ISSUE 12   July 20, 2006


If glyphosate resistant horseweed isnít bad enough a paraquat-resistant horseweed was identified in the Mid-Atlantic States by university weed scientists. In 2003, a small percentage of horseweed plants survived sequential paraquat applications in a commercial field. This is the first documented case of paraquat-resistant horseweed in the Mid-Atlantic states and the second reported case in the United States for this species.

University of Wisconsin Extension Weed Specialist, Chris Boerboom conducted a survey and found that, in soybeans, 63% of the people said that common lambsquarters was their most troublesome weed and that common lambsquarters was ranked at the top from survey respondents across the state.

I suspect that since Ohio, Wisconsin other mid-west and plains states are observing common lambsquarters escaping through glyphosate and have reported possible resistance that we might have similar populations in ND. Common lambsquarters is one of the top 10 weeds in ND and glyphosate use has been high with application in no-till, preplant, preharvest, and in-crop use in RR corn, soybean, and canola. Growers and consultants at winter meeting and in phone calls have reported this weed escaping glyphosate. This summer I have observed common lambsquarters escaping full rates of glyphosate in my research studies and in some growers field. The suspicious thing is that there are several desiccated lambsquarters carcasses all around one uneffected or slightly affected 6 or 8 inch common lambsquarters plant.



We are aware of many application or environmental reasons why common lambsquarters is not consistently controlled with glyphosate. However, we have continuing concerns about the potential for common lambsquarters to become resistant to glyphosate in ND for many reasons that we have discussed previously. We are interested in determining if there are lambsquarters populations in ND that are starting to show resistance to glyphosate. As the Canadians have shown - "If you search hard enough you can find it" - we suspect that resistant gene may be found in ND.

If you have common lambsquarters that has escaped control of a glyphosate application that does not seem to have a logical explanation, please email Rich Zollinger at If the case is compelling, we would like to arrange to collect a sample with mature seed for further testing.

These plants would have survived at least one or two glyphosate applications at the full rate of 22 fl oz/A of the 4.5 lb ai/gal products or 1 qt/A of the 3 lb ai/gal products.

Consider the following points in making your assessment. First, rule out any environmental and application factors that can lead to glyphosate performance problems. Some factors that may reduce glyphosate performance include:

  • incorrect rate of glyphosate for weed size or species
  • rainfall prior to complete glyphosate absorption
  • weather-stressed weeds (drought, cold, etc.)
  • incomplete spray coverage of weeds below the canopy
  • reduced glyphosate activity with early morning or late evening applications
  • weeds emerging after the glyphosate application
  • If all of the following points exist, the potential for a true resistance case increases.



    There is not a public testing service at NDSU to conduct routine screening of weeds for herbicide resistance, such as testing wild oat for ACCase resistance or foxtail for ALS resistance. A screening service is available through Michigan State Universityís Diagnostic Services.

    Steven Gower, MSU, writes "Confirming herbicide-resistant weed populations is the first step of any resistance management program. Verification will provide producers with the knowledge to implement the best possible management strategies, with the ultimate goal of preventing or limiting the spread of herbicide-resistant weeds."

    If herbicide resistance is suspected in any weed species, samples may be submitted to MSU Diagnostic Services for a herbicide resistance screen test. In most circumstances, a whole plant pot assay established from seed will be our standard test for herbicide resistance confirmation. Mature, high quality seed or seedheads should be collected from suspicious plants in late summer or fall and submitted in a paper bag or envelope. Do not seal plants or seed in plastic!

    Fees associated with herbicide-resistant weed testing are $75 per sample per herbicide site of action (i.e. ACCase inhibitors, ALS inhibitors, Photosynthesis inhibitors). Each additional site of action is $30 per sample."

    Please contact Steven Gower (517-432-9693, with any questions regarding resistance confirmation or sample collection. Samples can be mailed to:

    Michigan State University
    Diagnostic Services
    101 Center for Integrated Plant Systems
    East Lansing, MI 48824-1311
    Attn: Steven Gower

    Richard Zollinger
    NDSU Extension Weed Specialist

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