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Corn+Soybean Digest

Glyphosate Resistance Rising

Doug Even doesn't like what he's seeing. Although his Roundup Ready corn and soybeans are yielding more bushels than ever, it's taking up to twice as much glyphosate herbicide to kill waterhemp and other weeds. Resistance blooms.

The eastern South Dakota grower is like growers over much of the Corn Belt and other production areas. They're seeing Roundup Ready crops with cantankerous weeds that aren't fully controlled by over-the-top herbicide applications.

In its 2005 report, the Herbicide Resistance Action Committee (HRAC) indicated there were 13 biotypes of weeds identified as glyphosate resistant. That was small compared to more than 300 weeds worldwide that were resistant to certain herbicides, including more than 90 with acetolactate synthase (ALS) resistance.

But since Roundup Ready is dominant in corn and soybean production, glyphosate resistance is the biggest thorn for growers fighting weeds. Growers who rotate corn and soybeans with Roundup Ready cotton in the South face added pressure from Palmer Amaranth, or pigweed.

“Such shifts in weed populations to more tolerant weeds are already resulting in increased weed control costs due to additional herbicide applications or increased glyphosate rates,” says Stevan Knezevic, agronomist at the University of Nebraska-Lincoln.

“We have suspect fields where marestail (horseweed) has shown apparent glyphosate resistance,” says Brian Olson, area agronomist in northwest Kansas for Kansas State University. “Where you'll see resistance is where guys have used multiple applications of glyphosate a year with continuous Roundup Ready crops.”

THERE HAS NOT been confirmed glyphosate resistance in any plants in Iowa, says Bob Hartzler, Extension agronomist at Iowa State University. “But I think that's primarily because we haven't had an active program seeking them. I'm sure that we have glyphosate-resistant horseweed, but I'm not sure about the other species.”

Those are common responses from corn and soybean agronomists in a cross section of Corn Belt states (Kansas, Nebraska, Iowa, Illinois, Missouri and Ohio). And that worries growers like Even, who has been sold on glyphosate technology for years.

“All our corn and soybeans are Roundup Ready,” says Even, Elkton, SD. “Our corn yielded 149 bu. in 2007 and our soybeans yielded 46 bu. Our yields have really increased over conventional corn.”

However, his field's marestail, waterhemp and buckwheat are getting more difficult to control. “Before, we could get by with the smaller rate of (Roundup) WeatherMax — about 22 oz./acre,” says Even. “Now, we're using 32 oz.”

He adds that lambsquarters, once controlled with a 22-oz. application, now thrive at that level. “We used double the rate at 44 oz. in 2007,” he says. “That controlled them.”

MARESTAIL HAS BEEN confirmed as glyphosate resistant for several years in Illinois, and in 2007 waterhemp was confirmed as glyphosate resistant, says Bryan Young, weed science professor at Southern Illinois University. “The management problem and area infested will continue in severity with time,” he says.

“Growers haven't taken the approach of diversifying weed-management tactics outside the glyphosate realm, since they perceive it to be a less profitable endeavor — at least in the short run.”

In Missouri, several central and northwest counties now have waterhemp resistant to glyphosate, says Kevin Bradley, University of Missouri Extension weed scientist. “Much of this is likely due to our relatively high soybean-to-corn ratio in this state,” he says. “Almost all are Roundup Ready and receive continuous applications of glyphosate over and over.

“Marestail seems to be an increased problem. Common ragweed (which has shown glyphosate resistance in Arkansas) hasn't increased much, as far as we know,” Bradley says.

Ohio is seeing both glyphosate and multiple resistance problems, notes Jeff Stachler, Ohio State University Extension agronomist.

“Glyphosate-resistant horseweed can now be found in at least 70% of counties in the western half of Ohio,” he says. “So it is spreading. Worse yet, the multiple (glyphosate plus ALS) resistance in horseweed in Ohio continues to spread.”

Giant ragweed, common ragweed and common lambsquarters are showing resistance or more difficulty in controlling in Ohio. “The level of resistance in giant ragweed is increasing over time,” says Stachler. “The original populations had a low level, but the new ALS-plus-glyphosate population has a much higher level, and the original population is showing increased levels and frequency of glyphosate resistance.”

In Ohio, common ragweed populations are resistant to glyphosate plus ALS and ALS plus PPO inhibitors. “More common lambsquarters continue to survive glyphosate-only treatments, but the level is low enough that adequate control may be obtained with proper glyphosate application.”

Should you alter your chemistry? Weeds thriving after receiving a shot of glyphosate should be alarming, the weed specialists agree. But what can be done to save the benefits of Roundup Ready? There are alternative over-the-top chemistries and a different herbicide-resistant crop, namely LibertyLink.

The main glyphosate manufacturer, Monsanto, has advised growers to consider an alternate mode of action. “Monsanto indicates that growers should consider putting down a pre-emergence herbicide on Roundup Ready corn,” says Olson.

