No. 11 Article 4/June 6, 2008

Considerations for Postemergence Herbicide Applications in Corn

Cornfields across areas of Illinois will soon be treated with various postemergence herbicides to control a broad spectrum of weed species. The forecasted high temperatures will accelerate the growth of emerged weeds, making timely applications of postemergence herbicides a bit more challenging. This article highlights a few considerations for postemergence herbicide use in corn.

Herbicide application timing. The governing principle of postemergence herbicide programs is that crop and weeds can coexist for a critical period of time 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. Thus, the goal of postemergence weed management should be to remove interference from the corn crop before the weeds reduce corn grain yield. The key to success is determining when the weeds should be removed by applying postemergence herbicide(s).

Unfortunately, no one can accurately predict which specific day after planting or emergence weeds begin to reduce corn 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 through weed interference. This interval for corn is often recommended to be before weeds exceed 2 to 4 inches in height. If weeds are allowed to remain with the crop past this size range, the risk of crop yield loss substantially increases. Apart from preserving crop yield, another advantage of removing weeds at these suggested sizes is that small weeds are usually much easier to control than large ones.

Staging the corn crop. The labels of most postemergence corn herbicides include application restrictions based on a maximum corn size (specified as corn height or as leaf or collar number, or sometimes both). For product labels that indicate a specific corn height and growth state, be sure to follow the more restrictive of the two. If these restrictions are not followed, there can be substantial injury to the crop that may lead to yield reductions. Adverse environmental conditions (such as prolonged periods of cool air temperatures) can sometimes result in corn plants that are physiologically older than their height would suggest, so be sure to accurately assess plant developmental stage by leaf/collar number in addition to plant height.

Corn response. Corn plants under stress may be more prone to injury from postemergence herbicides. Stress can arise from a number of factors, including cool temperatures and wet soils. Be sure to consult the product label when selecting spray additives to include with postemergence herbicides. Many labels suggest changing from one type of additive to another when the corn crop is experiencing stressful growing conditions. Attempting to save a trip across the field 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.

Herbicide-insecticide interactions. Labels of several postemergence corn herbicides (most commonly ALS-inhibiting herbicides but also some HPPD-inhibiting herbicides) include restrictions with respect to applying the product to corn previously treated with certain soil insecticides. Be sure to consult the respective herbicide label for other restrictions and limitations.--Aaron Hager

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