Over the last two growing seasons (2020 and 2021), weather and soil conditions across southern Wisconsin were favorable for early soybean planting (see USDA NASS data below). Under suitable environmental conditions, planting soybean early is a recommended strategy to increase the crop’s yield potential (See Dr. Shawn Conley’s blogpost: “Just the Facts Jack: Soybean Planting Date, Seeding Rate and Seed Treatment Recommendations”).
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Can I skip a PRE-emergence application in early planted soybeans and control weeds POST only? We recommend against that. The use of an effective PRE-emergence herbicide is the foundation for chemical weed control in soybeans. Limited effective POST-emergence herbicide options are available for control of herbicide-resistant weed species such as waterhemp and giant ragweed.
Can I delay my PRE-emergence application in early planted soybeans? Yes, one could do so by selecting herbicides with soil residual activity that can be sprayed before crop emergence instead of herbicides that must be sprayed within 3 days of crop planting, particularly when the soybean crop is being planted early in the spring, is not expected to emerge for several weeks, and weeds are not actively emerging due to low soil temperatures. Keep in mind that PRE-emergence herbicides are effective if timely incorporated (by light tillage and/or precipitation) and active in top soil solution as weeds germinate and emerge.
With all that in mind, we developed the following handy table displaying the application window of commonly used herbicides with soil residual activity in soybean cropping systems.
Click to find the handy table developed by Arneson et al.: “Application Window for Soybean Herbicides with Soil Residual Activity”.