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Get control of weeds in wheat stubble

Published on -7/20/2014, 11:19 AM

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Due to the abundance of rainfall in our area and across the entire state, many fields of wheat stubble in Kansas have rather large broadleaf and grassy weeds actively growing at this time. These weeds are using moisture and nutrients that would be available for a subsequent crop. It is a good idea to control these weeds before they set seed.

Kochia and Russian thistle are day-length sensitive and will begin to flower toward the end of July and into August, thus will need to be controlled shortly. Controlling kochia and Russian thistle now is important to prevent seed production.

Weeds growing now in wheat stubble fields, without crop competition, set ample seed -- which will be likely to cause a problem in following crops. It especially is important to prevent seed production from happening on fields that will be planted to crops with limited options for weed control, such as grain sorghum, sunflower or annual forages. It especially is difficult to control broadleaf weeds in sunflower and grassy weeds in sorghum that emerge after crop emergence. Preventing weed seed production ahead of these crops is essential. Seed of some weed species can remain viable for several years, so allowing weeds to produce seed can create weed problems for multiple years.

If the field will be planted to Roundup Ready corn or soybeans, producers might decide they can just wait and control any weed and grass seed that form now and emerge next season with a post-emergence application of glyphosate in the corn or soybeans. However, with the concerns about the development of glyphosate-resistant weeds, kochia, Palmer amaranth and waterhemp, it would be far better to control these weeds now in wheat stubble. That way, other herbicides with a different mode of action can be tank-mixed with glyphosate to ensure adequate control.

Producers should control weeds in wheat stubble fields by applying the full-labeled rate of glyphosate with the proper rate of ammonium sulfate additive. As mentioned, it also is a good idea to add 2,4-D or dicamba to the glyphosate. Tank mixes of glyphosate and either 2,4-D or dicamba will help control weeds that are difficult to control with glyphosate alone, and will help reduce the chances of developing glyphosate-tolerant weed populations.

Often dicamba or 2,4-D tank mixes with glyphosate might not perform well under the drier conditions of western Kansas, especially on kochia and Russian thistle; however, this year with the improved moisture conditions, we might find glyphosate tank mixes will work well. If drought and heat stress set in, however, using Gramoxone with atrazine (atrazine is synergistic with Gramoxone) has been a more effective treatment than glyphosate/dicamba or glyphosate/2,4-D.

Several have asked about the addition of atrazine for residual weed control in fallow. Although atrazine provides residual control of weeds, it is best applied later in the fall. At this time of year, atrazine residual is quite short and will not provide adequate control of fall emerged weeds or winter annuals. An application of atrazine needs to be made in the fall (mid-October through November), depending on the weeds being targeted. Also, keep in mind atrazine antagonizes glyphosate -- just the opposite of the synergistic effect of atrazine and Gramoxone. Do not apply atrazine with reduced rates of glyphosate.

A good reference for weed control in Kansas is the 2014 Chemical Weed Control for Field Crops, Pastures, Rangeland and Non-cropland book printed by K-State Research and Extension, which can be accessed at www.ksre.ksu.edu/bookstore/pubs/SRP1099.pdf or stop in your local K-State County Extension office and grab a copy.

* Information provided by Curtis Thompson, Extension agronomy weed management specialist.

Stacy Campbell is agriculture Extension agent in Ellis County.

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