Is Glyphosate Herbicide Losing Effectiveness?

As glyphosate-resistant weed populations become economically significant, this tool may become less useful.

At last week's annual Integrated Crop Management Conference at Iowa State University, ISU Extension weed scientist Mike Owen presented an update on the status of weed resistance to the widely used herbicide glyphosate.

Glyphosate was introduced to the marketplace in the mid-1970s, a nonselective weed control tool for use in conservation tillage crop systems. Acreage treated with glyphosate in Iowa exploded upon the development and adoption of transgenic crop varieties, namely the Roundup Ready and related technologies.

Glyphosate tolerant soybean varieties were commercially introduced in 1996 and became immediately popular as an economically effective weed management tactic in reduced tillage systems and in managing weeds that have developed resistance to acetolactate synthase (ALS)-inhibitor herbicides. Currently, more than 90% of soybeans planted in Iowa are glyphosate resistant.

Known populations of resistant weeds

Whenever a crop management system changes, there are likely to be changes in pest and weed populations. "The prevalence of glyphosate use likely will, in time, change the populations of weeds we manage in fields," says Owen. "Development of glyphosate-resistant weed populations of several species has been reported in soybean fields in several states, and it is likely that there are glyphosate-resistant weeds present in Iowa. I think if we looked hard enough and in the right places, we could find resistant weeds in Iowa."

Here's a list of known populations of glyphosate-resistant weeds in the U.S. Midwest--by state and date of discovery :

Marestail: Tennessee (2001), Kentucky (2001), Missouri (2002), Arkansas (2003), Nebraska (2006)
Common ragweed: Arkansas (2004), Missouri (2004)
Common waterhemp: Missouri (2005)

ISU evaluating glyphosate cropping systems

As glyphosate-resistant weed populations become economically significant, glyphosate may become a less effective weed management tool. To that end, Owen and other ISU weed scientists have undertaken several long-term research studies to evaluate the impact of glyphosate-based crop production systems on weed communities and specific research on the evolved glyphosate resistance in common waterhemp and horseweed. These long-term studies are being conducted on ISU research farms.

At this point, no changes in weed populations have been observed in the experimental areas. But anecdotal information suggests that common waterhemp, common lambsquarters, giant ragweed and Asiatic dayflower problems are escalating in Iowa, possibly due in part to glyphosate-based crop production systems. "Our research will continue to investigate changes in weed communities and the impact of the glyphosate-based crop systems," he says.

Watch your fields for resistant weeds

"Iowa farmers should monitor their fields for glyphosate-resistant weed populations," says Owen. "Remember, weed escapes in glyphosate-treated fields are often caused by use of low application rates of glyphosate relative to the size of the weeds. The low application rates allow the bigger weeds to escape."

Weed escapes also can result because of exposure to doses of glyphosate due to physical constraints. For example, dust on the leaves of weeds or poor spraying patterns result in poor control and escaped weeds. Weed escapes are also sometimes caused by rain immediately following application of glyphosate, washing the herbicide off the leaves.

"Farmers also need to keep in mind that development of weed resistance to this herbicide can be delayed or avoided by rotating the use of glyphosate with other chemistries--in lieu of multiple glyphosate applications," says Owen.

ISU Extension weed scientists are watching for resistant weed species in Iowa. In addition, several studies are ongoing that examine weed control efficacy that employ combinations of tillage, planting date and crop rotations. Two ISU publications address this topic: "Issues in Weed Management for 2006", PM 1898, is available at www.extension.iastate.edu/Publications/PM1898.pdf and the "2006 Herbicide Guide for Iowa Corn and Soybean Production", is also online at www.extension.iastate.edu/Publications/WC94.pdf.

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