Kochia control in corn and grain sorghum

Producers should start this winter in planning their program for controlling kochia in corn and grain sorghum. The spread of glyphosate-resistant kochia populations throughout western Kansas, and the difficultly growers have had controlling these populations, suggest that perhaps control measure should begin prior to emergence of kochia.

Huge flushes of kochia emerge in late March and into April. Applying 16 oz/acre of Clarity/Banvel/generic dicamba with a pound of atrazine around mid-March, before the kochia has emerged, could provide significant control of early flushes of kochia. The addition of 2,4-D will help control winter annual mustards that have emerged. This will make subsequent kochia control measures discussed in this article more effective. When treating kochia with burndown or in-crop herbicides, spray early when the plants are about 1 to 3 inches tall. 

Control in corn

Kochia control in corn should always include a burndown application in April, shortly after the first kochia has emerged. A combination of glyphosate and dicamba will control small kochia, and almost all other existing weeds and grasses, at that time. If producers wait until later so they can apply the burndown and preemergence herbicide in the same application, the kochia will be larger and they may not get complete kochia control. If that occurs, the surviving plants will go on to cause problems throughout the growing season. Tillage prior to planting in a conventional till system can also be very effective.

The label for Clarity states that no more than 32 oz/acre can be applied per season. If 8-16 oz/acre is applied in March as an early preplant and 8 oz/acre is applied as a burndown ahead of corn or sorghum planting that still allows for an 8 oz application in-crop, which is often more than what is used in-crop.

After the early April burndown treatment, the next step would be to use a preemergence herbicide. Atrazine, or a product containing atrazine, should be included with this application. Even if there are triazine-resistant populations of kochia present, atrazine will still help control a number of other weed species.

Good options to include in a preemergence application for control of kochia (and other weeds) include:

* A chloroacetamide/atrazine premix. Examples of chloroacetamide-atrazine premixes include Bicep II Magnum, Cinch ATZ, Guardsman Max, Propel ATZ, Bullet, Harness Xtra, Keystone, Volley ATZ, FulTime, and others. New options in this class of herbicides that could be used include Zidua and Anthem plus atrazine or Anthem ATZ, which has atrazine included. The active ingredient, pyroxasulfone, in these herbicides has the best activity on kochia of the choroacetamides. If triazine-resistant kochia is present, then one of the other options below would be the better choice.

* An HPPD herbicide. Examples of HPPD herbicides include Lexar EZ or Lumax EZ (premixes of Callisto, Dual II Magnum, and atrazine), Corvus (a premix of Balance Flexx and thiencarbazone methyl), and Balance Flexx. Corvus and Balance Flexx should be mixed with atrazine.

* Verdict, which is a Kixor-powered combination of Sharpen and Outlook herbicides, has activity; however, the residual may be a little too short. Verdict should be mixed with atrazine.

Balance Flexx, and Corvus cannot be applied on coarse-textured soils with shallow (25 feet or less) groundwater. Always consult the labels for details.

If kochia becomes a problem after the corn has emerged, there are several postemergence herbicide options. In Roundup Ready corn, glyphosate should be used even though resistant populations of kochia may be present. It is also a good idea to add one or more herbicides with a different mode of action to the glyphosate. This will not only help control any glyphosate-resistant populations present, but will also help prevent the development of glyphosate-resistant populations of kochia where such populations do not yet exist.

Possible glyphosate tankmix partners would include Status, Impact/Armezon, Callisto, Laudis, Starane, Starane NXT, or Starane Ultra. If an HPPD-containing herbicide was used in the preemergence application (Lexar, Lumax, Corvus, or Balance Flexx), it would be a good idea not to use this mode of action in the postemergence treatment to help reduce the chances of HPPD-resistant weeds developing. Another option in Roundup Ready corn is Halex GT plus atrazine. Halex GT is a premix consisting of a high rate of glyphosate, Dual II Magnum, and Callisto. Atrazine should be added to this product to get the best season-long control of kochia. With Impact, Callisto, Capreno, and Laudis, producers should include a half-pound of atrazine.

In conventional corn, any of those postemergence herbicides mentioned above as tankmix partners with glyphosate can also be used alone, without the glyphosate tankmix partner. Halex GT cannot be used on conventional corn since it contains glyphosate. 

Liberty can also be used as a postemergence treatment if the corn is Liberty Link. Liberty alone will not control kochia, however. For kochia control, Liberty should be tankmixed with Status or other more effective postemergence products.

It should be noted that Balance Flexx and Corvus can be applied either preemergence or postemergence up through the 2-leaf stage of corn. If applied postemergence to corn, Balance Flexx and Corvus must be applied with atrazine only. No glyphosate or other adjuvants can be used. These products can do an excellent job of controlling kochia throughout the season if they are tankmixed with at least 1 lb/acre of atrazine. These products require moisture for soil activation, however they do have foliar activity.

Lumax EZ and Lexar EZ, which are best used as preemergence treatments, can also be applied early postemergence up to 12-inch corn when weeds are very small. Although waiting until this stage before application may work for controlling kochia, it is risky. Also, to get adequate grass control, these products must be applied preemergence to the grass.

Control in grain sorghum

There are fewer herbicide options for controlling kochia in grain sorghum than in corn, although there is a wider window available for sorghum than corn to make burndown applications prior to planting. Grain sorghum is planted later than corn, allowing more flushes of early-emerged kochia to be controlled with burndown treatments. Effectiveness of control during this time period is essential as in-crop options become limited.

This later planting of sorghum relative to corn requires producers to make two burndown applications of glyphosate-plus-dicamba before planting. This will control the largest two flushes of kochia emergence of the season. Producers who take advantage of this opportunity often have very good kochia control, although glyphosate-resistant kochia could complicate the issue. Producers who try to cut corners and do not control the early flushes of kochia when they have a chance often have problems with kochia in their sorghum later in the season.

To get the best control of kochia with the burndown treatments of glyphosate and dicamba, the kochia should be sprayed when plants are 2 to 4 inches tall and actively growing. Kochia plants one  inch tall or less that have not started to elongate and plants taller than six inches often are more difficult to control, especially under conditions of environmental stress.

If a flush of kochia emerges close to the time of grain sorghum planting, producers could combine a burndown treatment with a preemergence herbicide such as a chloroacetamide/atrazine premix, Lexar EZ, or Lumax EZ. Another option for burndown and early season residual control of kochia prior to emergence of sorghum would be Sharpen or Verdict. Sharpen provides no grass control. Methylated seed oil should be added to Sharpen for optimal burndown activity. Sharpen can be used at the 2 oz rate in sorghum. Verdict would provide some residual grass control. However, at the 10-oz rate, it should be combined with G-Max Lite or Guardsman Max for improved residual weed control. 

If a flush of kochia reaches 4 to 6 inches in height and grain sorghum planting is still a week or more away, producers should strongly consider making a burndown treatment before sorghum planting while the kochia is controllable, and then making the preemergence application as a separate treatment.

If a postemergence application becomes necessary in grain sorghum, good options include Huskie+atrazine+AMS, dicamba+atrazine or Starane-containing products. If Starane is used, it would be best to add a half-pound of atrazine.