Issue No. 11, Article 1/June 5, 2009
Postemergence Herbicide Applications: Timings and Tank-Mixes
Postemergence herbicides are integral to an integrated weed management program. Applying herbicide after crops and weeds have emerged allows you to identify the weed species present and assess the infestation so you can tailor herbicide selection for each field. Compared with soil-residual herbicides, postemergence herbicides minimize interactions with factors associated with soil (such as soil texture and organic matter content), but they tend to magnify interactions with 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 prior to absorption into the plant, be able to reach the site of action within 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.
The goal of a postemergence weed management program should be to remove weed interference from the corn crop before the weeds reduce corn grain yield. The key to success is determining when the weeds should be removed cia application of the postemergence herbicide(s). Unfortunately, no one can accurately predict which specific day after planting or emergence that 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 from weed interference. The 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 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.
Tank-mixing two or more postemergence herbicides can provide several advantages over single-product applications. Perhaps one of the most obvious is that the spectrum of weeds controlled can be broadened. Before the adoption of glyphosate-resistant crops, tank-mixing postemergence herbicides was common. Grass-control herbicides were often tank-mixed with broadleaf-specific herbicides to create a "one-pass" tank-mix. Tank-mixes generally have been less common in the past decade, as glyphosate alone has been an effective product for control of many broadleaf and grass weed species. However, with the occurrence of glyphosate-resistant weed populations and weed species inherently less sensitive to glyphosate, it will become increasingly common to tank-mix products with glyphosate to control these challenging species. In glyphosate-resistant corn, tank-mixing growth regulators (such as dicamba or 2,4-D) or HPPD inhibitors (such as mesotrione, topramezone, or tembotrione) with glyphosate can improve control of glyphosate-resistant waterhemp and other tough-to-control broadleaf weed species, such as morningglory and giant ragweed. Be sure to follow all label restrictions and additive recommendations when tank-mixing postemergence herbicides.
The labels of most postemergence corn herbicides include application restrictions based on a maximum corn size (specified as corn height, 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, quite often 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 (leaf/collar number) in addition to plant height. Also be sure to follow the more restrictive corn growth stage listed when two or more products are tank-mixed. For example, glyphosate can be applied broadcast to glyphosate-resistant corn through the V8 stage or until corn is 30 inches tall when applied alone, but only to corn 12 inches tall when tank-mixed with atrazine.
Corn plants under stress conditions 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 under stressful growing conditions.Aaron Hager