resistance will continue to grow due to the sheer number of acres being treated with glyphosate.”

To date, 18 states have confirmed the presence of glyphosate-resistant weeds.

Finding a weed that's naturally resistant is merely a numbers game. Continue to use the same chemical with the same mode of action year after year, and the chances of selecting a weed with resistance increases substantially.

New twist to an old problem

Weed resistance is not a new phenomenon. When the first modern herbicides were introduced in the mid 1940s, there wasn't much concern about weed resistance because it wasn't on the radar of weed scientists.

Move ahead to the mid 1970s, and the first instances of weeds resistant to triazine herbicides were discovered. However, that also coincided with the boom in herbicide development of different modes of action. The most famous was Monsanto's introduction of Roundup in 1976.

But today there is a difference. Take waterhemp, for example. Several states have documented cases of waterhemp populations that are resistant to PPO inhibitors. For these populations, the only remaining herbicide choice for postemergence control in soybeans is glyphosate. However, Missouri, Illinois and Kansas have now documented cases of glyphosate-resistant waterhemp.

Aaron Hager, assistant professor and weed science extension specialist at the University of Illinois, explains, “Hypothetically, if we had a waterhemp biotype resistant to PPO inhibitors and glyphosate, soybean farmers would have no postemergence control options, other than cultivation. That's significant.”

That also has most farmers worried. In grower surveys conducted by Bayer CropScience, more than half of the respondents said they are concerned about weed resistance. “That's a big jump from even a few years ago,” says Andy Hurst, product manager for Liberty herbicide and LibertyLink traits for Bayer CropScience. “And a third of the growers surveyed felt they had resistant weeds on their farm.”

Agricultural chemical companies also are concerned about weed resistance. They have millions of dollars invested in the underlying chemistry. And many millions of dollars have been used to develop and market today's most popular herbicide-resistant crops.

The resistance issue is larger, and concern is higher, in the Corn Belt.

“As we look across a large part of the Corn Belt, the occurrence of herbicide-resistant weeds continues to increase virtually every year, either in acreage infested or selection of resistant biotypes in other weed species,” Hager says.

Multiple modes of action

A key management strategy to combat weed resistance is diversification — developing a program that incorporates several herbicides with various modes of action. “What growers can do is develop a multiyear weed management plan for their fields,” says Chuck Foresman, weed resistance strategy manager for Syngenta. “A single mode of action herbicide used year after year is not sustainable. There needs to be diversity in your weed-control package. We tell growers to strive for at least three modes of action in each crop and include the use of preemergence, residual herbicides.”

Companies have been busy packaging various products with a diversity of herbicides with alternative modes of action. Often it's reliable chemistry that's been in the market for several years but may have fallen out of favor due to the widespread adoption of glyphosate as the herbicide of choice, especially over glyphosate-resistant field crops.

Companies are offering increased flexibility in their trait products to give growers additional weed-control options. Products such as AgriSure 3000GT offer tolerant traits to both glyphosate and Liberty, along with rootworm and corn borer protection. “Producers then have the option of using the herbicide mode of action that will give them the best control, and they can rotate between two different modes of action,” says Bruce Battles, agronomy marketing manager with Syngenta.

Bayer is expected to receive EPA registration for its Laudis herbicide for the 2008 crop season. Laudis is a new postemergent corn herbicide that can be tank mixed with glyphosate or Liberty for additional weed-control options.

Companies and universities have launched educational programs and campaigns to help producers develop effective weed resistance management. Web sites like and continue to update producers on the latest strategies.

Hager says the most effective way to manage resistance is to never get it. “But we've obviously reached the point where herbicide-resistant weeds infest a very large number of acres,” he says. “The key is to find products with alternate modes of action to control the resistant weeds.”

Hurst says interest in Liberty and the LibertyLink herbicide-tolerant technology has increased dramatically as producers look for alternative weed-control options. “We've seen more use of Liberty than ever before,” he says. “In 2007 there was an exponential growth, and we expect that trend to continue.”

LibertyLink has been associated with all of the Herculex Bt traits, as well as the AgriSure CB trait on the market. “The number of available hybrids with LibertyLink technology is increasing and will continue to do so,” Hurst says.

In addition, the LibertyLink technology is on target to be introduced in soybeans in 2009.

The goal is to rotate the chemistry on corn and soybean fields to a different mode of action to extend the lifespan of both herbicide-tolerant traits.

Scouting and proper application remain critical tools in the weed resistance management toolbox.

“Scouting is more than finding what weeds are out there,” Battles says. “Now it's a matter of looking at the crop stage and timing of applications.”

Hager says that regardless of the postemergence herbicide control, current recommendations are to target weeds in the 3- to 5-in. growth stage, not at 8 to 12 in. “Targeting early gives you an opportunity to go out and determine if you've done an effective job of weed control,” Hager says. “You can also determine if you need to adjust your herbicide application to get weeds that escaped the first treatment. If you wait too long, those weeds can become too large to effectively control.”

It may also give you a better idea if you have a problem with herbicide-resistant weeds. “If you know you have a problem, there are options you can consider to control those weeds,” Hager says. “If the weeds are up and growing, it may be too late in some situations.”

Dave Ruen, product technology specialist with Dow AgroSciences, says a good weed-control program is to use multiple modes of action to prevent weeds from escaping and potentially developing resistance. “If producers do a good job of managing early-season weeds with a soil-applied preemergence product, they are getting a broad spectrum of foundation control,” he says.

Basic preemergence programs that include acetamides such as acetochlor, one of three active ingredients in SureStart herbicide or the active ingredient in Surpass, in Roundup Ready corn, can help control small-seeded weeds that may be resistant or increasingly tolerant to glyphosate, for example.

Dealers and custom operators

One reason glyphosate has become so popular is its ease of use. However, with a larger number of herbicide formulations, along with increased interest in preemergence herbicide treatments, producers could become interested again in asking custom applicators and dealers to help with weed-control issues.

“Retailers and custom applicators can provide a wide range of options and match up the herbicide program to the crop system,” says James Reiss, vice president of agricultural business for Precision Laboratories. The company recently introduced Simplyx, an adjuvant the company says helps maximize performance of glyphosate tankmix partners.

Retailers and custom applicators are equipped to handle preemergence herbicide applications. “Growers may have their equipment set for glyphosate application, and that's not the same as what's needed with Liberty,” Hurst says. “Custom applicators can cover more ground, with the right chemistry, when producers may be busy with other spring chores.”

As producers adopt multiple herbicide programs, retailers are an excellent source of advice and counsel. “Dealers understand all the chemistry that's available and can advise on effective use of the herbicides,” Foresman says.

Increasing the herbicide rate may seem like an option, but when a weed is resistant, more chemical isn't always a viable solution. Hager at the University of Illinois studied a field of glyphosate-resistant waterhemp. Normal application of 22 oz. to the acre of WeatherMax did not control the weed, and some plants even survived after being treated with 2.7 gal. to the acre. “Our field results in 2007 suggested even the maximum in-crop application rate of glyphosate did not effectively control this waterhemp population,” Hager says.

Protecting the current chemistry and maintaining the value of herbicide-tolerant traits is the ultimate goal in weed resistance management, because a new herbicide with a novel mode of action isn't around the corner.

“We're not seeing a lot of new offerings, and I don't expect a new active ingredient for the postemergence soybean market in the foreseeable future,” Hager says.

But even if a new active ingredient were discovered today, it's likely that it would take a minimum of 10 years to weave its way through the regulatory approval process. Ten years may not seem like a long time for the industry, but for producers battling weed problems, it can be an eternity, especially with today's grain prices.

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