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Do Your Soybeans Need 2 Postemergence Herbicide Applications?
Because some weeds — such as marestail, Palmer amaranth and waterhemp — are capable of germinating well into the growing season, a second postemergence application may be necessary to protect yield and reduce weed seedbank buildup.
Ask yourself these questions before making a late-season herbicide pass:
- How tall are the weeds? Weeds that escaped earlier applications may be too large for consistent control, or they may be hardened off from the initial application and less susceptible to the herbicide.1 If weeds are taller than your soybean plants, it’s unlikely that a second postemergence application will provide effective control or better results.
However, applying a second postemergence application in fields with a new flush of weeds that are 2 to 4 inches tall can help you ensure the weeds don’t go to seed. - Did weather impact herbicide efficacy?If there was excess rainfall or a complete lack of rainfall after your first postemergence pass, herbicide efficacy may have been affected. A second postemergence application in better weather conditions will likely yield better results in this scenario.
- Is herbicide resistance suspected?Herbicide resistance is one possible reason for weed escapes, and it should be further investigated if suspected. However, if there are multiple species of weed escapes, it’s unlikely a resistance issue and more likely a weather or spray decision issue.
If you’re looking at making a second postemergence pass, choosing chemistries that can be sprayed on the crop at this time is important. Work with your local crop protection retailer or Corteva Agriscience representative to evaluate herbicide options and spray timing. Growth stage of the crop as well as calendar dates will affect your herbicide options.
For more tips, listen to this Brownfield Managing for Profit episode.
1 Hartzler, B. 2009. Late-season herbicide applications in soybean. https://crops.extension.iastate.edu/cropnews/2009/07/late-season-herbicide-applications-soybean
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