By Lizabeth Stahl, Extension Educator - Crops, and Jeff Gunsolus, Extension Agronomist - Weed Science
With very tight windows of opportunity to plant this year, preemergence herbicides may not have been applied as planned. Application of a residual herbicide prior to planting or emergence of the crop, in both corn and soybean, is a great weed management strategy overall and also a key tool in managing against herbicide resistance. What are some of our options if soybeans emerged before a preemergence herbicide application was made?
Most soybean preemergence herbicides with a site of action (SOA) of #14 must be applied within 3 days of planting. Note that many companies now list SOA numbers on the front page of the herbicide label. Products with this SOA that must be applied within three days of planting and prior to crop emergence include: Authority products, Enlite, Gangster, Optill, Sharpen, Fierce, and Valor. Warnings on the herbicide labels vary slightly, but essentially you can expect severe crop injury if these products are applied after soybeans have begun to crack or emerge. An exception of a product with this SOA is Prefix: This product can be applied from cracking through the 3rd trifoliate stage of soybean.
Herbicides such as Dual II Magnum, Outlook and IntRRO have a wider window of application, and can be applied through the 3rd trifoliate in soybean. Another herbicide within this chemistry, Warrant, can be applied through the R1 stage of soybean, although the optimal time of application is the V2 to V3 stage of soybean. Note that these herbicides will not control emerged weeds.
FirstRate is another herbicide option that can be applied postemergence, up to 50% flowering in soybean, although the optimal time of application is preemergence within 2 days after planting.
Be sure to check herbicide labels for further information about restrictions and rates when applying products postemergence, and for any restrictions on additives, carriers and tank mix partners.
For Corn: See the article "Soil residual herbicide options after corn emergence" for information and links to resources that address this issue in corn at: