Mustard plants such as field pennycress, bushy wallflower, flixweed, tansy mustard, and blue mustard in wheat fields often are not noticed until the mustards start to bloom in the spring. As a result, farmers often do not think about control until that time. Although it is possible to get some control with spring herbicide applications, mustards are much more difficult to control once they have flowered and often have already reduced wheat yields by then.
To minimize yield losses, mustards should be controlled by late winter or very early spring before the stems begin to elongate or bolt. If mustards are present in the fall, they can be controlled by various active ingredients. You can look for products containing Group 2 ALS-inhibiting herbicides.
Most ALS-inhibiting herbicides control winter annual mustards very well. However, there are populations of bushy wall flower (treacle mustard) and flixweed in Kansas that are ALS-resistant and cannot be controlled by these products. Alternative herbicides will be needed to control these populations. Also, be aware that some of the Group 2 herbicides have long rotation intervals to other crops.
Group 27 herbicides can control ALS-resistant mustards. Other options to control ALS-resistant mustards are Group 4 herbicides.
Group 27 and Group 4 herbicides have little to no residual activity, thus will only control weeds that have emerged and are actively growing. Applying them with fertilizer in January or February when weeds are dormant will not provide good mustard control. In addition, special care should be taken to ensure wheat is fully tillered when 2,4-D is applied to avoid reducing tillering.
In the late 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 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 is often applied too late to be effective on blue mustard.
Flixweed and tansy mustard should be treated when they are no larger than two to three inches across and two to three inches tall. As these plants become larger, 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.
Crop rotation with corn, grain sorghum, soybeans, or sunflowers is a good way of managing mustards as long as they are controlled in the spring before producing seeds. Crop rotation will usually result in a gradual reduction of mustard populations in the future as the seedbank in the soil decreases.
Always consult the herbicide label for the most current use requirements.
Stacy Campbell is a Crop Production Extension agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension.