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Corn+Soybean Digest

As Glyphosate Resistance Grows in Waterhemp, Soil Residuals May Help Control It

Mike Kuhns used a two-pass glyphosate program from the time he started growing Roundup Ready soybeans in 1996. For years, he applied 1.5 pints of glyphosate/acre. “It always worked before,” says the Mason, IL, farmer, who raises no-till corn and soybeans and produces hogs with his father and brother.

But three or four years ago, “we started seeing a lot of waterhemp it didn't control,” he says. “We've been steadily increasing the rate of Roundup.”

In 2008, “we did two applications of Durango — 36 oz./acre followed by 48 oz./acre three weeks later — and still we missed some waterhemp.” In 2009, glyphosate again “did a poor job” controlling waterhemp and marestail, Kuhns says.

The Kuhnses' waterhemp troubles intensified when they added Roundup Ready corn to their rotation. At first, waterhemp escapes were a problem in only a couple of fields, Kuhns says. “Now, it's every field I've got.”

George Soltwedel, agronomist at Effingham Equity in Altamont, IL, has been working with the Kuhnses for many years. “It's pretty obvious that they have resistant waterhemp. And it's not just them. It's all around here.”

THROUGHOUT THE MIDWEST, waterhemp is rapidly becoming resistant to glyphosate, just as it did to earlier herbicides, says Kevin Bradley, University of Missouri Extension weed scientist. Waterhemp populations have evolved to withstand atrazine, ALS-inhibiting herbicides and PPO-inhibiting herbicides.

Today, the stage is set for “a worst-case scenario,” says Aaron Hager, University of Illinois Extension weed scientist. ALS-inhibiting herbicides are obsolete for waterhemp control in the Midwest because of widespread resistance. If the weed also becomes resistant to glyphosate and PPO inhibitors, farmers who plant conventional or Roundup Ready soybeans will have no postemergence herbicide options for waterhemp control, Hager says. University of Illinois trials show that soybean yield losses from uncontrolled waterhemp average 19% after six weeks of growth.

Glyphosate-resistant waterhemp populations have been confirmed in Illinois, Missouri, Kansas, Iowa and Minnesota. In Missouri, 45 out of 88 waterhemp populations randomly surveyed in the northern half of the state were resistant to glyphosate. “We have at least 28 counties in Missouri with glyphosate-resistant waterhemp today,” Bradley says.

In 2008 in southern Illinois “the control of emerged waterhemp with only glyphosate was the worst I have observed” — that is, until 2009, says Southern Illinois University weed scientist Bryan Young. “My confidence in using glyphosate as a grower's only waterhemp management tool is gone.”

Young has also confirmed the existence of PPO-resistant waterhemp in 21 counties in south-central and western Illinois, an area “much greater than previously doc-umented or suspected.” Kansas, Iowa and Missouri have confirmed PPO-resistant waterhemp, too. So growers can't assume that the PPO-inhibiting herbicides they used in the 1990s will still be effective on waterhemp if glyphosate stops working, Young says.

Already, a few populations of waterhemp have been discovered that can tolerate three different herbicide modes of action. Illinois reported biotypes resistant to ALS inhibitors, triazines and PPO inhibitors. Missouri and Iowa have reported biotypes resistant to ALS and PPO inhibitors and glyphosate. Just recently, Hager adds, a waterhemp population from western Illinois was found to be resistant to four different herbicides — atrazine, lactofen, imazamox and glyphosate.

Iowa State University weed scientist Mike Owen frequently sees fields with oblong-shaped patches of waterhemp — evidence that seed from escaped weeds is being distributed by the combine.

“Typically, there are dead plants next to the live plants,” he says. “This should scream out impending concerns about resistant populations.”

The best way for growers to stem the threat from resistant waterhemp is to diversify their weed chemistry, Hager says. In particular, “we need to rethink the utility of soil-applied herbicides, and not just as a set-up for glyphosate.”

IN UNIVERSITY OF MISSOURI trials with glyphosate-, ALS- and PPO-resistant waterhemp, a pre-emergence herbicide applied before soybean planting reduced late-season waterhemp density by 97%, Bradley says. That's compared to a single postemergence application of glyphosate plus a diphenyl ether.

In 2009, for the first time in a decade, Mike Kuhns treated his soybean fields with a pre-emergence residual herbicide, spraying Prefix (s-metolachlor + fomesafen) on June 1. “It kept the fields clean for about five weeks,” Kuhns says.

In 2010, Kuhns will apply a pre-plant burndown of 24 oz./acre of Rascal plus 2 oz./acre of OpTill (saflufenacil + imazythapyr). He'll follow that at about the third trifoliate stage with 22 oz./acre of Roundup Power Max plus 20 oz./acre of Prefix. (A 30-day interval is required between applications of OpTill and Prefix.) This program lets Kuhns cut his glyphosate use in half while hitting weeds with four modes of action for about $34/acre.

To help you manage glyphosate-resistant waterhemp, Hager recommends a five-step integrated approach:

  1. Apply the full rate of soil-residual herbicide no sooner than seven days before planting or no later than three days after planting. Using a full rate prolongs residual control. That's important because waterhemp germination and emergence extend late into the growing season.

  2. Apply glyphosate alone at regular rates when waterhemp is 3-5 in. tall. Non-resistant waterhemp less than 5 in. tall is sensitive to glyphosate at 0.75 lb. acid equivalent/acre, Hager says. Don't jack up the rates. University of Illinois 2007 trials showed that increasing the rate on glyphosate-resistant waterhemp did not improve control.

    Hager doesn't recommend tankmixing glyphosate and a PPO inhibitor without knowing how the population would respond. There's the potential for antagonism between herbicide classes, and questions about additives. And if PPO-resistant waterhemp is present, tankmixing can complicate a resis-tance diagnosis.

  3. Scout seven days after applying glyphosate.
  4. If waterhemp control is inadequate, apply a PPO-inhibiting herbicide at full-labeled rates as soon as possible.
  5. Re-scout within 10-14 days. If any waterhemp plants have survived, remove them before flowering. Research suggests that some resistance traits can be transferred by pollen. And female waterhemp plants produce prodigious amounts of seed ? from 250,000 to more than 1 million seeds/plant.
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