Improve Corn Herbicide Programs through Effective Weed Scouting

By: Mark Waddington, Bayer, Selective Herbicides Product Development Manager
resistance is changing effective weed control and scouting message with field in background

The rise of herbicide resistance has prompted a renaissance in weed scouting. More than just walking the fields, scouting has evolved into a yearlong routine.

Whether you've just finished scouting or you’re about to head out again, knowing the weed pressures you face this year can better inform your decisions next year. Here's what I recommend to scout in a way that effectively boosts your corn herbicide program.

What to Watch for When Scouting Weeds

Weed scouting doesn't stop once a weed is spotted. The best weed-scouting programs include keeping a careful eye on whether weeds are developing resistance to chemistries, recognizing if weed shifts occur in the field and identifying weeds at various stages.

There are several signs to look for when checking your fields after spraying that can help determine whether or not weeds are becoming resistant:1,2

  • Check the other weed species listed on the product label. Are these other weed species being effectively controlled with the same herbicide or herbicide group, while one species is not? Multiple weed species are unlikely to develop resistance at the same time.
  • Herbicide failure may be present in some parts of the field and not others with no other reasonable explanation. This could look like a patch of one species of weed that is uncontrolled and spreading.
  • Check your field records to see if the same herbicide or herbicide group was used on the field year after year. Note any reduction in effectiveness on a particular weed species that may have happened in the previous year.
    • If there have been previous issues to control the same species with the same herbicide or herbicide group, resistance may be developing.
  • Some healthy weeds may be mixed with other, effectively controlled weeds of the same species.

If you suspect weed resistance, collect any plants and weed seed samples and send to a lab for testing and confirmation.

When scouting, you may see that certain weeds appear at different times of the year, depending on the growing degree days (GDD) they require for growth. Because of this, it's imperative to continue scouting throughout the growing season. The National Corn Growers Association (NCGA) offers a helpful reference page for identifying how and why specific weeds show up at different times of the year.


  • Did you know, for example, that giant ragweed needs only 150 GDD to emerge, and it typically shows up before you’ve turned any wheels for spring planting?4
  • Waterhemp, on the other hand, likely won’t show up until after the corn emerges, as those plants typically need more than 350 GDDs.
  • Giant foxtail falls somewhere in between – typically 150 to 300 GDDs.5

Monitor Your Fields to Develop an Effective Weed Control Program

As you scout fields this summer, it’s important to note which weeds are appearing when. Keep that in mind as you consider your weed control program year-to-year.

For example, if your field has early emerging weeds, such as lambsquarters, Corvus® herbicide is a valuable tool to control the weeds and help get your field off to a clean start. With two sites of action (Groups 2, 27), Corvus is labeled to control more than 65 grass and broadleaf weeds.

If a weed like the foxtail species shows up after your corn has emerged, consider a postemergence application of a product like DiFlexx® DUO herbicide. With two sites of action (Groups 4, 27), DiFlexx DUO provides powerful postemergence of a broad range of the toughest weeds, including glyphosate-resistant weeds. 

Before applying any herbicide, please read the entire label for best possible results and to confirm that the product is effective against the weed you need to control. Not every product is suitable for every situation, and correct application technique will help ensure the best results.

It’s never too early to develop your scouting plan. It is surprisingly easy to forget details from one year to the next, so you should document your scouting year-to-year. But most importantly, remember to use the resources at your disposal to scout correctly and make well-informed decisions in your weed control program for years to come.

Weed Identification Resources for Your State

For more weed identification, scouting and control recommendations specific to your location, most states provide weed management guides. (Note: Most are free online, but some do have an associated fee.)

Read More about Managing Weeds in Corn

©2019 Bayer Group. Always follow grain marketing and all other stewardship practices and pesticide label instructions. Bayer, the Bayer Cross, Corvus and DiFlexx are registered trademarks of Bayer Group. Corvus is a Restricted Use Pesticide. Not all products are registered in all states and may be subject to use restrictions. The distribution, sale, or use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. For additional product information, please call toll-free 1-866-99-BAYER (1-866-992-2937) or visit our website at Bayer CropScience LP, 800 North Lindbergh Boulevard, St. Louis, MO 63167.

Works Cited

  1. Alberta Agriculture and Forestry, and Policy and Extension Services Branch. “Herbicide Resistance in Weeds - Frequently Asked Questions.” Alberta Agriculture and Forestry, 2 May 2014,$Department/deptdocs.nsf/all/faq14845.
  2. “Herbicide-Resistant Weeds and Their Management.” Composting with Worms | OSU Extension Catalog | Oregon State University, Extension & Experiment Station Communications, 1 Apr. 2011,
  3. Kendig, Andrew, and Fred Fishel. “Herbicide Resistance in Weeds.” University of Missouri Extension,
  4. “Giant Ragweed.” Native Plants and Ecosystem Services, Michigan State University | College of Agriculture & Natural Resources,
  5. “Giant Foxtail.” Native Plants and Ecosystem Services, Michigan State University | College of Agriculture & Natural Resources,
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