Fall Applied Herbicides

Fall herbicide applications can be a great first step to set the stage for effective weed management in next year’s corn and soybean crops. The two goals of fall-applied herbicides are: 1) to control existing winter annuals, biennials, and cool-season perennials, and 2) to minimize the chance of those weeds producing seeds.

Fall 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. Applications should be made to actively growing weeds before a heavy frost or freeze. Review the cut-off date and soil temperatures on the herbicide labels before application. The majority of herbicides recommend application while the soil is cool, but not frozen. For those planning to apply herbicides with fall fertilization, the recommended soil temperature for herbicide application is below 50° F on actively growing vegetation.

Fields that are heavily infested with winter annual weeds such as marestail, henbit, and chickweed are good candidates for a fall burndown herbicide application. Winter annual weeds will emerge in the fall after harvest and complete their life cycle in the spring and early summer. Weed control is often more difficult in the spring because of weather conditions and the potential for larger weed size. If allowed to grow in the spring, winter annual weeds can form a thick mat on the soil surface which can interfere with tillage, crop establishment, and soil warming from the sun.

A single marestail plant (Figure 1) can produce as many as 200,000 seeds, so it is vital that growers initiate a herbicide control program when marestail is small. Marestail is generally easier to control in the fall when they are small before they bolt or shoot a main stem in the spring. Bolted marestail is difficult to control with a spring burndown herbicide application.

Glyphosate-resistant marestail is of particular concern. Cropping systems with glyphosate- resistant weeds like marestail should include an effective tank-mix partner with glyphosate for fall burndown. Adding a second herbicide with an effective site of action will improve weed control and reduce selection for glyphosate-resistant weeds. Applications should be made during periods of mild weather when marestail is actively growing.

 

Figure 1. Marestail rosette (left) and bolting (right).

Fall herbicide application alone is not a comprehensive approach; a majority of your weed management plan should be targeted to spring/summer treatments. Recommended weed management plans 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 herbicide in the tank -mix.

Benefits of fall-applied herbicides include:

  • Reducing weed pressure and helping prepare fields for planting and emergence.

  • Controlling winter annuals in the fall can help to increase both soil temperature and moisture at planting time.

  • Helping to spread out the spring workload.

  • Reducing early spring infestation from insects such as black cutworm and armyworm by removing weeds that are attractive egg-laying sites.

See your local Monsanto Crop Protection Dealer or visit RoundupReadyPLUS.com for more information.​​​​

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