With wheat harvest largely behind us, it is time to consider post-harvest weed control in wheat stubble if you haven’t already. Weeds present before harvest have recovered from cutting and are growing in the absence of crop competition. Any weeds that have germinated since harvest still have time left to cause problems. Weeds should be controlled as soon as possible to prevent soil moisture use and seed production. Controlling seed production to reduce weed populations next season is important when rotating to crops with limited options for in-crop weed control. From a disease standpoint, early control of volunteer wheat and grass weeds is important to break the green bridge and reduce spread of wheat streak mosaic virus into next year’s crop.
Kochia and Russian-thistle tend to be the most problematic species right now. Consistent heat and moisture stress on the large weeds found this time of year make challenging conditions for achieving good herbicide efficacy, particularly with glyphosate. The potential for glyphosate resistance in kochia and other weeds is an important consideration. For this reason, glyphosate should be avoided entirely for post-harvest use if possible. A notable exception to this guideline is when targeting grasses or volunteer wheat, where glyphosate is still a good choice when tank-mixed with other products.
If avoiding glyphosate entirely isn’t feasible, tank mixing with additional modes of action active on the target weeds is essential. Another practice to improve herbicide efficacy is to spray only when temperatures are 80 to 85 F or less. When temperatures rise above this, many plants start to shut down, which can reduce the amount of herbicide taken into plants. When weeds are under drought stress, it may be best to wait for a rain or a period of cooler temperatures to restore active growth before spraying.
Dicamba and 2,4-D are generally dependable options for broadleaf control alone, together, or tank-mixed with glyphosate or other chemistries. Occasional reports of poor kochia control indicates that at least some low-level resistance to these herbicides may be present. Often, this sort of emerging resistance is only expressed in plants under stress – typical conditions for post-harvest applications. Again, tank mixing with multiple modes of action effective on the target weed is the best response. Paraquat is a very effective contact herbicide with a unique mode of action. Post-harvest applications are one of the best uses for paraquat, and a good opportunity to bring it into rotation for herbicide resistance management.
While large, stressed weeds are generally less susceptible to herbicides than small, actively growing ones, contact products are less affected by weed stress and size than most systemic products. If a period of dry, hot weather follows application, efficacy of contact herbicides is generally enhanced rather than diminished. Contact herbicides kill only plant tissues they cover though, so good coverage is critical for performance.
However it is accomplished, control of weeds in winter wheat stubble within a few weeks of harvest is important.Click here for more information about glyphosate resistant kochia: http://msuextension.org/publications/AgandNaturalResources/4602.pdf