Preemergence herbicide programs for corn
There are several preplant and preemergence residual herbicides available for corn. 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 22-24 of the 2013 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1081). See: http://www.ksre.ksu.edu/bookstore/pubs/SRP1081.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 grasses (except Johnsongrass and shattercane) 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. 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.
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.
In fields with normal weed and grass infestations (no herbicide resistance problems), a reduced-rate of an acetamide/atrazine premix product applied preemerge, followed by a postemerge-applied herbicide can still do a good job. The purpose of the low-rate preemerge treatment is to kill the easy weeds (common annual grasses and pigweeds), get corn off to a head start, keep the weed infestation at a manageable density, and buy time for the post application.