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7 Steps for Managing Resistant Weeds in Corn, Soybeans

Patches of uncontrolled waterhemp can take over entire fields in a year or two without an effective weed resistance management plan
<p> Patches of uncontrolled waterhemp can take over entire fields in a year or two without an effective weed resistance management plan.</p>

If there’s any good news in the herbicide-resistant weed situation in Iowa, it’s that most Iowa growers still have a little time to stay out in front of the issue...but maybe not as much time as you’d like.

Mike Owen, Iowa State University weed scientist, said in our January 24, 2013 On-Farm Advance that a weed management strategy to eliminate or prevent herbicide resistance is something you needed for your farm yesterday, not tomorrow. And he reminded us recently that glyphosate is just the latest herbicide weeds have become resistant to.

Atrazine-resistant kochia, lambsquarters, Pennsylvania smartweed and giant foxtail were documented in 1985, 1989, 1990 and 1992, respectively. Shattercane, cocklebur and wild sunflower with resistance to ALS herbicides have been documented in Iowa, starting in 1992. There’s a complete listing in Mark Storr’s 2013 On-Farm Network Conference presentation.

He laid out a five-step program for managing weed resistance, which includes:

  1. Identify the target weeds
  2. Include multiple, effective sites of action (SOA)
  3. Incorporate SOAs that are at low risk for developing resistance
  4. Layer residual herbicides both pre and post
  5. Don’t let weeds escape to produce seed

We'll add a couple more:

  1. Rotate crops to allow use of different modes (or sites) of action over a two-year period
  2.  Use cover crops to provide competition for late fall or early spring weeds

Whether you’re growing corn or soybeans, Storr recommends using herbicides that incorporate at least two different sites of action (modes of action), based on information compiled by University of Wisconsin herbicide specialists. This is not just a soybean problem. Canopy height differences may make it more apparent in soybeans than in corn, but once the problem develops, resistance becomes a problem for both crops.



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Iowa Soybean Association On-Farm Network replicated strip trials comparing different weed management programs since 2008 have not yet detected a yield advantage from switching away from a two-pass glyphosate herbicide program. But keep in mind that these trials have not been done on fields where there was a known, serious resistant-weed situation. We did see yield increases in 2012 trials that also used a fungicide, no matter which herbicide programs were being compared.

In the course of this work over the past five years, we have been able to document a few serious problems in soybean fields and a few weeds that appear to be resistant were found in others. Long-term crop and herbicide use data for individual fields could be helpful in anticipating where to expect resistant weeds.

In addition to the five-step (plus our two) program above, we’d urge you to participate in 2013 On-Farm Network trials involving herbicides/weed management systems. There are three trial projects we'd like to call to your attention. These are:

  1.  Monsanto high intensity – pre followed by post glyphosate and fungicide vs glyphosate on soybeans
  2.  Syngenta – post residual herbicide and fungicide vs glyphosate
  3.  Pioneer – pre herbicide vs glyphosate (protocol and details coming soon).

Information and protocols for the specific trials are available in the 2013 trials section of the On-Farm Network website. For more information, contact Tristan Mueller by email or by phone at 515-334-1075. 


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