Some areas in Kansas have received just enough rainfall to have rather large broadleaf and grassy weeds actively growing in harvested wheat stubble at this time. These weeds are utilizing 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 usually begin flowering in late July and early August, and set seed shortly after that. Controlling kochia and Russian thistle now is very 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 is especially 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 is especially 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 may decide they can just wait and control any weed and grass seed that form now and emerge next season with a postemergence application of glyphosate in the corn or soybeans. However, with the concerns over the development of glyphosate-resistant weeds, it would be far better to control these weeds and grasses 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 is also a good idea to add 2,4-D or dicamba (unless there is cotton in the area) to the glyphosate. Do not apply the growth regulator herbicides around cotton. 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 that glyphosate-tolerant weed populations will develop.
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/winter annuals. An application of atrazine needs to be made in the fall (early October into November), depending on the weeds being targeted. Also, keep in mind that atrazine antagonizes glyphosate. Do not apply atrazine with reduced rates of glyphosate. Atrazine can be synergistic with Gramoxone; however, the spectrum of weeds controlled with this combination will be less than with glyphosate. An application of atrazine may limit subsequent crop selection.