제초제 내성 풀이 늘어난다

Monsanto's Mayank Malik took attendees to a test plot that showed Dicamba-tolerant soybeans. (Terry Anderson/Midwest Producer)

DAVID CITY, Neb. - If farmers didn't already know that there's a problem with herbicide-resistant weeds, they certainly did after a weed management field day on July 12.

Researchers and experts from university and commercial levels hammered home the fact that weeds are increasingly more resistant to herbicides and that farmers shouldn't expect any new product anytime soon. Instead, they should incorporate best management practices into their operations.

"There's no silver bullet, no chemistry, no molecule coming," said Leo Charat, BASF biology manager. "EPA is not holding the research back. All companies go through the testing. Companies are looking for that new molecule. There are things that would work but they don't have the tolerance of the crops."

He said if a new molecule was found today, it would be 7 to 10 years before it would be available to users.

Jason Norsworthy from the University of Arkansas told the crowd of about 300 farmers, crop consultants, seed and chemical sales personnel, and government employees that 370 species worldwide are resistant to herbicides.

In 1996, 19 herbicides were used on at least 5 percent of the soybean acres, he said. By 2005, that was reduced to just one herbicide. In 1986, 18 to 20 companies brought 96 new herbicides to the marketplace. Now fewer than 20 herbicides were introduced by companies that could be counted on one hand.

"This is not a local or regional problem," he said. "It's a national problem."

Norsworthy said a prime example of how herbicide-resistant weeds can get out of control quickly was Palmer amaranth in Arkansas. As few as five or six years ago, it wasn't a problem. In 2005, Arkansas saw its first confirmation of glyphosate-resistant Palmer amaranth.

"It's a myth that it's a Southern weed," Norsworthy said. "It's a problem in Kansas and moving north. It grows quite well in Iowa, northern Illinois and Michigan."

He said the northern move is because the seed is carried from the south on cottonseed hulls and moves on combines and cotton pickers.

In Arkansas last year, Palmer amaranth caused 5 percent yield loss, worth $71 million to the farm economy. This year, 32 percent of soybean acres will be cultivated just to control the weed.

Residual herbicides are a must, he said, and season-long control requires multiple residual herbicides, spraying every 14 days until the bean field has a full canopy.

"If I don't have a herbicide activated when the weed seed emerges, I've not killed a sufficient number of weeds."

Timely applications are crucial, he said, because spraying two days late can make the difference on whether to even bring the combine to the field. once Palmer amaranth gets to be 4 inches tall, it will grow 3 inches a day under good growing conditions. A commercial sprayer could take as much as a week to get to a field.

"If it's over 4 inches, we're pulling in a disk," he said. "Most farmers are buying their own sprayers if they're losing the crop."

As much as 80 percent of the soybean and cotton acres in Arkansas will be chopped this year, just to get rid of the weeds.

"You've got to prevent seed production," he said. "You've got to go out and get that one weed." That weed and every weed needs to be destroyed, brought out of the field and either buried with a backhoe or burned.

Phil Stahlman, a weed scientist at Kansas State University, told how kochia has become herbicide resistant and less manageable by shifting in the last 15 years to a later season emergence.

Kochia is not native but has adapted well to the western United States and has been spreading west and north.

Aaron Franssen with Syngenta said controlling water hemp has been "a train wreck, ignored to the point we can't do anything."

Four locations have confirmed herbicide resistance, including a significant find in Nebraska, he said.

Stevan Knezevic, at University of Nebraska-Lincoln weed expert, said giant ragweed is an annual that acts like a perennial "and we're having a hard time killing it. We burn it to the ground, but it keeps growing from the secondary buds."

He said it takes just one plant per field to survive and spread seeds, and in a few years the field has a major problem.

"Every chemical we use has a chance to become resistant," he said. "Guys will say 'I don't care if it's not on my farm.' We're seeing species develop multiple resistances."

BASF's Charat said best management practices means changing the way farmers do things, such as timing of applications or changing the type of nozzles.

"If you have 6,000 acres, you may have six BMPs that need different programs," he said. "There are a lot of things in a BMP that we have to adapt to."

More about Herbicides

• ARTICLE: $4.8 billion benefit attached to atrazine use in U.S.
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• ARTICLE: Envisioning tomorrow's weed control and how to get there
• ARTICLE: Crops Briefs: Report: Some dryland acres doubled value since 2006