Postemergence Corn Herbicides--Some Precautions to Consider

May 12, 2000
The time to apply postemergence corn herbicides has arrived in many areas of Illinois. Before the postseason gets into full swing, there are a few precautions to keep in mind.

Each year we receive a few calls and questions about applying postemergence corn herbicides in a liquid nitrogen solution carrier in hopes of saving a trip across the field. The most common carrier people ask about is 28% UAN solution. While applying high rates of UAN by itself can cause corn injury, adding a postemergence herbicide can greatly increase corn injury. Most postemergence corn herbicide labels restrict application with UAN as a 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.

While the majority of postemergence corn herbicides have application restrictions with respect to maximum corn size (either specified as height, leaf number, or sometimes both), some also have restrictions on minimum corn size for application. For example, the Spirit (prosulfuron + primisulfuron) label indicates that broadcast over-the-top applications should be made when corn is between 4 and 24 inches in height. The postemergence corn herbicides that have minimum corn-size restrictions were discussed and listed in Table 4 of the "Considerations for Early-Season Weed Control in Corn" article that appeared in issue no. 5 of the Pest Management and Crop Development Bulletin.

Can postemergence corn herbicides be tank mixed with insecticides? The best answer is to consult the respective herbicide and insecticide labels because of a wide variation in tank-mix restrictions. Keep in mind that most all postemergence ALS-inhibiting herbicides have restrictive intervals with respect to application before or after applications of certain organophosphate (OP) insecticides. Table 4 is a summary of label information for several postemergence ALS-inhibiting herbicides with respect to time intervals before or after foliar applications of OP insecticides.

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 adverse conditions, such as low nighttime air temperatures, excess soil moisture, dry soil conditions, and so on.

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 nonphytotoxic 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, its ability to metabolize the herbicide generally occurs well 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.

With the warm air temperatures experienced this past week, keep in mind that several postemergence corn herbicide labels have application restrictions based on air temperature. For example, the Marksman label indicates that applications should not be made when air temperatures above 85°F are expected the day of application. High air temperatures enhance the possibility of volatilization of certain herbicide formulations. 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.

The addition of an ammonium nitrogen source to postemergence herbicides has become very popular. The two most common ammonium sources are 28% UAN and spray-grade ammonium sulfate (AMS). In many cases these two ammonium sources can be used interchangeably. However, there are some postemergence corn herbicides that call for the addition of one or the other, so check respective herbicide labels carefully.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague