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Expert on resistant weeds researching Arkansas case

When Paul Neve arrived at the Newport, Ark., research station in mid-July, pigweed was healthy, abundant and herbicide-resistant.

The increasing threat of glyphosate-resistant weeds in the state is why Neve's hosts, Arkansas Extension weed specialists, had invited the resistance expert for a tour of the state.

They are especially keen to see what Neve's computer modeling and predictions will say about where the state's weed problem is headed and what solutions might be.

First, though, Neve needed to assess and gather data from Arkansas fields. The modeling will be done after he returns to his teaching position at Warwick University in Birmingham, England.

At the Jackson County Extension office following the plot tour, Neve discussed what he'd just seen with Extension and Syngenta researchers.

Ken Smith was eager for Neve to know that the pigweed-infested soybean plots he'd just left were not an aberration. “This is the beast we're dealing with,” said Smith, Arkansas Extension weed specialist. “The encouraging thing is some of the (treated) plots look good. But to get them in that shape will require a change in how we think about farming — particularly farming soybeans. I think we'll make that change. Our farmers are resilient enough to do it.”

It won't be the first time Mid-South farmers have changed farming practices. “Some aren't old enough to remember when Treflan first came to market. People said, ‘It's crazy to bring to market a product that has to be incorporated in the soil.’ But, three years later, Treflan was the leading herbicide.”

Neve, a UK native, was doing research in Australia shortly after glyphosate resistance became a problem in ryegrass.

“I was working with Steve Powles (see He's truly the world expert on these things.

“I recognized a need for developing computer models. While they're miles away from the real field situations, models enable us to project into the future or try to see what are the main drivers, or main methods, of reducing selection pressure for resistance.”

The main advantage of the models, said Neve, is eliminating time-consuming field trials. By the time 10 years of field trials are completed and an answer found, a weed could run riot.

“These models can run a number of scenarios that provide almost instantaneous answers.”

Modeling works, said Bob Scott, Arkansas Extension weed specialist. “When I was working with American Cyanamid, a big concern was resistance developing in Clearfield rice. They created four-, six-, and eight-year models looking at crop rotations with Clearfield rice to see how long the technology might perform well.”

The modeling showed that in a rotation with Roundup Ready soybeans, “the technology was good almost indefinitely. The models even showed two years of Clearfield rice and one year of Roundup Ready beans would work well.”

However, plug a continuous Clearfield crop into the model and the Clearfield technology crashed in five or six years.

“Many of the answers that come from models aren't surprising,” said Neve. “What they do is put some numbers on the actual, studied assumptions.”

One key to resistance is the frequency of the mutation, said Les Glasgow, Syngenta technical brand manager. “If you go back, say 10 years, to look at glyphosate resistance, you probably wouldn't see it. At that time, conventional wisdom said the frequency of mutation was extremely low. What wasn't taken into account was how extensive and intensive the use of one mode of action would be.”

In graduate school when Roundup Ready soybeans were introduced, Scott remembered “Monsanto talking about a 40 percent, or 50 percent, market share. I don't think anyone, in his wildest dreams, imagined that in a few years Roundup Ready would be at 99 percent market share in three crops.”

Another thing came out of the Australian weed research: not every glyphosate application is equivalent. In Australia, glyphosate resistance is still a relatively minor problem. Models show the reason for that is glyphosate is used only as a burndown herbicide.

“The lack of resistance is due to two things,” said Neve. “First, only a portion of the population is being sprayed. Everything that emerges after the crop doesn't get sprayed with glyphosate. Second, even when used as a burndown, even if resistant survivors pop up, other herbicides normally will gain control.

“The point is one application of glyphosate at burndown is not equivalent to an application in the crop.”

The number and size of Arkansas pigweed “is very striking,” said Neve. “In Australia, ryegrass is bad. But it's nothing compared to the numbers from pigweed. Basically resistance is a numbers game. Say one in 100 million seeds will be glyphosate-resistant. Well, if one plant is producing 500,000 seeds, it won't take long to come across resistance.

“Having seen the amount of seed production here, it's obviously a problem. If you were writing a recipe for glyphosate resistance, the ingredients are already in place here.”

In getting farmers to focus on weed resistance, Chuck Foresman, Syngenta's head of weed resistance strategies, said current commodity prices “are working in our favor. Growers will want to protect their investments.

“We're talking about one weed. But we must watch others very closely: giant ragweed, waterhemp and common ragweed. I think it's probably time to start working on grasses — barnyardgrass, particularly.”

Keith Driggs, Syngenta technical support representative, said the rise of glyphosate-resistant pigweed can be traced to where Roundup Ready technology was first used. “The worst pigweed areas currently are where Roundup Ready beans were first adopted.”

“That's true,” added Scott. “It was adopted even when there was a big yield drag in Roundup Ready varieties.”

And then there's the issue of farm infrastructure based on Roundup Ready crops. Driggs predicted that could make dealing with resistant weeds even more difficult.

“Growers have set aside that (incorporating) equipment and labor. Growers have adjusted farm size, equipment and strategy for farming to Roundup Ready.

“If we need to go back to incorporating Treflan and other things, it'll be hard. People are selling off their hooded sprayers because of Flex cotton. People will have to be wrenched into doing what they need to.

“Labor isn't available, either. The problem is more than just doing things differently. Many, intentionally, are no longer capable of (reverting to earlier cropping practices). It will be a major adjustment.”

It isn't just soybeans under resistance pressure. Between resistant pigweed and horseweed, “our cotton industry is threatened tremendously,” said Scott.

Some gins are surely watching the steady march of resistant weeds and the loss of cotton acres to corn. “They're on the margin and need those acres to keep running. Once a gin is closed, it's hard to fire it back up. Georgia, where resistant weeds are a raging problem, knows that all too well.”

Something else to add to the mix: the percentage of soybeans in rows is dropping. “It's all about convenience and ease. Everyone is going with broadcast or drilled beans,” said Driggs. “They can plant a huge acreage quickly — throw it out there, spray a good rate of glyphosate, maybe do it again, and that's it. Now, we may have to go back to hooded sprayers and rowed soybeans.”

And the resistance isn't always to a single mode of action. “Stacked resistance weeds are developing in the landscape,” said Foresman. “That makes weed scientists' jobs tough. Someone describes a problem. How do you offer a remedy? It's hard to know if a stacked weed is out there.”

The weed scientists looked unfavorably at reduced-rate herbicide programs. “Farmers cannot go with reduced rates,” said Foresman. “They must use full rates and ensure the weeds get an ample dose.”

Reduced rate programs have been in Arkansas for years. Love them or hate them, “those programs have saved soybean farmers lots of money and still cleaned up fields,” said Scott. “Whether or not those were even partially responsible for the resistance, last fall, (Smith) and I decided to remove the reduced-rate recommendations (from Extension literature).”

In the end, though, it all comes down to what farmers are willing to do. With all the weed science done, with all the competition studies, “it's amazes me what we still find in fields,” said Glasgow. “Even if we could kill a 14-foot tall pigweed, why would anyone allow it to grow to that size and let so much yield melt away? That's mind-blowing.”

Researchers are hopeful Neve's modeling work on Arkansas weeds will yield results by year's end. For more, see and

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