Controlling Large Weeds, Do You Feel Lucky?

Controlling Large Weeds, Do You Feel Lucky?

Heavy rains in Iowa have made timely postemergence weed control difficult. What can you do to control large weeds?

The heavy rains across the state the past few weeks have made timely postemergence weed control difficult, even in fields treated with preemergence herbicides. This leads to questions as to what can be done to insure control of weeds larger than specified on herbicide labels. Bob Hartzler, Iowa State University Extension weed management specialist, offers the following answer.

MANAGING BIG WEEDS: The principles that apply to managing large waterhemp (shown here) in soybean fields can be applied to control other weeds that are larger than specified on herbicide labels.

"Keep in mind that if there was a way to improve the consistency of a herbicide on large weeds, the manufacturer probably would have included that practice on the label," he says. In the following article, Hartzler focuses on managing large waterhemp in soybean fields, but many of the principles are the same with other weeds, he notes.

Herbicide labels require application to small waterhemp
The occurrence of waterhemp resistant to Group 2 (ALS inhibitors) and Group 9 (glyphosate) herbicides greatly complicates waterhemp management. The remaining effective postemergence options for waterhemp are the Group 14 herbicides aciflourfen, fomesafen and lactofen.

These three herbicides are very similar in their performance on waterhemp when applied at equivalent rates. They are contact herbicides that require application to small waterhemp (maximum of six leaves) for consistent control. While application to weeds beyond the size restriction is not a violation of the label, the manufacturer is not responsible for performance.

Can mixing multiple herbicides improve performance?
One approach to enhance performance is to add multiple herbicides (herbicide cocktails, witches' brews, etc.) to the spray tank in the hope an additive or synergistic response will improve performance. Unfortunately, products registered for use in soybean (such as Cadet, 2,4-DB, any Group 2 herbicide) that can be added to a Group 14 herbicide have minimal activity on waterhemp, thus little or no benefit exists in using these mixes in terms of improving waterhemp control.


The addition of these products may, however, help control other weeds on which they have good activity, such as velvetleaf. Remember, a combination of multiple herbicides can increase the likelihood of significant crop injury compared to the products individually.

Your best bet is to improve coverage of the target
The tactic with the greatest likelihood of improving control of large weeds is to adjust the sprayer to improve coverage of the target weeds. Steps to consider include:

1) Higher spray volumes

2) Slower tractor speeds

3) Nozzles that produce a smaller range of droplet sizes and

4) Lower boom height

Another consideration is the impact late applications may have on the selection of new herbicide resistant biotypes. Large weeds that survive these applications have essentially been treated with a sub-lethal herbicide dose, the same process as when below-labeled rates are used. This has been shown to contribute to resistance evolution within weed populations.

Weigh cost of application and potential for crop injury
"Although many people have had success at killing weeds larger than specified on the label, the variability in herbicide performance increases rapidly with increasing weed size," notes Hartzler. "You need to weigh the cost of the application and the potential for crop injury against the likelihood of successfully controlling the weeds. It is never easy to concede to weed control failures, but in certain situations it is necessary to recognize that an effective chemical control option is not available."

For more weed management information, visit

TAGS: Extension
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.