Meet Weed Enemy No. 1

The weeds aren't playing fair near Equality, Ill. DTN found soybean fields infested with overgrown waterhemp, velvetleaf, marestail and cocklebur. Palmer amaranth was ganging up on the crop, too. So was Jimsonweed.
Waterhemp isn't hard to find these days in the Midwest. It's hard to find the soybeans in this field near Equality, Ill. (DTN photo by Pamela Smith)
Sure the field was droughty, but the sprayer was still sitting in the field and some weeds showed evidence of die back and subsequent re-growth. An observer could assume the farmer threw up his hands and decided the weeds had won.
To be fair, some fields in the region were clean as a whistle, but too many resembled the tangled mess of resistant weeds farther south in cotton country.
University of Illinois weed scientist Aaron Hager isn't surprised. "Our worst fears are coming true with waterhemp," Hager told DTN. "Waterhemp plants resistant to multiple herbicide families are becoming increasingly common in Illinois. We are screening waterhemp at no cost to our growers this summer. The best thing you can do now is prepare a plan for next year."
A recent survey by BASF shows that waterhemp has become the No. 1 weed to watch throughout the Midwest. One in five growers listed waterhemp as the top glyphosate-resistant weed they expect to show up on their farms during the 2012 season. Marestail and lambsquarter ranked second and third on the list, respectively.
Dan Westberg, technical market manager at BASF, wasn't surprised by the results either. A similar survey in 2010 called out common lambsquarters as the main worry, followed by marestail and ragweed species. Waterhemp ranked fourth only two years ago.
"In 2011, we saw glyphosate-resistant waterhemp explode across the Midwest," Westberg said in news release on the survey. "It was a tipping point for farmers and another sign that we have to think beyond glyphosate alone for weed control."
Westberg noted that waterhemp comes by its notorious ways naturally. "Waterhemp is a prolific seed producer that can create detrimental seed banks farmers must deal with for years," he said. "Waterhemp also emerges throughout the season, so it's a weed that is poised to spread like wildfire -- which makes the resistant populations especially dangerous." BASF released two new residual herbicides in 2012 to help growers control glyphosate-resistant weeds. The company is also working with Monsanto to deliver a dicamba-resistant trait.
Westberg noted that of the 10 states that now have confirmed glyphosate-resistant waterhemp, three have waterhemp with resistance to multiple sites of action. Growers interested in learning more about where weed resistance is located can monitor it at www.weedscience.com.
Hager adds that not every waterhemp plant that survives an application of glyphosate is resistant to it. He says the following four factors, in combination, might lead you to suspect that a waterhemp population is indeed glyphosate-resistant: The appropriate rate of glyphosate (plus proper adjuvants) was applied at the appropriate weed growth stage. Environmental conditions during and after application were conducive for good glyphosate activity. Plants that survived the glyphosate application are found next to plants that were controlled. The field has a history of glyphosate use.

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