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Killer weeds

When one door closes, another one opens.

Killer weeds

When one door closes, another one opens.

Most people figure that’s an optimistic way of looking at life. People say it to someone who has been kicked down to help them get back up. The ones getting up repeat it to themselves.

Problem is, the flip side of that is Ian Malcolm telling the foolish folks in “Jurassic Park” that “life finds a way.”

In his case, he’s talking about dinosaurs. In agriculture, the term “opportunistic weed” suddenly is relevant. It’s just a coincidence that one of the ALS herbicides is named “Raptor.”

This isn’t news: A weed will find a way to live.

Key Points

• Manage herbicides to avoid cross-resistance.

• Write down a long-term herbicide management plan.

• Consider tillage or “extreme” cover crops.

“What would Charles Darwin have thought about herbicides being applied on weeds?” asks Steve Powles, director of the Western Australia Pesticide Resistance Initiative. “He would have said, ‘Nature will win.’ ”

In a Macon County, Ga., field in 2004, a Palmer amaranth found a way to live by resisting glyphosate.

In that same county — and in counties across a couple of states — Palmer pigweed now also resists ALS herbicides, such as Staple and Envoke.

All told, the International Survey of Herbicide Resistant Weeds now lists 346 resistant biotypes, 195 species, in more than 330,000 fields.

Read that again.

So now, even though everyone who grows row crops is a candidate for post-traumatic stress disorder from the war on pigweed, another front is open, and the soldiers must roll to protect their flank. The new fronts are: several different resistant weeds in one field and weeds that are resistant to more than one chemistry.

Though the focus in much of the Southeast is on glyphosate-resistant Palmer amaranth, 16 other weeds also are resistant to glyphosate.

University of Tennessee Extension weed scientist Larry Steckel spends most of his days focused on marestail, also called horseweed.

“It is the most widespread glyphosate-resistant weed,” Steckel points out. Resistant marestail started in Tennessee in 2001 and went from bad to worse in 2003. Now a weed that once was controlled at burndown, and emerged no later than June, shows up in a field at all times during a summer crop.

And some marestail is glyphosate- and ALS-resistant.

ALS-resistant weeds also are rampant. Cross-resistance is growing, and weed scientists fear resistance to PPOs is coming soon.

“In a four-year rotation we could spray 16 PPOs,” says University of Georgia Extension weed scientist Eric Prostko. “We’re overusing the PPOs.”

Survival, however, is possible for growers who plan their battle.

Write down a battle plan

The first step, Prostko says, is to write down a resistance management plan that covers the entire crop rotation. If the rotation is peanut, corn, cotton and cotton, then the plan should cover four years. As the plan is followed, notations should be made regarding exactly what was used and when.

While scientists throw around terms like “mode of action” and sometimes say “protoporphyrinogen oxidase” instead of PPO, all a grower has to know is which number is on the bag. Regardless of the company or the active ingredient, all commercially available chemicals have a number on the bag that identifies the mode of action. Growers don’t have to know that a 2 is an ALS inhibitor; they just need to make sure they don’t follow a 2 with another 2.

When applying chemicals, Powles says, use high rates.

“We should use herbicides at high rates infrequently,” he says. “Be very scared of this resistance mechanism [in plants]. ... We have plants that are resistant to herbicides that have not yet been discovered.”

In addition to rotating crops and chemicals, growers also must decide whether to use tillage or “extreme” cover crops, which means growing a cover such as rye to 6 or 7 feet tall, killing it in spring and stripping the summer crop into heavy residue. Both accomplish what’s needed to destroy Palmer pigweed seeds: Bury them 4 inches deep.

“We don’t do what we’ve always done: Just spray herbicides,” says University of Georgia Extension weed scientist Stanley Culpepper. “We’re using an integrated approach to battle a pest that is ‘whupping’ us.”

One of the most effective tools against herbicide-resistant weeds is tillage. Steckel remembers losing most of the no-till acres in his area in 2005-06. Using Dicamba, however, growers now are holding on to the financially and environmentally preferred no-till and reduced-tillage practices.

“The key is to get [resistant horseweed] before it gets to Coke can size,” Steckel says.

Using the size of the weed to determine when to spray is essential to control, notes Bayer CropScience technical representative Keith Rucker.

When making a herbicide application, calibrate the equipment and check all the nozzles.

“It’s all about the system,” Culpepper says. “It isn’t any one thing that we do.”

PPO Herbicides



Authority First

Authority MTZ



Canopy XL







Flexstar GT




Goal Tender

















Ultra Blaze


Valor XLT

ALS Herbicides



Authority First


Basis Gold






Canopy EX

Canopy XL



Celebrity Plus



Cimarron Max

Cimarron Plus













Harmony Extra

Harmony GT



























Steadfast ATZ




Synchrony XP


Valor XLT


Weeds on the Web

For more information on killing resistant weeds, visit these Web sites: — University of Georgia — sponsored by Monsanto Co. — sponsored by the Herbicide Resistance Action Committee, the North American Herbicide Resistance Action Committee and the Weed Science Society of America — sponsored by Syngenta — National Corn Growers Association

How do you control resistant weeds in your fields?

