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Burndowns Can’t Always Withstand Cold Temperatures

November 22, 2017
This agronomic image shows soybeans ready for harvest.

As harvest wraps up, prepare for next season with a burndown herbicide.

As you start to plan ahead for planting next season, you’ll also need to prepare for pre-season weed control. While burndown herbicides will clear weeds already in your fields, winter temperatures can affect the efficacy of fall burndown herbicide applications.

The effect of cool weather on weed control can vary depending on the target weed, herbicide used and rate of application. For weeds like waterhemp and Palmer amaranth, a fall application will not help with control, as these weeds do not emerge until May.

For marestail, a fall application may be critical for optimum weed control. The University of Nebraska Extension says most marestail in Nebraska is fall-emerging and is better controlled in the fall when the weed is still young.

The ideal temperature for applying most post-emergence herbicides is 65 to 85 degrees Fahrenheit. Herbicides can still be effective when applied at 40 to 60 degrees, but they will take longer to work as the cooler weather slows down the absorption rate. If it is below 40 degrees or cloudy for a period of time after the application, weed control will most likely be reduced.

Frost can also impact the efficiency of burndown herbicides. Frost damage puts weeds under stress, which makes them poor targets for herbicide application. After frost, you should wait a few days for active growth and new leaf tissue to begin applying the burndown herbicide.

According to the Purdue University Extension, winter weeds, like purple deadnettle, can serve as an alternate host to soybean cyst nematode, the top U.S. soybean pest. Scout fields now for winter annual weeds and remove them prior to planting.

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