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Multiple, timely herbicide applications are the key components of a successful weed management strategy. Fall applications to control already existing weeds can be a great first step to set the stage for effective weed management in next year’s corn and soybean crops.
The primary goals of fall-applied herbicides are to control winter annuals, biennials, and cool-season perennials that are present while they are still small and manageable and while the weather is more conducive to herbicide applications, and also to minimize the chance of those weeds producing seeds. Fall herbicide applications also give you a head start on preparing the field for planting next season and can reduce early spring infestation from insects, such as black cutworm and armyworm, by removing weeds where they overwinter. A weed species of major concern is marestail and a fall herbicide is considered an essential component of effective marestail control.
Figure 1. Fall-applied herbicide applications targeting winter annuals and dandelion can help to provide a weed-free seedbed in the spring (treated area on the right).
Generally, burndown herbicides are recommended in the fall. In some situations, a burndown plus a residual herbicide may be needed to control some fall weeds. Cropping systems with glyphosate-resistant weeds like marestail should include an effective tank-mix partner with their glyphosate for fall burndown. Adding a second effective site of action will improve weed control and reduce selection for glyphosate-resistant weeds.
Fall herbicide treatments may be applied anytime after harvest depending on weed species and size. In general, winter annuals and biennials may be controlled well into December, but perennial control usually drops off in late November. Make sure the weeds are actively growing when the application is made. If weeds were cut off during harvest, ensure that new leaves are actively growing before applying an herbicide. Review the cut-off date, soil temperatures, and plant-back restrictions on the herbicide labels before application. The majority of herbicides recommend application while the soil is cool, not frozen. For those planning to apply herbicides with fall fertilization, remember that the recommended soil temperature is below 50 °F and weeds should be actively growing.
A fall herbicide application alone is not a comprehensive weed management plan. Fall-applied residual herbicides may not persist long enough to provide adequate in-crop weed control. The application of residual herbicides in the fall does not replace the need for residual herbicide applications in the spring. Recommended weed control programs for glyphosate-tolerant crops include a fall application if needed, a spring preemergence application of a residual herbicide, and one or two in-crop applications of a Roundup® brand glyphosate-only agricultural herbicide with at least one additional site of action in the tank.
Loux, M. 2007. The ABCs of fall herbicide treatments. C.O.R.N. Newsletter 2007-32. Ohio State University Extension; Loux, M. 2011. Fall herbicide treatments. C.O.R.N. Newsletter 2011-33. Ohio State University Extension; Nice, G. et al. 2006. Fall applied herbicide for corn and soybean. Purdue University Extension. 171003114657