Give Beans a Weed Break

February 21, 2010 06:00 PM

Pam Smith, Farm Journal Seeds & Production Editor
Sooo…your soybean fields used to be a weed free picture. Now, some 14 years after the introduction of Roundup Ready technology, you notice a few weeds are getting obstinate?
Mark Loux, weed scientist at The Ohio State University, says growers who have maintained effective management of Roundup Ready crops practice more effective management of glyphosate. "Some of the problems with ineffective control are likely due to resistance, and some are due to mismanagement of glyphosate—especially rate, timing, number of applications and failure to include other herbicides in an attempt to reduce costs.”
Instead of forecasting doom and gloom, he recommends that growers try to preserve the use of the glyphosate as a tool. He says Ohio growers that get more consistent control, reduce selection for glyphosate resistance and preserve soybean yield tend to:
  • Rotate soybeans with corn and/or wheat or another crop.
  • Use herbicides other than glyphosate for weed control when planting the rotational crop of corn or wheat.
  • Apply herbicides the previous fall and/or before planting in the spring to ensure that soybeans are planted into a relatively weed free seedbed.
  • Include 2,4-D ester in fall or spring preplant, in addition to glyphosate or other herbicides.
  • Use residual herbicides in fall or spring to reduce early-season weed populations, weed competition with the crop and reliance on glyphosate for control of summer annual weeds.
  • Apply the first postemergence glyphosate treatment when weeds are relatively small.
  • Use a higher glyphosate rate or include another postemergence herbicide where populations of tough weeds (such as giant ragweed) are present. A later than desirable application results in large weeds and/or there is a history of less adequate weed control.
  • Scout fields within 3 weeds of the initial postemergence glyphosate application and retreat with glyphosate where late-emerging weeds are observed or where weeds are not completely controlled with the first application.
Loux reminds growers that they have the option of using Ignite in a Liberty Link system to rotate trait systems. "This can be an effective tool to manage glyposate-resistant populations of giant and common ragweed and horseweed, especially those populations that are resistant to ALS inhibitors,” he says.
In a Liberty-Link system it's still recommended to follow guidelines with regard to a weed free start, use of residual herbicides and weed size for first and second post emergence applications.
Want to know what weeds are resistant in your state? Here's a list.

You can email Pam Smith at

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