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Issue No. 13, Article 6/July 2, 2010

Screening Waterhemp for Glyphosate Resistance

Waterhemp populations resistant to glyphosate will become increasingly common across Illinois. We suspect that resistant biotypes can be found in many areas across Illinois, but we would like to improve our understanding of just how widespread they are at present.

Postemergence applications of glyphosate to soybean fields are occurring at a rapid pace, given the recent good weather. We anticipate that glyphosate-resistant waterhemp plants will become noticeable about 7 to 10 days after application. We invite Illinois producers to send us waterhemp samples for herbicide-resistance testing. If you have a population that you suspect is resistant to glyphosate, we will test any samples you send us for resistance not only to glyphosate, but also to the PPO and ALS inhibitors.

Follow these directions for sampling:

  1. Select five waterhemp survivors in the field.
  2. Remove the top inch or two from each plant (with young, newly emerged, healthy leaves) and seal it inside in a sandwich-sized plastic zipper bag. Use a separate bag for each plant.
  3. Package the bags in an envelope and send by overnight delivery to Chance Riggins; 320 ERML; 1201 W. Gregory Dr.; Urbana IL 61801. Ideally, samples should be sent the same day they are collected, but if necessary they can be stored for a day or two in a refrigerator (but do not freeze them).
  4. Include with the samples your contact information, any details about the herbicide history in the field, and the field location (GPS coordinates if possible; at minimum, indicate in which county the field is located).

With the recent delays in applying postemergence herbicides, many large waterhemp plants (>12 inches tall) will likely be difficult to control. Please keep in mind the following points that might lead you to suspect glyphosate resistance in a waterhemp population:

  • The appropriate rate of glyphosate (plus proper adjuvants) was applied at the appropriate weed growth stage.
  • Environmental conditions during and after application were conducive for good glyphosate activity.
  • Plants that survived the glyphosate application are found next to plants that were controlled.
  • The field has a history of glyphosate use.

We will not charge you for the testing, but please understand that we can't promise when results will be provided. Also, because of the way we conduct the resistance tests, a result of "sensitive" does not rule out the possibility that the plant actually is resistant, but by a mechanism different from what we are testing for. If you have any questions, contact Pat Tranel (217-333-1531; tranel@illinois.edu).--Pat Tranel and Aaron Hager

Authors:
Aaron Hager
Pat Tranel

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