Physical Removal May Be Only Option For Super Weeds

Once herbicide-resistant weeds grow past five inches, they become extremely difficult to control.

Published on: Jul 16, 2013

By Aaron Hager

The volume of inquiries about how to control large (taller than 12 inches) horseweed (a.k.a. marestail) and waterhemp in soybean has remained consistent over the past 10 days. 

The answer can be summarized as follows: there are no post-emergence herbicides that will consistently control these very large weeds in soybean, especially if these weeds are resistant to glyphosate.

Glyphosate-resistant waterhemp can be controlled by foliar-applied PPO inhibitors (such as lactofen (Cobra), fomesafen (Flexstar) or acifluorfen (Ultra Blazer)) in conventional or glyphosate-resistant soybean varieties, or by glufosinate (Liberty) in glufosinate-resistant (Liberty Link) soybean varieties. 

Physical Removal May Be Only Option For Large, Herbicide-Resistant Super Weeds
Physical Removal May Be Only Option For Large, Herbicide-Resistant Super Weeds

However, it is very important to remember that these herbicides do not extensively translocate within the weed following their absorption through the leaf surface, and control of large weeds is often not as consistent as control of small (5 inches or less) weeds.  Reducing application rates of these herbicides often results in reduced waterhemp control. 

In our research, we have observed less control of glyphosate-resistant waterhemp with tankmixes of glyphosate and fluthiacet (Cadet) or 2,4-DB compared with tankmixes of other PPO-inhibiting herbicides.  Keep in mind that PPO-resistant waterhemp biotypes are very common, and these biotypes are sometimes also resistant to glyphosate. 

The only effective methods to control waterhemp plants resistant to PPO inhibitors and glyphosate include physically (i.e., weed hook or hoe) or mechanically (i.e. cultivate) removing the plants from the field.

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Double-crop soybean tips
The following text, written by Dr. Mark Loux, extension weed scientist at the Ohio State University, provides excellent information about managing marestail in double-crop soybean.

A weed free start is the most critical aspect of a weed management program for double-crop soybeans.  This can be challenging to achieve where glyphosate-resistant marestail are present after wheat harvest. 

With regard to the control of weeds that can emerge after double-crop soybean planting, and the entire weed control system, the following approaches can be considered.

1.  Plant any type of soybean, and include a residual herbicide with the burndown treatment so that POST herbicides are not needed.  A good strategy in Roundup Ready or nonGMO soybeans even where POST treatment is needed, since POST marestail control might be impossible in these systems.  Residual herbicides used at this time of the year should be restricted to those that have little or no carryover risk – such as metribuzin, Valor, or low rates of chlorimuron or cloransulam products.

2.  Plant a LibertyLink soybean, and apply Liberty POST as needed.  Probably the best option for control of later-emerging marestail or plants that regrow after the burndown, assuming that there is any Liberty available.

3.  Plant a Roundup Ready soybean and apply glyphosate POST.  Should work for most weeds, but not a good choice if the POST application needs to control marestail.

4.  Plant a nonGMO soybean and apply conventional POST herbicides (Flexstar, Fusion, Select, etc) as needed.  This system has the most potential for soybean injury, but seed may be cheaper than the other systems.  Not a good choice if the POST application needs to control marestail.

Aaron Hager is a weed specialist with the University of Illinois. This article originally appeared in The Bulletin. For the complete version, click here.