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Glyphosate resistance flares up

Agriculture.com Staff 05/06/2008 @ 9:24am

Add Minnesota to the list of states where glyphosate-resistant giant ragweed and common waterhemp are besting the most common means for control.

University of Minnesota Extension agronomist and weed scientist Jeffrey Gunsolus confirmed this week that both species appear to be resistant to around four times glyphosate's labeled use rate. The giant ragweed was discovered in McLeod County in a 40-acre field and the common waterhemp was found in Renville County.

These discoveries add Minnesota to growing lists of states where glyphosate resistance has been discovered, though combined with relatively low levels, this latest discovery doesn't mean the end is nigh for glyphosate. In addition to Minnesota, glyphosate-resistant giant ragweed has been found in Arkansas, Tennessee, Ohio, Indiana and Kansas. Resistant common waterhemp has been confirmed in Missouri, Illinois, Kansas and Minnesota, according to Gunsolus.

"If the pattern of continuous reliance on glyphosate for weed control continues, it is likely that glyphosate resistance will follow the pattern of many other cases of herbicide resistance and increase at an exponential rate -- slow at first followed by an increase in frequency," he says. "However, since the levels of glyphosate resistance are fairly low (four times) and it has taken approximately eight to 10 years for selection of resistant weeds to occur, I would not assume that the battle is lost."

That's not to say resistance isn't a concern. It's quite the opposite, according to Marysville, Ohio-based certified crop adviser (CCA) Travis Rowe. The CCA with Champaign Landmark, Inc., says his farmer-customers are showing more attention to the issue and making adjustments to their operations to help prevent resistance issues.

Gunsolus advises considering these points, in planning your weed management program, to avoid glyphosate resistance:

  • Utilize other modes of action through use of a preemergence herbicide or a tank-mix partner.
  • Consider alternating Roundup Ready crops with LibertyLink technology or a conventional hybrid or variety.
  • As the growing season moves into high gear don't forget to scout your fields approximately 10 to 14 days after your first glyphosate application to detect weed escapes.
  • If weeds survived, try to determine why. Was the failure caused by misapplication, poor weather, poor timing, or later weed flushes? Get on these problems early while there is still some time in the current growing season to address them.
  • Keep in mind that many postemergence herbicides have crop size restrictions.
  • If your glyphosate applications have failed to control the same weed species, in the same area of the field for several years you may have a resistant weed problem. The sooner you act on a potential problem, the better.
  • Other weed species that are difficult to consistently control in glyphosate dominated cropping systems are: Common lambsquarters, common ragweed and in no-till fields -- horseweed.

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