It has been a difficult spring and summer for weed control. This issue is not that the weeds in general have not been controlled by the herbicides, particularly with herbicides that are applied postemergence. But rather the issue has been finding the opportunities to make the applications. It's been a year when the weather was often too wet to get into the field to spray. Many fields have been weedy too long. As a result, growers have lost an indeterminable percent of their potential soybean yield and, consequently, profitability. That's how Iowa State University Extension weed scientist Mike Owen sizes up the situation for 2010. Iowa has a lot of weedy soybean fields this year. "Despite the general susceptibility of weeds to most herbicides, an increasing number of growers are discovering weed species that are no longer responding to herbicides," he points out. "The evolution of herbicide-resistant weed biotypes continues to increase at an increasing rate."
Resistant weeds are more of a problem than most growers admit
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. Furthermore, populations of common waterhemp that no longer are controlled by the HPPD inhibitor herbicides, such as Callisto, have been identified in Iowa." The problems of herbicide-resistant weeds have become so noteworthy that 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 soybean production in Iowa more than most growers are willing to admit!" It is important to recognize that weeds have historically been and will continue to remain the most economically damaging pest complex that Iowa soybean growers face, he says. Farmers lose more money due to weeds than any disease complex, despite effective management tactics being available.
Glyphosate-resistant weeds identified in more locations in Iowa
Glyphosate-resistant common waterhemp, giant ragweed, and marestail has been identified across the state and the number of locations is increasing rapidly, says Owen. ISU has generated data confirming many of these locations, and grower complaints provide anecdotal evidence on many more fields. Kansas State University has recently announced the occurrence of glyphosate-resistant kochia populations. "Soybean growers in southwest Iowa need to be observant to make sure this problem has not evolved in Iowa, and if it has, take immediate action to manage the problem," he adds. While there is little that can be done this year to resolve these herbicide-resistant weed problems, it is critical to make plans to mitigate the problems for next year. "You need to recognize that weeds will evolve resistance to any management tactic or crop production practice that is used repeatedly," says Owen. "Thus, cropping strategies that diversify soybean production, and specifically weed management, must be included in your production plans for 2011."