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Options for Postemergence Grass Control in Corn

May 24, 2002
In the next couple of weeks, grass control in corn could be a major challenge for some producers in the state. Fields where postemergence grass control measures may need to be taken are fields where a soil-applied grass herbicide may have been on for a number of weeks prior to planting or may have failed due to excessive precipitation. Additionally, there also have been a number of corn acres planted that have not received a soil-applied grass application due to untimely rains. In these situations, what options do we have for postemergence grass control in corn? There are currently a number of herbicides that are available to control grasses in these situations. Some of these options require the use of a special corn hybrid, such as Roundup Ready (RR), Liberty Link (LL), or Clearfield (CL). Other products, such as Accent, Accent Gold, Basis, Basis Gold, Celebrity Plus, Option, Steadfast, and atrazine, do not require the use of a special corn hybrid. When making a decision on what postemergence grass control option you may want to use, there are a number of things you need to consider.

The first question to ask is "What type of corn hybrid did I plant?" If a herbicide-resistant crop wasn't planted, whether it be a Roundup Ready, Liberty Link, or a Clearfield hybrid, it narrows your options to using only herbicides that do not require the use of a herbicide-resistant/-tolerant hybrid. These options generally consist of products that contain one of three sulfonylurea active ingredients: the active ingredient nicosulfuron (Accent), the active ingredient primi-sulfuron (Beacon), or the new active ingredient foramsulfuron (Option). These active ingredients are ALS-inhibiting herbicides and will not control grass species that are resistant to this class of herbicides. As mentioned in issue no. 7 of the Bulletin, there are some isolated incidences of ALS-resistant shattercane in certain areas of Illinois.

Another question that needs to be asked is "What are the grass species that I am trying to control and what sizes are they?" Atrazine plus crop oil controls a number of grass weed species, as long as they are less than 1.5 inches in height. The herbicide Basis provides control of foxtails, barnyardgrass, and fall panicum 2 inches in height or less. Other grass herbicides control grasses ranging up to 20 inches in height, depending on the herbicide and target grass species. A complete listing of the postemergence grass herbicides and maximum grass heights for these herbicides can be found in Table 1.

Along with knowing the size of the target grass species, it is important to realize that there are maximum corn heights that are listed for many of these herbicides. If these corn heights or stages are exceeded, a number of these herbicides can cause significant corn injury that may result in a reduction in yield. These corn heights should be monitored carefully. For example, the herbicides Accent Gold and Basis Gold may be applied to corn up to 12 inches tall or at the V6 stage, whichever is more restrictive. Table 1 lists the maximum corn heights to which these herbicides can be applied. Remember that a number of these herbicides also have maximum leaf stages also listed on their labels that may be more restrictive.

Another important consideration is the environmental conditions at the time of the postemergence application. Over the last week, we have seen some extremely cool temperatures, especially in the evening. Many of these postemergence grass herbicides warn against applications in cooler weather. For example, the Accent label warns against applications made during or immediately following periods of large day/night temperature fluctuations or where temperatures do not exceed 50°F. Applications during these times may result in lack of weed control performance and also can cause crop injury. It is important to consult labels for these environmental precautions.

Finally, adjuvant selection is also very important when using these different herbicides for postemergence grass control. The appropriate adjuvant selection increases the activity of the herbicide on a particular weed species, as well as helps prevent injury to the crop. For example, the new herbicide Option requires the use of methylated seed oil (MSO) and a nitrogen source for maximum grass control. Adjuvant selection options can vary with the herbicide, tank-mix partner, and environmental conditions. Always consult the herbicide label for the appropriate adjuvant selection, and remember that these herbicides should not be applied in a liquid fertilizer carrier or severe corn injury can occur.--Christy Sprague and Aaron Hager

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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