Weed Management Issues

Growing continuous corn limits growers to fewer herbicide options than growing corn in rotation with soybeans or another crop. In addition, corn grown continuously can lead to increased crop residue levels, which can decrease the efficacy of many soil-applied herbicides and favor certain weed species that thrive in an environment of higher residue and greater soil surface moisture. Consequently, certain annual grasses, johnsongrass (Sorghum holepense (L.) Pers.) and certain small-seeded broadleaf weeds can be more problematic in continuous corn.

If using soil-applied herbicides, use full rates to compensate for the effects of greater residue to best manage weeds in continuous corn. If plans include greater reliance on post-emerge herbicide applications, ensure that weeds are not taller than 6 in. before making applications. In the long run, a combination of pre-emergence and postemergence weed control strategies will usually result in the most effective weed control.

Weed management concerns in second-year corn will be influenced by the performance of the previous year’s weed management program. In 2004, for example, early planting and subsequent wet conditions diluted soil-applied herbicides, resulting in widespread instances of giant ragweed (Ambrosia trifida L.), burcucumber (Sicyos angulatus L.) and giant foxtail (Setaria faberi Herrm.) breaking through the soil-applied treatments. In 2005 and 2007, lack of rainfall to activate soil applied herbicides resulted in widespread instances of poor control giant foxtail, lambsquarter (Chenopodium album L.) and giant ragweed.

In 2006 and 2007, many growers waited until weeds were excessively large before making postemergence herbicide applications, and weed control failures were obvious. The fields with moderate to high densities of weeds that emerged with corn and were not controlled until the V3 corn stage or when weeds were in excess of 4-6 in. tall likely suffered significant yield losses and allowed weeds to produce seed. In addition, many growers apparently reduced their use of residual herbicides in corn production. Consequently, late-season of grass weeds such as crabgrass (Digitaria sanguinalis (L.) Scop.), barnyardgrass (Echinochloa crusgalli (L.) Beauv.) plus broadleaf weeds such as waterhemp (Amaranthus tuberculatus (Moq.) Sauer.) and redroot/smooth pigweed (Amoranthus retroflexus L., Amaranthus hybridus L.) were very evident.

Fields with such weed escapes leave behind a good supply of new weed seed in the soil seed bank. Furthermore, giant ragweed, burcucumber, waterhemp and crabgrass have relatively long emergence periods in Indiana and two-pass weed control programs are always more successful on these weeds.

Mitigate the risk of poor giant ragweed and burcucumber control by adjusting weed management plans to include the use of postemergence herbicides that provide residual activity on these weeds. Shifting atrazine use from preplant to postemergence application will extend the residual window of activity and reduce late-season weed emergence. Callisto, Hornet and Peak (Spirit) products also provide foliar and residual activity on these weeds, unless the giant ragweed is ALS resistant, and would be well-suited to use as postemergence treatments.

For better control of late-emerging grass weeds and some small seeded broadleaf weeds, consider adding a reduced rate of an amide metolachlor (Dual and other formulations), acetochlor (Degree or Surpass and other formulations), dimethenamid (Outlook) or flufenacet (Define) to the postemergence herbicide treatment. Amide herbicides will not control emerged grass weeds. If grass weeds have emerged, a postemergence grass herbicide will be required to control them. All of the chloroacetamide products listed above are labeled for application to emerged corn.

Mitigate the risk of yield loss due to late postemergence herbicide treatments by using residual herbicides at planting and making postemergence treatments before the V3 growth stage. Use the WeedSOFT® Yield Loss Calculator (Univ. of Nebraska, 2006) to assist in your understanding of the impact of early-season weed competition on corn yield.

Glyphosate-Resistant Weeds

Glyphosate-resistant marestail (aka horseweed, Conyza canadensis) is widespread in southeast Indiana and southwest Ohio and effective postemergence control of marestail with glyphosate alone in this region is unlikely (Loux, et. al., 2006). In addition, glyphosate-resistant marestail has now been documented in 15 states in the U.S. In 2006 and 2007, we observed frequent giant ragweed and lambsquarter control problems with glyphosate in soybean and corn. Lambsquarter biotypes with elevated tolerance to glyphosate have been reported in Indiana and Ohio. Purdue and Ohio State weed scientists have conducted extensive field and greenhouse experiments on giant ragweed biotypes with elevated tolerance to glyphosate and have documented populations that show a low level of resistance to glyphosate.

Mitigate the risk of glyphosate-resistant weeds by including a variety of herbicide modes of action, especially on weeds that are most problematic to control with glyphosate alone. If glyphosate-resistant corn was grown in a particular field in the previous year, one should also strongly consider using herbicides that rely on other modes of action on the most problematic weeds to reduce selection pressure for glyphosate-resistant weeds. This is particularly important in fields where the grower has noticed increased difficulty in controlling giant ragweed and common lambsquarter. Marestail, lambsquarter and giant ragweed are effectively controlled by many postemergence herbicides in corn. The most effective control of these weeds are usually provided by dicamba, 2,4-D, Hornet, or Callisto-based products containing atrazine, provided the applications are made before weeds are 6 inches tall.