At the advice of his consulting agronomist, Even says he has tried new herbicide chemistry at preplant. “We used some Harness herbicide in preplant,” he says. “We had some grass pressure. We'll probably try some more of it. One year isn't a fair test.”

Young says Illinois area growers should consider 2,4-D or dicamba growth regulators to control glyphosate-resistant horseweed. ALS-inhibiting herbicides like Firstrate or Classic, as well as a Photosystem I distruptor such as Gramoxone Inteon, should be looked at. Liberty, a GS inhibitor, could work in controlling glyphosate-resistant weeds, he says. So could Photosystem II inhibitors like atrazine or metribuzin, as well as PPO inhibitors like Valor in a soil residual control application.

For glyphosate-resistant waterhemp in soybeans, Young says growers should consider residual control with acetamide herbicides like Dual and Intrro, and DNA herbicides like Prowl or Treflan. PPO herbicides like Valor, Spartan, Authority First, Sonic, Valor XLT, Envive, Flexstar, Cobra and Ultra Blazer can also work, he says. He adds that “in corn, use HPPD herbicides such as Callisto, Impact or Laudis; atrazine or Princep; or growth regulators.”

YOUNG PRECLUDES ALS herbicides for waterhemp control since “we assume that all waterhemp in Illinois is already ALS resistant.”

The best alternative weed management program “is becoming a more difficult question to answer due to the presence of multiple resistance,” says Stachler. “Obviously, we are recommending that 2,4-D is always included in a burndown program. The reasons are twofold: one, an alternative site of action, but two, a reduction in the number of plants at the time of postemergence herbicide applications.”

He says that in Ohio, 60-90% of common and giant ragweed had emerged for the season in 2007 at the time of the burndown application. “This shows the importance of a burndown application and the need for herbicides with alternative sites of action,” he says.

“We suggest using at least 0.75 lb./acre Gramoxone in the burndown instead of glyphosate. We are recommending that residual herbicides be used in all soybeans, whether fields are tilled or not,” Stachler says. “We also recommend products with the broadest spectrum of control, which usually include products containing the ALS herbicides — Classic or FirstRate — plus Valor, metribuzin or sulfentrazone.

“Many different residual products can be used to control common lambsquarters, and more herbicides are effective for common ragweed compared to giant ragweed,” he says.

In Missouri, Bradley says alternative weed control methods should include “use of soil residual herbicides that are effective on the species in question.” He also recommends a rotation with corn the following year with use of residual herbicides.

NEBRASKA'S KNEZEVIC SAYS it's “easy to fall into a trap of over-using glyphosate — versus combinations of pre-emergence herbicides or tankmix partners — when one Roundup Ready crop is grown after another.

“Using various weed-control tools is not a new thing,” he says “We only forgot about it since the introduction of Roundup Ready crops. Changing modes of action in your herbicide program is also one of the basic ideas in an integrated weed management (IWM) program, especially to combat weed resistance/tolerance issues.

“I believe that Roundup Ready technology has a fit under the umbrella of an IWM system, and the value of this technology can be preserved only by proper management and reduced overuse,” he adds.

Knezevic says soil-applied herbicides would provide additional modes of action, thus reducing a chance for weed resistance. They would provide a longer comfort zone early in the season by delaying the critical time for weed removal and reducing the need for multiple glyphosate applications later on in the season, he notes.

Iowa State's Hartzler doesn't recommend a specific mode of action. He encourages growers who believe they are facing glyphosate resistance problems to “pick products that have good activity on dominant weeds in their fields.”

Cultural practices can help control resistance problems. “Do everything you possibly can to get a competitive crop canopy,” says Hartzler, adding that “a good seedbed, quality seed, appropriate row spacing and planting densities” could also help counter glyphosate resistance.

Stachler says crop rotation, as well as delaying planting in “fields with the highest weed pressures or fields with the most survivors following herbicide applications,” should help growers better manage resistance problems. “Farmers must stop growing a single crop continuously,” he stresses. Growers also need to change their weed management strategies sooner, rather than waiting for the system to break completely.

Young says rotation that would allow for greater herbicide diversity and tillage, if possible, can help thwart resistance.

Bradley says that if soybean growers suspect they have a glyphosate-resistant weed like waterhemp, “rotate away from continuous Roundup Ready soybeans altogether.

“For example, rotate to a conventional corn hybrid and use alternative herbicides in that system for a year or two,” he says.

But with the production ease of glyphosate systems of corn and soybean production, going back to conventional hybrids or varieties may be difficult for many. However, it may be the only alternative in some rotations. Growers should consult with their regional Extension staff to determine the best programs to counter resistance locally.

“Just because a certain herbicide application fails, it doesn't mean it is resistant,” says Olson. “Farmers need to look at all aspects of the application process and not just assume resistance. Application timing, actively growing weeds, using ammonium sulfate and adding surfactants, if needed, are all important in allowing glyphosate the opportunity to provide adequate weed control.”

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