Control Mustards in Your Winter Wheat

Look carefully, and you may notice tiny mustard weeds in your winter wheat fields. Now is the time to take care of them, says Dallas Peterson, weed specialist at Kansas State University's Department of Agronomy. 

Many farmers don't think about controlling mustard species until spring -- when they really begin to take off. "It is still possible to get some control at that time with herbicides, but mustards are much more difficult to control at that stage and often have already reduced wheat yields by then," Peterson says.

To keep yield losses to a minimum, mustards should be controlled by late winter or very early spring, before the plants begin to bolt or stems elongate.

If winter annual broadleaf weeds are present in the fall, they can be controlled with any number of ALS-inhibiting herbicides, including Ally, Amber, Finesse, Affinity, Rave, Olympus, or PowerFlex. Huskie, 2,4-D, and MCPA can also provide good control of most mustards if the weeds are at the right stage of growth and actively growing, and if the wheat is at the correct growth stage. Dicamba and Starane are not very effective for mustard control.

Here are some control tips, based on the species you are after:

• Blue mustard: Pehaps the most difficult winter annual broadleaf weed to control because it bolts very early; herbicides should be applied in late February or early March. Blue mustard is more difficult to control than tansy mustard with 2,4-D because blue mustard has often already bolted by the time 2,4-D can be safely applied to wheat. Thus, 2,4-D often is applied too late to be effective on blue mustard.
• Flixweed and tansy mustard: Treat before they reach 2 to 3 inches across and 2 to 3 inches tall; control thereafter decreases dramatically. Ester formulations of 2,4-D and MCPA are more effective than amine formulations. 
• Field pennycress: Easier to control than other species, herbicide applications made before the pennycress bolts are usually effective. Wheat should be fully tillered before applying 2,4-D, or tillering will be inhibited and wheat yields may be decreased.

Most ALS-inhibiting herbicides control winter annual mustards very well, although there are populations of treacle mustard and flixweed that are ALS-resistant and cannot be controlled by these products.

Alternative control measures will be needed to control these populations. The best approach is to use other herbicides such as 2,4-D, MCPA, or Huskie as an alternative or in a tank-mix with the ALS herbicides. MCPA can be applied after the wheat is in the three-leaf stage; but 2,4-D should not be applied until after wheat is fully tillered -- which often doesn’t occur until spring. Huskie can be applied between the 1-leaf and flag leaf stage of growth. None of these herbicides has much residual control, so the majority of weeds need to be emerged and actively growing at the time of treatment.

Some producers commonly apply ALS herbicides with fertilizer in January or February. Unfortunately, MCPA, 2,4-D, and Huskie are most effective when applied to actively growing weeds, so application when weeds are dormant may not provide good control.  As a result, if an ALS-inhibitor tank-mix with one of these herbicides is applied to dormant ALS-resistant mustards in the winter, poor control can be expected.

ALS-resistant bushy wallflower seems to be present in a number of fields in central Kansas. ALS-resistant flixweed has only been confirmed in the Saline county area, but may start to show up elsewhere. Producers should watch for cases of poor control, and consider alternative herbicides or herbicide tank-mixes to help prevent or manage ALS-resistant weeds. 

Crop rotation with corn, grain sorghum, soybeans, cotton, or sunflowers is a good way of controlling the mustards as long as they are controlled in the spring prior to producing seed. Crop rotation will usually result in a gradual reduction of mustard populations in the future as the seedbank in the soil gradually decreases.