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Issue No. 7, Article 7/May 7, 2004

A Plethora of Corn Postemergence Concerns

With more than 90% of the corn crop in the ground and some of it already emerged, the time to apply postemergence corn herbicides has arrived in many areas of Illinois. Before we get caught up in the hustle and bustle, here are a few precautions to keep in mind. In an earlier article in the Bulletin (issue no. 5, April 23, 2004, "Dry Soils and Soil-Applied Herbicides"), we identified some options for weed control on those acres where the soil-applied herbicides were not applied or are not working to their full potential due to lack of rainfall.

One thing to keep in mind when planning postemergence herbicide programs and rescue applications is the maximum corn size (specified either as height or leaf number, or sometimes both) at which these herbicides can be applied. Quite often, if these restrictions are not followed, there can be substantial injury to the crop that may lead to yield reductions. So it is always important to keep these maximum sizes in mind when planning postemergence herbicide applications. We will discuss some of these maximum sizes in the following text. Table 2 lists maximum corn sizes and comments on all postemergence corn herbicides, while Table 3 lists minimum corn sizes for certain postemergence corn herbicides.

2,4-D-containing products. Herbicides that contain 2,4-D can be broadcast applied until corn is 8 inches in height. Applications to corn larger than 8 inches require the use of drop nozzles to avoid placing the herbicide into the corn whorl. Also, do not spray corn between tassel and dough stage with 2,4-D; however, this herbicide can be used as a harvest aid after corn is in the dent stage. Shotgun, a premix containing 2,4-D, requires drop nozzles when corn is greater than 8 inches; however, the maximum height for applications with drops is 12 inches due to the atrazine component. Ester formulations of 2,4-D are more prone to volatilize than amine formulations, so use an amine formulation if air temperatures are expected to approach 80 to 85°F. Many broadleaf plants are susceptible to 2,4-D, so take precautions to avoid drift out of the target area.

Atrazine-containing products. Herbicides that contain atrazine must be applied before corn is 12 inches tall. These products include Basis Gold, Buctril + atrazine, Liberty ATZ (Liberty-Link corn), ReadyMaster ATZ (Roundup Ready corn), and Shotgun. Marksman also contains atrazine, but the size limit is 8 inches or 5-leaf corn, whichever is more restrictive.

Dicamba-containing products. Herbicides that contain dicamba have rate cutoff restrictions. The application rates for Banvel and Clarity must be lowered if applications are made to corn over 8 inches tall or more than 5-leaf corn. Maximum corn size at lower rates depends on the proximity and stage of soybeans and other susceptible plants. The maximum corn size is 24 inches if nearby soybeans are over 10 inches or are blooming; if soybeans are smaller or are not in proximity, the maximum corn size is 36 inches. Distinct and Celebrity Plus have a minimum corn size of 4 inches, and Distinct rates change based on corn size (6 oz/acre for corn between 4 and 10 inches and 4 oz/acre for corn between 10 and 24 inches). NorthStar also has a minimum corn size of 4 inches and a maximum of 20 inches or V6 corn, whichever is more restrictive; if drops are used, the limit increases to 36 inches. Remember, many plants are susceptible to dicamba, and certain precautions need to be made to avoid drift to these species.

A few questions always arise 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. Table 2 also lists the postemergence herbicides that can be combined with liquid nitrogen. DO NOT apply postemergence corn herbicides in a liquid fertilizer carrier, as severe corn injury can occur.

Are you thinking about tank-mixing corn herbicides and insecticides? You must consult the respective herbicide and insecticide labels because of the wide variation in tank-mix restrictions. Keep in mind that almost all postemergence ALS-inhibiting herbicides have restrictive intervals with respect to application before or after applications of certain organophosphate (OP) insecticides. Table 4 summarizes label information for several postemergence 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, and dry soil conditions, to name a few.

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.

Air temperature also needs to be considered. Several postemergence corn herbicide labels have application restrictions based on air temperature. 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.Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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