Source: Dallas Peterson, Weed Management Specialist and Curtis Thompson, Weed Management Specialist, Kansas State University Extension
As of mid-March, kochia was emerging in wheat and wheat stubble in western Kansas. Much of the wheat stands in that area are thin and the early growth is below average. As a result, it is possible for kochia to grow taller and to be even more competitive than usual this year, reduce yield, and pose a serious problem by harvest time if left uncontrolled.
Challenges to getting good control of kochia
It is not always easy to control kochia in a standing wheat crop, however. There are four big challenges to kochia control in wheat:
- There are many populations of kochia with resistance to either ALS-inhibitor herbicides, atrazine, or glyphosate. There may even be some populations resistant to dicamba.
- A majority of kochia emerges early in the spring, but some emergence can extend over a period of weeks or months. A herbicide applied early in the spring will need to have residual activity to be effective on later-emerging kochia. Several ALS-inhibitor herbicides have good residual activity, but are ineffective on ALS-resistant kochia.
- Dicamba, a non-ALS herbicide and one of the more effective products on most populations of kochia, must be applied before the jointing stage of wheat, meaning that later-emerging plants may not be controlled.
- Most other non-ALS herbicides that can be applied at a later growth stage of wheat are primarily contact herbicides that require thorough coverage to be effective, and this can be difficult to achieve when the wheat canopy gets larger and covers up some of the kochia present. Two exceptions are Huskie and Starane. Those two products can be applied at later growth stages of wheat, are translocated (Starane more so than Huskie), and are effective on kochia.
At one time, the most effective way to control kochia in the wheat crop was an application of a sulfonylurea herbicide with residual activity. This is still a good approach where the kochia populations present are susceptible to ALS-inhibitor herbicides. The advantage of using this type of herbicide is that several sulfonylureas have both foliar and residual weed control, and thus can provide good control of susceptible populations through wheat harvest. Also, the sulfonylurea herbicides provide a broad spectrum of broadleaf weeds. Tank-mixing a growth regulator herbicide with a sulfonylurea herbicide is generally recommended to add additional control and to help reduce the risk of developing ALS-resistant weeds.
If populations of kochia resistant to ALS-inhibitor herbicides are present in wheat, getting good control becomes a little more difficult.
One option is to apply dicamba, or a premix that includes dicamba, such as Rave (Amber + dicamba) or Pulsar (Starane + dicamba). These products have to be applied before the jointing stage of wheat, and thus may miss later-germinating kochia. Dicamba alone has some residual soil activity, but not as much as most sulfonylurea herbicides. Rave will have residual activity from the Amber, but since Amber is a sulfonylurea herbicide, it would not provide any residual control of kochia populations that are resistant to ALS-inhibitor herbicides. Both ingredients in Pulsar have limited residual activity.
Another option producers have is Starane, which is effective on kochia. Like dicamba, Starane is a growth regulator herbicide, but it can be applied up to the early boot stage of wheat. Starane also has limited residual activity, so good coverage is still important for control. Starane is weak on mustard control.
Huskie is also effective on kochia. It is a broad-spectrum herbicide effective on most broadleaf weeds in wheat, and can be applied up to the boot stage of wheat. Huskie also has limited residual activity, so producers will need to make sure kochia plants are thoroughly covered with Huskie to get the best control. Ideally, the Huskie should be timed for application after the majority of kochia has emerged, but before the wheat canopy has become thick.
Buctril and Aim can control kochia and can be applied at later stages of wheat development, but both are contact herbicides with little or no residual activity. Consequently, both products have the same challenges as Starane and Huskie in terms of getting good coverage. Getting thorough coverage is even more critical with Buctril and Aim, since they are true contact herbicides and not translocated in plants.
Control in wheat stubble after harvest
If kochia has not been completely controlled in the wheat crop, then it may be present at the time wheat is harvested. In most cases, the kochia plants will have grown taller than the wheat canopy and will get "topped" by the combine as the wheat is harvested.
If kochia has been topped, producers should wait until some regrowth has occurred before applying herbicides in the wheat stubble to control it. A combination of glyphosate plus either dicamba or Starane may be the most effective treatments to control kochia in wheat stubble. Even if kochia populations are resistant to glyphosate, the tank-mix combinations with dicamba or Starane will probably provide good control. Some 2,4-D can be added to the mixture to help with control of other broadleaf weeds, although 2,4-D generally will not help much in controlling kochia.
To improve the chances of getting good control after wheat harvest, apply the postharvest treatments in the morning hours or after the field has received some moisture, not when the kochia plants are under maximum stress. Use the highest labeled rate of glyphosate, and make sure to add ammonium sulfate and any necessary surfactants.