The main winter annual broadleaf weeds in wheat in Kansas are the mustard species and henbit. These species germinate in the fall or early winter, overwinter, then bolt in the spring.
With a couple of exceptions, the winter annual broadleaf weeds can be controlled relatively easily with the array of herbicide options available for wheat. The exceptions are:
- ALS-resistant populations of bushy wallflower (treacle mustard) and flixweed (tansy mustard)
- Late applications
ALS-resistant populations of bushy wallflower and flixweed
Bushy wallflower (treacle mustard) and flixweed (tansy mustard) populations resistant to ALS-inhibiting herbicides (such as Finesse, Glean, Ally, Express, Affinity, Amber, Olympus, PowerFlex or Beyond) have been confirmed in central Kansas in recent years. This class of herbicides has been around for many years and has historically provided excellent control of mustard species in general. A complete failure to control one of the mustard species with an ALS-inhibiting herbicide is probably a pretty good indication that an ALS-resistant population has developed.
ALS-resistant bushy wallflower was first confirmed in Marion county in 2005, and based on field reports, seems to be fairly common in Marion, Dickinson, and Saline counties, but also may be present elsewhere. It was highly resistant to all ALS-inhibiting herbicides evaluated.
ALS-resistant flixweed was first confirmed from Saline County in 2007. ALS-resistant flixweed still seems to be confined to that general area. Some ALS-inhibiting herbicides still had some activity on the resistant biotype, but history tells us that if we switched to another ALS-herbicide, it wouldn’t be long until we had resistance to those products as well.
ALS-resistant mustard populations likely developed in continuous wheat fields where either Glean or Finesse was routinely applied in combination with a topdress fertilizer application and in the absence of a tank-mix partner such as 2,4-D or MCPA.
Fortunately, the ALS-resistant mustards can still be controlled with a timely application of 2,4-D, MCPA, or the new herbicide Huskie. These products can be tank-mixed with ALS herbicides or used as an alternative to ALS herbicides. MCPA and Huskie can be safely applied to wheat in the fall through spring after the wheat has 2 leaves. However, 2,4-D should not be applied until wheat is fully tillered, which generally does not occur until sometime in spring.
These herbicides are active primarily through foliar uptake and have limited soil activity, so ideally they should be applied to plants with viable foliage and when temperatures are 50 degrees or higher to achieve optimum performance. If applying MCPA or 2,4-D with topdress nitrogen fertilizer, the ester formulation needs to be used as amine formulations are not compatible with liquid N fertilizers.
Starane or dicamba products generally do not provide very good mustard control, so wouldn’t be very good choices to help control ALS-resistant mustards.
Many producers like to wait to apply broadleaf herbicides until winter or early spring, for a variety of reasons. This normally works well for control of mustard species, but is less effective for henbit control. Winter annual broadleaves do not generally cause much yield loss if left uncontrolled in the fall. However, these weeds should be sprayed in early spring when they are actively growing, but before they begin to bolt.
Producers who wait too long before making a herbicide application can run into a problem if weather conditions such as high winds or wet weather prevent timely application. If the herbicide application is delayed too long, weeds can become too large to control effectively and yield loss may occur.