The days when Roundup at burndown, Roundup in the planter and Roundup over the top as a complete herbicide program are gone. Growers now are using integrated approaches to kill weeds, especially in those areas where resistant weeds complicate management, curb yield and create havoc at harvest.

Tillage and residual

Waynesboro, Ga., grower Henry Hopkins III tried Prefix on his soybeans in 2009 and saw near-perfect control of Palmer amaranth. He tank-mixed Gramoxone with Prefix and applied Prowl at plant. Hopkins conventionally tills most of his fields because he has a broad range of soil types. He also subsoiled and bedded some soybeans, a departure from his usual practice of drilling beans.

“I think they came off better,” Hopkins says. “It cost more and the no-till drill is easier, but I think the subsoiler worked better.”

Flex the Reflex

Helena Chemical Co’s. Waynseboro, Ga., facility manager discovered the value of Reflex; renewed his faith in Cadre, Valor and atrazine; and sometimes was forced into a salvage treatment using Harmony SG.

George Tedder figures about half of the 70,000 acres his facility treats are infested with glyphosate- and/or ALS-resistant weeds. He recommends a pint of Reflex at plant in cotton and soybeans. The label allows up to 1.5 pints.

What growers need to be prepared for as they go back to older chemistries and tougher new compounds is crop burn.

“With this pigweed, you can stand a lot more burn than we used to tolerate,” Tedder says.

Tedder also recommends a fall burndown, particularly behind corn. Atrazine, Tedder says, takes out pigweed during the season, but weeds spring up after harvest.

“You harvest the corn in August, and pigweeds come up as thick as hair on a cat’s back,” he says.


Georgia growers spent $11.4 million hand-weeding Palmer amaranth on 799,000 acres in 52 counties, reports University of Georgia Extension weed scientist Stanley Culpepper. The average cost was $26 an acre, with some paying as little as $3 and others as much as $100.

The growers hand-weeded to deplete the seed bank in their fields. It’s aggressive. It’s expensive.

Culpepper believes the practice will pay off. “If we’re aggressive for three to four years, I think we can turn the tide on this pest.”

Expert: Resist gov’t regulation

Louisiana crop consultant Blaine Viator gave Syngenta weed resistance strategies manager Chuck Foresman a new middle name during a weed resistance conference in February: “Toll Ya So”

Viator says colleges once taught that glyphosate resistance wasn’t possible. Foresman was the first person Viator heard say it was — and he said it in 2004.

If that gives Foresman oracle status, then those listening at the Syngenta Southern Field Crops Consultants meeting need to act on his message in Miami.

“If we don’t jump on top of this — if we don’t jump on diversity — there’s some folks in Washington who want to jump in and help us,” Foresman said.

The reason government is interested is the same reason Foresman “despises” weed resistance.

“It takes food and clothing off people’s backs,” he said.

Syngenta figures by 2013, the U.S. will have 38 million acres infested with resistant weeds. That’s a 43% increase per year.

Foresman isn’t concerned only about glyphosate.

“Most growers do believe there’s going to be new technologies to fix the problem,” he said.

He thinks so, too. But those also must be used carefully.

“Science is going to prevail,” Foresman said. Resistance management, he said, “is about sustaining these excellent technologies we have.”

Stop it or grow it

Some might call a list of things to do to stop weed resistance and things to avoid a list of “Dos and Don’ts.” The issue here, however, is whether to let those weeds grow or stop them. So here’s a list of “Stop Its and Grow Its.” This list is compiled from interviews and presentations in the Southeast this winter, starting at the Beltwide Cotton Conferences and continuing through the Syngenta Southern Consultants Conference and the Bayer CropScience Southeast Technical Conference.

Stop it

Write down a weed resistance plan for an entire rotation — preferably three to four years.

Use full labeled rates. Many believe using reduced rates of glyphosate contributed to creating resistant weeds.

Make timely applications. Since timely post-emerge applications are not always possible, a preemerge application is a must.

Rotate crops.

Rotate herbicides. Herbicide packages have a number on them to indicate the mode of action. Don’t follow a 6 with another 6. Don’t use the same number over and over again in a year — whether it’s the same crop or the next one.

Pay attention to changes in weed response to herbicide applications.

Calibrate equipment.

Check, check and triple-check nozzles.

Grow it

Base a weed management program on the neighbor’s plans.

Wait for a company to come out with “the next glyphosate.”

Underestimate a weed’s resilience.


NOTHING LESS: When a herbicide achieves 90% control, weed scientists call that “excellent.” “That’s not enough with Palmer pigweed,” says Auburn University weed scientist Mike Patterson. “You’ve got to have 100% control.” Patterson suggests a three-year program that includes using a moldboard plow in the fall followed by heavy cover in the first year and then heavy cover the next two years.

This article published in the April, 2010 edition of SOUTHERN FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.

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