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Issue No. 14, Article 1/June 27, 2008

Where to Begin?

Many weather-induced challenges continue to plague Illinois farmers across large areas of the state. In particular, timely implementation of weed management practices has been difficult in both corn and soybean fields. As mentioned in previous articles, it's nearly impossible to offer any "one size fits all" suggestions given the broad range of crop sizes. The following thoughts are provided for corn and soybean.

1. The ability to find cornfields where no preemergence herbicide was applied has been all too easy. A recent journey through the central Illinois countryside revealed a surprisingly high frequency of cornfields approaching the V6 growth stage that showed no evidence that any weed management treatments have been applied. Given the ample soil moisture, densities of grass and broadleaf weeds are very high in some fields, and these weeds are often in excess of 4 to 6 inches tall. Fields such as these should be treated as soon as possible to avoid additional loss of crop yield potential. It's nearly impossible to know with certainty how much potential already has been lost in these instances, but the range of 6% to 10% might be close. Using very simple math and an assumption of a 150-bushel corn crop, a 10% loss of yield potential equates to 15 bushels; that value multiplied by $6 a bushel comes out to a loss of $90 per acre because weeds were allowed to compete too long with the crop. If the plan at the beginning of the season was to save money by reducing or eliminating a preemergence herbicide, the amount of money seemingly saved up front (perhaps $20 to $30 an acre?) might have been far exceeded by the loss of yield potential.

2. Reports of crop injury following the application of postemergence herbicides have been common thus far in the season. The adverse growing conditions encountered soon after corn emergence persisted into the beginning of the postemergence herbicide application window, and we speculate that these conditions probably resulted in a relatively thin corn leaf cuticle. A thin cuticle would allow for more rapid herbicide uptake, especially when postemergence treatments included additives such as crop oil concentrates or tank-mixes included herbicides with an oil-based formulation. Also, saturated soils can place the corn crop under additional stress. Crop injury is often more common when postemergence herbicides are applied to plants under stress. For an excellent discussion of some physiological effects of crop stress caused by excessive soil moisture, please see the article by Dr. Emerson Nafziger, "Picking Up the Pieces after a Rough Start to the Season," in issue 13 (June 20, 2008) of the Bulletin.

3. Be sure to scout cornfields before applying postemergence herbicides to accurately determine the crop's growth stage. Adverse environmental conditions, such as prolonged periods of cool air temperatures or wet soils, can result in corn plants that are physiologically older than their height would suggest, so be sure to accurately assess plant developmental stage (i.e., leaf/collar number) in addition to plant height. Pay close attention to the maximum corn stage listed on the respective herbicide label, and do not apply the product if corn exceeds the labeled stage. If you are tank-mixing two or more products, follow the most restrictive corn growth stage listed on any of the tank-mix component labels. For example, glyphosate can be applied broadcast to glyphosate-resistant corn (RR2) up to 30 inches tall, but if it is tank-mixed with Harness or Harness Xtra, the maximum corn height is 11 inches.

4. Many soybean fields were planted without prior tillage or herbicide application to control existing vegetation. Many of the winter annual weed species commonly present in no-till fields have completed their life cycle, but several species of summer annuals have made significant growth to date. While it is advisable to control these weeds before soybeans emerge, better control might be achieved if the burndown herbicide application is delayed for two or three days following planting. The physical disturbance caused by the planting operation may cause the weeds to "shut down" their growth for a time, resulting in less herbicide absorption. Allowing a few days to elapse between the planting operation and the application of the burndown herbicide might result in overall better control of the existing vegetation.--Aaron Hager

Aaron Hager

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