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Issue No. 7, Article 9/May 6, 2005

Postemergence Corn Herbicides--Considerations to Keep in Mind

The adverse growing conditions across much of Illinois the past couple of weeks have slowed corn development. If there is a potential bright spot to the cool air and soil temperatures, perhaps it is that soil-applied herbicides appear to be holding back weeds reasonably well. With the return to more average air temperatures over the next few days, attention will likely focus on applications of postemergence corn herbicides. The potential for corn injury from postemergence herbicides will probably remain high for several days after air temperatures return to more average levels.

Postemergence herbicides are integral to an integrated weed management program. Applications made after crops and weeds have emerged allow for identification of the weed species present as well as the severity of infestation, so herbicide selection can be tailored to the particular field. Postemergence herbicide applications minimize interactions of the herbicide with factors associated with soil (such as texture and organic matter content) but often magnify interactions between the herbicide and prevailing environmental conditions. To achieve weed control with postemergence herbicides, the herbicide must come in contact with the target, be retained on the leaf surface before absorption into the plant, be able to reach the site of action in the plant, and finally induce some phytotoxic response. If for any reason one or more of these steps is restricted or limited, the level of weed control can be expected to decline.

Plant age and size, relative humidity, soil moisture, and temperature are factors that influence absorption of post-emergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend applications be made when target weeds are small and caution of reduced effectiveness if applications are made to larger plants. More and more postemergence herbicide labels are also cautioning users to delay applications if weeds are under "adverse environmental conditions." Examples of such conditions include prolonged periods without significant precipitation (dry soil) and low air temperatures. On the other hand, high relative humidity, adequate soil moisture, and moderate to warm air temperatures all favor enhanced herbicide absorption. Remember that if conditions favor rapid herbicide absorption into weeds, conditions are also favorable for enhanced absorption into the crop, which may result in crop injury.

Almost all postemergence corn herbicides have application restrictions with respect to maximum corn size (specified as height, leaf number, or sometimes both). Table 4, reproduced from the 2005 Illinois Agricultural Pest Management Handbook, summarizes information for many postemergence corn herbicides, including maximum corn size for broadcast applications.

Keep in mind also that some postemergence corn herbicides have application timing restrictions based on minimum corn size. 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 high. Table 5 lists the postemergence corn herbicides that have minimum corn size restrictions on their respective labels.

With high fuel costs, saving trips across a field can potentially add up to significant savings. Attempting to save a trip by applying a postemergence corn herbicide with a liquid nitrogen fertilizer solution (such as 28% UAN) as the carrier is not advisable. 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 applications 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.

Can postemergence corn herbicides be tank-mixed with insecticides? For the best answer, consult the respective herbicide and insecticide labels, because tank-mix restrictions vary widely. Keep in mind that almost all post-emergence ALS-inhibiting herbicides have restrictive intervals with respect to application before or after applications of certain organophosphate (OP) insecticides. Table 6, reproduced from the 2005 Illinois Agricultural Pest Management Handbook, summarizes label information for several postemergence herbicides with respect to time intervals before or after foliar applications of OP insecticides.

--Aaron Hager and Dawn Nordby

Aaron Hager
Dawn Refsell

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