By Dallas Peterson, Weed Management Specialist
Mustards are a common broadleaf weed in wheat throughout Kansas. Unfortunately, growers often do not notice these weeds in their fields until they start to bloom in the spring. As a result, growers often don’t think about control until that time. Although it is still possible to get some control with herbicides at that time, mustards are much more difficult to control at that stage and often have already reduced wheat yields by then.
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.
In the later winter or early spring, blue mustard is perhaps the most difficult of the winter annual broadleaf weeds to control because it bolts very early. To be effective on blue mustard, herbicides typically need to be applied to 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 should be treated when they are no larger than 2-3 inches across and two to three inches tall. As these plants become larger, the control decreases dramatically. Ester formulations of 2,4-D and MCPA are more effective on tansy mustard and flixweed than amine formulations. Field pennycress is easier to control than tansy mustard or flixweed. 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 in Kansas now 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 3-leaf stage; but as mentioned above, 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 growers 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 could occur.
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. Growers 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 managing 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.