University of Illinois

No. 9/May 21, 1998

Postemergence Corn Herbicides:
Some Precautions to Consider

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

We have received several calls and questions about applying postemergence corn herbicides in a liquid nitrogen solution carrier in hopes of saving a trip across the field. The carrier that people most commonly ask about is 28 percent UAN solution. Although 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 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. Refer to Table 15 for label restrictions with respect to foliar use of OP insecticides. DO NOT apply postemergence corn herbicides in a liquid fertilizer carrier, as severe corn injury can occur.

Table 15. Herbicide label restrictions with respect to foliar applications of OP insecticides

Foliar-applied OPinsecticides
HerbicideDo not apply _ days beforeherbicide applicationDo not apply _ days afterherbicide application
Accent Gold73
Basis Gold73
Scorpion III77
PermitNo information on labelNo information on label

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.

Can postemergence corn herbicides be tank-mixed with insecticides? The best answer is to consult the respective herbicide labels because there is 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 15 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 conditions such as low nighttime air temperatures, excess soil moisture, dry soil conditions, etc. Much of the corn crop has been under stress conditions for a large part of the growing season, so it may be more susceptible to injury from postemergence herbicides.

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 for 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 temperatures above 85 degrees F are expected the day of application. High air temperaturesenhance 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 most common ammonium source is 28 percent UAN, but spray-grade ammonium sulfate (AMS) has also become quite popular. Some recently registered postemergence corn herbicides call for the addition of an ammonium nitrogen source but do not allow the addition of AMS, so check respective herbicide labels carefully.

Aaron Hager ( and Marshal McGlamery (mmcglame@, Department of Crop Sciences, (217)333-4424