Herbicide resistance is already a common issue in southern Illinois, one which presses northward each year. Without careful management, it’s only a matter time before resistance problems break out across the entire state and rob even more fields of yields.

In the third episode of “Management Matters,” Mark Bernards, assistant professor of agronomy, crop science and weed control at Western Illinois University, gives tips on how to recognize, control and prevent herbicide resistance. He also discusses four herbicide-resistant weeds that should be of greatest concern to Illinois soybean farmers.


Bernards’ Forecast

  • Herbicide resistance is already common in southern Illinois—so much so that there is a “zero tolerance” policy in place.
  • Central Illinois farmers can expect more resistance in the next two years and northern Illinois farmers a year or two following.

Resistant Weeds to Watch

The following weeds have already shown resistance to multiple modes of action and are apt to develop more:

  • Common waterhemp – especially in western parts of the state
  • Palmer amaranth – primarily in southern Illinois but spreading rapidly
  • Marestail (horseweed) – has a particular resistance to ALS inhibitors
  • Giant ragweed – while not yet common in Illinois, surrounding states have issues

Recognize, Prevent, Control

  • Investigate further if one weed survives following a spraying, or if previously easy-to-control weeds survive and worsen.
  • Since weeds can spread via various sources (wind, animals, neighbors, contaminated seed sources), prevention must include diverse approaches:
    • Vary herbicides and use two or three herbicide modes of action
    • Rotate crops
    • Consider cover crops
    • Be vigilant, walk fields and do not allow weeds to go to seed
  • Consult the United Soybean Board’s Take Action resources, extension specialists and local agronomists for assistance.
  • Illinois growers can also use the checkoff-funded WeedMap Tool Kit to see confirmed herbicide-resistant weeds by county and resistant sites of action.

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