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Got Weeds? Evaluate Your Weed Control Program

By Lizabeth Stahl, Extension Educator in Crops and Jeff Gunsolus, Extension Agronomist, Weed Science

By the end of the growing season, it is not too hard to spot soybean fields where weed control was less than optimal.  Prior to harvest, waterhemp can be found towering over soybean canopies throughout Minnesota.  Taking some time to evaluate effectiveness of your weed control program now can help enhance future weed control and ultimately protect yield potential and enhance profitability in the long run. 

If your weed control was less than optimal this year, questions to ask yourself include:
  • Did I use a preemergence (PRE) herbicide?  PRE herbicides can greatly enhance control of weeds like waterhemp that emerge over a long period of time.  Use of PRE herbicide helps provide a wider window in which to make a timely postemergence application because early-emerging weeds are controlled.  Use of a PRE herbicide can also increase the diversity of chemistries used - using multiple sites of action is a key, long-term strategy to combat weed resistance.  Be sure, however, to use a PRE herbicide that is rated highly in control of the problem weeds in your field.  

  • How large were weeds at the time of application?  When glyphosate first came out,
    applicators and growers were often able to control waterhemp much taller than the maximum weed height listed on the herbicide label.  Consider those days over.  Repeated use of glyphosate over time has selected for weed populations that are tougher to control or even resistant to glyphosate.  This is not an issue unique to glyphosate, it has occurred with repeated use of other chemistries over time as well.  As a general guideline, smaller weeds are easier to control.  Target two to three inch weeds at application.  Using a PRE herbicide (see #1 above) helps make this goal easier. 

  • What herbicide(s) did I use postemergence?  Although not all weed escapes are due to resistant issues, resistance issues appear to be expanding throughout the region.  Waterhemp is a prolific seed producer with the potential to produce over 500,000 seeds per plant and up to 40% of seeds shed in a year can emerge the following year.  Considering these biological factors, one resistant plant can lead to an out-of-hand population within three years if the same herbicide continues to be applied.  Diversify the sites of action used postemergence, and be sure to select products rated highly in control of your problem weeds.  For example, Liberty herbicide used on Liberty Link corn or soybeans will control small waterhemp and provides a site of action different from glyphosate in the Roundup Ready system.

  • What rate of herbicide did I use?  Full-labeled rates are recommended, from an herbicide resistance management perspective, as well as to help provide maximum control. 

  • Would non-herbicide tactics help my weed control?  Timely cultivation can help enhance weed control and help in the management of resistant weeds.  Hand pulling may be a viable option too, particularly if weed escapes are in patches. 

Taking the time to evaluate the effectiveness of your weed management program this year can help you make decisions to improve weed control next year and beyond.  Note which fields had significant weed escapes this year and thus the highest contributions to the weed seedbank.  Target these fields first when making herbicide applications, and utilize a diversified weed management strategy.  For resources to aid in weed management decisions, check out the U of MN Extension Crops website at

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