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Corn+Soybean Digest

Multiply Your Modes | Multiple Modes of Action Are Becoming Necessary as Glyphosate Resistance Grows

Two 48-oz. applications of glyphosate should wipe out about any weed. But not the kochia that Randall Currie finally battled to death with multiple modes of action (MOA) in southwest Kansas.

Glyphosate-resistant kochia has also crept up on Brian Schafer, a southwestern Nebraska grower who shoots for 200-bu. corn and 70-bu. beans under irrigation. Even though he grew 100% Roundup Readycorn and soybeans in 2009, multiple MOA are usually the only ways to control kochia and other weeds.

Resistant weeds, whether kochia in Kansas, pigweed in Pennsylvania and dozens of others elsewhere, are likely here to stay. If you haven’t seen weeds that show resistance to glyphosate, ALS or other herbicide types, don’t hold your breath. Neither had countless other growers who counted on regularly dependable weed killers one too many times.

“We almost lost corn and sorghum test plots overrun with kochia,” says Currie, Kansas State University research weed scientist in Garden City. “Multiple herbicide modes finally killed it.”

Currie can’t tag kochia 100% glyphosate resistant, but it’s close to it. “It’s becoming more difficult to control with glyphosate in western Kansas,” he says, putting it mildly.

He tested various tankmixes in the region. “We determined that a good tankmix that would control kochia includes a very early application of dicamba (0.5-1 pt./acre) and atrazine (1-2 lbs./acre) to get them when they’re small,” says Currie. “That’s followed by an application of dicamba (0.5-1 pt.) and glyphosate (1 lb.). Starane has also provided excellent control.

“As seen by Bob Wilson (University of Nebraska weed scientist) and others, a good foundation treatment of atrazine tankmixed with one of several grass-control compounds has helped a great deal,” Currie says.

John Fenderson, Monsanto agronomist in Kiowa, KS, says glyphosate tankmixed with Status, either at a 2½-oz. half rate or 5-oz. full rate, “seems to be very popular” with growers looking to control tough broadleaf weeds. Also, Balance-Flexx at a 3-5½-oz. rate is popular as a pre-emergence treatment for broadleaf control.

Experiences with glyphosate and other resistance are being seen more and more. They started in the Deep South and traveled north. Schafer’s southwest Nebraska location has helped him dodge most resistance problems.

“Kochia, velvetleaf, shattercane, pigweed, marestail and foxtail are the main grassy weeds we see,” says Schafer. “Most are being controlled. But we’re seeing some kochia resistance now. We’ve seen three fields where it’s been resistant to glyphosate.”

Schafer often plants both conventional and Roundup Ready hybrids for diversification. But for 2009, he planted triple-stack corn hybrids with glyphosate resistance, as well as corn borer and rootworm resistance. He likes the weed- and insect-control qualities provided, but counts on multiple MOA for corn weed problems.

“We know we can’t depend solely on Roundup in corn and never really have,” he says, in describing his typical multiple MOA program.

If there are residual weeds and a preplant herbicide treatment is needed, he goes with Roundup or Roundup mixed with 2,4-D. But most herbicide is applied in post-treatments.

“We went with an application of Bicep (1 lb. atrazine + 0.78 lb. S-metolachlor), along with 1 lb. of Roundup,” says Schafer. “If you have Roundup-only fields, you can’t get to all of them at once, especially when you’re trying to cut wheat at the same time you need to spray. So rather than try to push it, we run another herbicide.

“We actually waited until the corn was 8-12 in. tall (in 2009) before we went in and sprayed. But that’s only because we had clean fields in most cases. If we have a weed problem, we address them field to field,” he says.

Schafer knows that resistance for weeds other than kochia could be lurking. “Ten years ago we never had any problem with marestail,” he says. “Now you really need to be on top of that one.”

Bill Johnson, Purdue University weed scientist, says that when planting Roundup Ready corn, multiple MOA should help corral glyphosate-resistance problems. “Use a residual herbicide preplant or pre-emergence and appropriately timed Roundup postemergence,” says Johnson. “This helps to protect crop yields from early season weed competition and reduces the number of weeds exposed to glyphosate.”

For conventional corn he recommends a similar program, “except for postemergence, we would use something other than glyphosate.”

Nebraska’s Wilson adds that his regional studies have shown that going with a pre-emergence herbicide followed by glyphosate can improve production.

“Once growers know their main weed problems, they can address them with a pre-emergence that would assure activity against those weeds, then come back with their regular glyphosate program,” he says.

“Eleven years of studies have shown us that a pre-emergence followed by glyphosate produces better yields than just glyphosate. The increased yield more than pays for the cost of the pretreatment.”

Wilson says the pre-emergence tankmix likely should include more than one MOA, which provides additional protection against future resistance problems.

According to, there are 16 weeds listed showing resistance to glyphosate worldwide. They include: Palmer amaranth, common waterhemp, common ragweed, giant ragweed, hairy fleabane, horseweed, junglerice, sourgrass, goosegrass, wild poinsettia, Italian ryegrass, rigid ryegrass, ragweed parthenium, buckhorn plantain, johnsongrass and liverseed grass.

But that’s a drop in the bucket. Overall, there are more than 330 resistant weeds worldwide. Most are in the ALS inhibitor class with 101 resistant weeds, followed by Photosystem II inhibitors, or atrazine, with 68. There are 28 resistant to 2,4-D and 24 resistant to paraquat. (For a complete list of resistant weeds, go to

Still, glyphosate-resistant weeds are often the biggest weed obstacles for growers, who overwhelmingly use Roundup Ready corn hybrids, and an even larger percentage of Roundup Ready soybeans. They have seen the tremendous benefits of the over-the-top glyphosate herbicides. But most realize their weed-control program may need tweaking.

“Weed control in glyphosate-resistant crops will continue to increase in complexity due to development of resistance or selection of weed species that can avoid glyphosate, late emergence or just inherent tolerance to the rates we are using,” says Johnson.

“So, getting the right residual on the field will increase in importance as we develop more weeds that escape our current practices in Roundup Ready systems,” he adds.

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