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Issue No. 10, Article 6/May 27, 2005

Postemergence Options for Grass Control in Corn

We have received a number of inquiries regarding options for postemergence grass control in corn. In many of these cases, the soil-applied grass herbicide failed because of lack of precipitation following the application. While there are numerous options for grass control, keep in mind that timely application is needed in order to achieve maximum yield potential.

To determine which herbicide is right for your field, you need to answer a few questions, the first being "What type of corn hybrid did I plant?" In the case of a herbicide-resistant corn hybrid, such as Clearfield (CL), Liberty Link (LL), or Roundup Ready (RR), your options can include those herbicides not labeled for broadcast application in conventional hybrids.

If a herbicide-resistant hybrid was not planted, the options for grass control become slightly more limited. Of the options that remain, most of them con-tain active ingredients that are ALS inhibitors. These herbicides include Accent, Accent Gold, Basis, Basis Gold, Celebrity Plus, Beacon, Equip, Steadfast, Northstar, Option, and Spirit. There are a few cases of ALS-resistant shattercane and giant foxtail in Illinois, and this should be taken into consideration when selecting the herbicide.

Another question to ask is "What grass species are present and how tall are they?" Table 2 lists the herbicides available for postemergence control of grasses and the species controlled. Control of grass weeds depends on the height of the grass at the time of application. Atrazine with the addition of crop oil can be applied to control many grasses less than 1.5 inches tall. Basis will control many grass species up to 2 inches tall. Other grass herbicides can control grasses ranging in height from 4 inches to 20 inches, depending on species and product (refer to Table 3 for grass weed height restrictions).

As with most herbicides, there are restrictions that pertain to crop height at the time of application. Table 3 lists the application restrictions for corn herbicides. The restrictions are based on either plant height or number of leaf collars, with empha-sis on using the more re-strictive of the two. Many agree that the leaf-collar method is more closely related to the physiological age of a plant than to plant height. With the recent cool weather, much of the corn throughout Illinois is stunted, reinforcing the need to count leaf collars. For a quick review on staging corn, refer to the Pocket Guide to Crop Development. If these corn heights or stages are exceeded, significant crop injury can occur, with subsequent reductions in yield. Basis can be applied to corn with two leaf collars or less. Atrazine and atrazine-containing products can only be applied to corn less than 12 inches tall or with less than five leaf collars. Accent, Steadfast, Spirit, and Northstar limit applications to corn less than V6 or 20 inches. Quite often the height restrictions for a herbicide can be extended if drop nozzles are used.

Another important consideration is environmental conditions at the time of application. We have had some extremely cool temperatures already this season. Many of the postemergence grass herbicide labels warn against applications in cold weather. For example, the Accent label warns that poor weed control and crop injury may occur if applications are made to plants under stress from abnormally hot or cold weather, drought, water-saturated soils, hail damage, or frost. The Celebrity Plus label warns that crop injury can occur under conditions of crop stress or rapidgrowth; it also states that applications made during or immediately after periods of extreme day/night temperature fluctuations or where daytime temperatures do not exceed 50°F may decrease weed control or increase crop injury. In these cases, delay application until temperatures warm and both the crop and weeds resume normal growth. Thus, it remains important to consult the labels for these environmental precautions.

The final consideration for post-emergence grass control is adjuvant selection. By selecting the appropriate adjuvant, you can increase the activity of a herbicide on a weed species and prevent crop injury. Adjuvant selection varies with individual herbicides, tank mixes, and environmental conditions. Liberty and Liberty ATZ require the addition of AMS only, while Option requires methylated seed oil (MSO) and a nitrogen source for optimum weed control. Keep in mind that these herbicides should not be applied in a liquid nitrogen carrier or severe corn injury can occur. Always refer to the labels for adjuvant selection.--Dawn Nordby and Aaron Hager

Aaron Hager
Dawn Refsell

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