Preemergence herbicide programs for corn
There are several preplant and preemergence residual herbicides available for corn. These herbicide programs are key to managing glyphosate-resistant and other difficult-to-control weeds. It’s important to know the strengths and weaknesses of each product in terms of the spectrum of weeds controlled. A table summarizing weed species response to various corn herbicides can be found on pages 23-25 of the 2014 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1099). See: http://www.ksre.ksu.edu/bookstore/pubs/SRP1099.pdf
For burndown applications in a no-till system on emerged grass and broadleaf weeds, an application of glyphosate and a product containing dicamba or 2,4-D may be critical. The choice between 2,4-D and dicamba will depend on weed species present. Dicamba products will be more effective on kochia and marestail. 2,4-D is more effective on winter annual mustards. The use of preemergence herbicides often provides control of weeds for several weeks. This can greatly improve the effectiveness of a postemerge herbicide application, and give the producer more leeway on post application timing.
Soil-applied residual herbicides for corn can be grouped into several basic categories.
Acetamides and acetamide/atrazine premixes. The main acetamide products used in corn include acetochlor, S-metolachlor, dimethamid-P, pyroxasulfone, and flufenacet, and many premix products containing one of these five active ingredients. In general, these products are very effective in controlling annual grasses (except shattercane and perennial Johnsongrass) and small-seeded broadleaf weeds such as pigweeds. They are much less effective in controlling small-seeded kochia or large-seeded broadleaf weeds such as cocklebur, devilsclaw, morningglory, sunflower, and velvetleaf. An exception are those products containing pyroxasulfone, Zidua and Anthem. These products have activity on kochia and the large seeded velvetleaf. There have been no cases of weed populations in Kansas developing resistance to the acetamides to date.
The acetamide products are most effective when applied with atrazine. Several atrazine/acetamide premixes are available and should be used instead of acetamides alone unless atrazine is not allowed. These premixes generally fit into two groups. A reduced atrazine rate and a full atrazine rate group of herbicide. Soil type, soil pH, and organic matter will determine whether the reduced or full rate atrazine herbicide is used. In past years, often because of cost, reduced rates of these products were applied to help manage heavy summer annual grass pressure, then followed up with a good postemergence herbicide program. With the increased occurrence of glyphosate- and other herbicide-resistant weeds, the use of reduced/setup rates greatly increases the risk of unacceptable control.