Resistant weeds a growing issue

It’s been a difficult year for weed control. The issue isn’t that weeds in general have not been controlled by the herbicides, particularly those applied postemergence. Rather, the issue was finding the opportunities to make the applications in a timely manner.

The weather was often too wet to get into the field to spray. Many fields were weedy too long. As a result, growers have lost an indeterminable percentage of their potential corn and soybean yield, and consequently, profitability.

That’s how Iowa State University Extension weed scientist Mike Owen sizes up the situation. Iowa has a lot of weedy corn and soybean fields this year.

The evolution of weed resistance to herbicides is showing up in more fields, too. “Despite the general susceptibility of weeds to most herbicides, an increasing number of growers are discovering weed species that are no longer responding to specific herbicides,” says Owen. “The evolution of herbicide-resistant weed biotypes is continuing to increase at an increasing rate across Iowa and the Midwest in general.”

Herbicide-resistant weeds

Specifically, Owen says, “we are seeing an increasing number of fields with common waterhemp that does not respond to glyphosate and the PPO inhibitor herbicides, such as Cobra and others. Furthermore, populations of common waterhemp that no longer are controlled by the HPPD inhibitor herbicides, such as Callisto and similar products, have been identified in Iowa.”

The problems of herbicide-resistant weeds have become so noteworthy that even Congress is starting to pay attention. The U.S. House of Representatives recently held a hearing where testimony about herbicide-resistant weeds was presented to the House Oversight Committee on Domestic Policy. Owen was one of the people who testified.

“In short,” says Owen, “the problems of herbicide-resistant weeds are complicating corn and soybean production in Iowa and the Midwest more than most growers are willing to admit.” He says farmers need to recognize “we now have herbicide-resistant weed populations for most of the herbicides used commonly in Iowa, and for all of the companies whose products are represented.”

It is important to understand that weeds have historically been and will continue to remain the most economically damaging pest complex Iowa corn and soybean growers face, he says. Farmers lose more money due to weeds than any disease complex, for example, despite effective weed management tactics being available.

Glyphosate-resistant common waterhemp, giant ragweed and marestail have been identified across Iowa, and the number of locations is increasing rapidly. ISU has generated data confirming many of these locations, and grower complaints are providing anecdotal evidence on many more fields. 

“You need to recognize that weeds will evolve resistance to any management tactic or crop production practice that is used repeatedly,” Owen advises. “Thus, cropping strategies that diversify corn and soybean production, and specifically strategies that diversify weed management, must be included in your production plans for 2011.”

The spread of weeds resistant to herbicides is bringing new scrutiny to the government’s regulation of biotech crops. At the hearing in Washington, D.C., attended by Owen, a congressman stated that USDA has been too quick to approve new varieties of herbicide-tolerant crops and other biotech products. Another person who testified proposed that the government restrict the use of herbicide-tolerant crops and impose a tax on biotech seeds to fund research and education programs.

Farmers need to act now

The problem with herbicide-resistant weeds is most prevalent in soybean and cotton fields in the Southern U.S., but it is increasing in other regions. Owen thinks Iowa might be only two years away from a very serious problem with glyphosate-resistant weeds.

Also, significant issues with ALS inhibitor herbicides already exist in essentially any field in Iowa that is infested with common waterhemp. Furthermore, issues with PPO inhibitor and HPPD inhibitor herbicides are increasing in Iowa. He says farmers have to quit relying so heavily on glyphosate, or any single tactic, to control weeds. “Farmers value the convenience and simplicity of these glyphosate-resistant crops without appreciating the long-term ecological and economic risks,” Owen notes.

Biotech companies are trying to deal with the weed resistance problem by engineering new crop varieties that will be immune to more than just glyphosate, but Owen says even those crop varieties will eventually run into weed resistance problems if farmers aren’t careful.


RUNAWAY RAGWEED: Herbicide resistance is spreading quickly, and resistant weeds are more of a problem than many growers admit, observes ISU weed scientist Mike Owen.

This article published in the September, 2010 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.