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Considerations with Postemergence Corn Herbicides

May 25, 2001
Many parts of Illinois have recently experienced several weeks of very dry soil conditions, but recent precipitation has undoubtedly eased some concerns. In other areas, wet soils have delayed postemergence corn herbicide applications. Dry soil conditions have reduced the effectiveness of several soil-applied corn herbicides and resulted in some postemergence herbicide applications being made sooner than anticipated. Excess precipitation in other areas may have moved soil-applied herbicides too deep in the soil to effectively control weeds. Seems like a feast-or-famine season.

The recent precipitation in dry areas will undoubtedly help crop growth and development but may also result in additional weed emergence, especially in cornfields where the soil-applied herbicide did not receive significant precipitation for several weeks following application. Cornfields should continue to be monitored closely for the next 2 weeks to determine if supplemental weed-management strategies are needed. Following are several considerations for postemergence corn herbicide applications that should be kept in mind whether you've had too little or too much precipitation.

1. We have received some questions about applying postemergence corn herbicides in a liquid nitrogen solution carrier in hopes of saving a trip across fields where nitrogen has yet to be applied. The most common carrier people ask about is 28% UAN solution. While applying high rates of UAN by itself can cause some corn injury, adding a postemergence herbicide can greatly increase corn injury. Most postemergence corn herbicide labels restrict application with UAN as the total carrier, but many allow a lower rate (usually 1 to 4 quarts per acre) of UAN to be added as a spray additive to enhance control of particular weed species, most commonly velvetleaf. Do not apply postemergence corn herbicides in a liquid fertilizer carrier, as severe corn injury can occur.

2. The recent problems with armyworms in cornfields may have some growers considering including a foliar insecticide with a postemergence corn herbicide application. Several postemergence corn herbicide labels contain restrictions or precautionary statements about this type of tank mix. Keep in mind that if a particular postemergence herbicide/insecticide tank mix is restricted, there may also be a waiting interval for sequential applications of the individual products. For example, the Distinct label indicates the product should not be applied in a tank mix with Lorsban 4E, Ambush EC, or Warrior EC. However, sequential applications may be made if applications are at least 7 days apart. Consult the respective herbicide and insecticide labels for additional information related to tank-mixing or the time interval required between sequential applications.

3. Corn plants under stress conditions may be more prone to injury from postemergence herbicides than when growing conditions are more ideal. Stress can arise from a number of factors, and an increasing number of postemergence herbicide labels are cautioning against making applications under conditions such as low nighttime air temperatures, excess soil moisture, dry soil conditions, and so on. For example, most DuPont postemergence corn herbicide labels indicate applications should be made when minimum nighttime temperatures are above 40°F and the maximum daytime temperatures are below 92°F to maximize performance and minimize the potential for crop injury.

Why is a crop under stress more likely to be injured from a selective herbicide? In the majority of cases, herbicide selectivity arises from the crop's ability to metabolize (break down) the herbicide to a nonphyto-toxic form before it causes much injury. For example, a grass herbicide used in corn cannot discriminate between giant foxtail and the corn crop. It attempts to control the corn just as it does the giant foxtail. When the corn is growing under favorable conditions, it is able to quickly metabolize the herbicide, generally before the corn is injured enough to express injury symptoms. If, however, the corn plant is under stress (which could be caused by a variety of factors), its ability to metabolize the herbicide may be slowed sufficiently to allow the herbicide to cause enough injury for symptoms to be manifested.

4. With the warm air temperatures, keep in mind that some postemergence corn herbicide labels have application restrictions based on air temperature. High air temperatures also increase the potential for certain herbicide formulations to volatilize. Volatilization is the process whereby a herbicide changes from a liquid state to a vapor phase. Vapors are easily moved by air currents and could potentially move out of the treated area and cause injury to nearby sensitive vegetation.

5. The selection of herbicide additives is often specified on the respective product label, but many postemergence corn herbicides allow use of a nonionic surfactant (NIS) or a crop oil concentrate (COC), with or without a nitrogen fertilizer. For many products, NIS is the preferred additive, but COC may be used under very dry conditions to enhance weed control. Using a COC instead of an NIS increases the crop-injury potential for several postemergence corn herbicides.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague


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

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