No. 5 Article 3/May 7, 2010

Timely Applications of Postemergence Corn Herbicides

Cornfields across areas of Illinois soon will be treated with various postemergence herbicides to control a broad spectrum of weed species. Postemergence herbicides are a key part of 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 that herbicide selection can be tailored to a particular field. Postemergence applications minimize the interactions of the herbicide with factors associated with soil (such as soil texture and organic matter content), but they often magnify interactions between the herbicide and prevailing environmental conditions. For weed control to be achieved 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 within the plant, and induce some phytotoxic response. If one or more of these steps is restricted or limited for any reason, 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 postemergence herbicides. Younger, smaller plants usually absorb herbicide more rapidly than older, more mature plants. Many postemergence herbicide labels recommend that applications be made when target weeds are small and warn of reduced effectiveness if applications are made to larger plants. Also, several postemergence herbicide labels suggest that users increase application rates or delay applications if weeds are growing under "adverse environmental conditions," such as prolonged periods without significant precipitation (dry soil) or 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, they also favor it into the crop, which may result in crop injury.

Timely applications of postemergence herbicides are always preferred over delayed applications, but each growing season seems to introduce unique challenges that sometimes interfere with timeliness. The governing principle of postemergence herbicide programs is that the crop and weeds can coexist for some critical period without resulting in yield loss. Numerous research trials conducted over many years have demonstrated that if weeds are removed within this critical period, crop yield is generally not adversely affected. So the goal of postemergence weed management (not necessarily the same as control) should be to remove weed interference from the corn crop before the weeds reduce grain yield.

The key to achieving this goal is determining when the weeds should be removed by applying the postemergence herbicide(s). Unfortunately, no one can accurately predict on which specific day after planting or emergence weeds begin to reduce yield. Weed scientists generally suggest an interval based on either weed size (in inches) or days after crop/weed emergence during which postemergence herbicides should be applied to avoid yield loss. But a problem with suggested intervals is that they are not extremely precise, and they likely vary from year to year and even field to field. For example, earlier research has reported that weed interference began to reduce corn yield as early as the 2-leaf stage and as late as the 14-leaf stage. Many weed scientists suggest applying postemergence herbicides before weeds exceed 2 to 3 inches tall. If they are allowed to remain with the crop past this size range, the risk of yield loss increases substantially.

Almost all postemergence corn herbicides have application restrictions with respect to maximum corn size (specified as height, leaf number, or sometimes both). And remember that some postemergence corn herbicides also have application timing restrictions based on minimum corn size. For example, the label for Status (diflufenzopyr + dicamba) indicates that broadcast applications should be made when corn is between 4 (V2) and 36 (V10) inches tall.--Aaron Hager

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