With very tight windows of opportunity to plant this year, pre-emergence 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 options if soybeans emerged before a pre-emergence herbicide application was made?

Most soybean pre-emergence herbicides with a site of action (SOA) of #14 must be applied within three 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 third 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 third 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-V3 soybean stage. Note that these herbicides will not control emerged weeds. 

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FirstRate is another herbicide option that can be applied postemergence, up to 50% flowering in soybean, although the optimal time of application is pre-emergence within two 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. 

Corn

Many, but not all, soil-applied corn herbicides can be applied after corn emergence.Products that must not be applied after corn emergence include Fierce, Sharpen and Verdict. Severe crop injury can occur if these products are applied postemergence.

If postemergence application of a soil-applied residual herbicide is allowed, be sure to check the herbicide label for any crop height restrictions. Maximum height restrictions can range from 5 in. to 40 in., depending on the product. A helpful table from the University of Illinois summarizes the maximum size corn can be for several of these products. In addition, be careful if you plan to use UAN as a herbicide carrier, as UAN can enhance foliar activity of products. An article from Iowa State University discusses this concern and lists recommended maximum UAN rates based on crop stage.

Also note that most residual soil-applied products will have little if any activity on emerged weeds. A postemergence product may need to be added for control of emerged weed seedlings. As always, be sure to check the herbicide label for tank-mix restrictions and additive recommendations.

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