Weed control in late planted corn

Make weed control in late planted corn a priority.

Figure 1. Corn yield in untreated vs. one-pass postemergence herbicide programs.

Last week I had the opportunity to speak with many growers at our annual Michigan State University Weed Tour, which took place July 10, 2019. The majority of conversations revolved around the topic of late planting due to the challenging season we are currently having. Due to this wet, challenging season, for many the window for applying a soil-applied preemergence herbicide immediately after corn planting was not an option. Given this reality, those who planned to use a soil-applied residual herbicide treatment in corn are forced to make postemergence applications to the weeds and corn.

With this topic at hand, I “mined” the last five years of the MSU Weed Science Program’s data for studies evaluating one-pass postemergence (EPOST) strategies for weed control in corn. Post-herbicide applications were made on V3-V4 corn (ranging from 6-12 inches tall) and weeds were 4 inches tall or less. One-pass postemergence herbicide programs yielded on average 38, 75, 150, 19 and 79 bushels per acre greater than untreated control plots in 2014-2018 (see the graph). For specific herbicide treatments used in these studies, visit the Annual Results tab of the MSU Weed Science website.

The main concern when making delayed postemergence herbicide applications in late planting seasons affected by difficult weather is making the application early enough to protect crop yield. If weeds compete with corn for too long yield loss will occur. In an optimal year, if a preemergence herbicide application has not been made the early postemergence herbicide application should be made when weeds are less than 4 inches tall. This year was far from optimal and weed sizes in many fields have surpassed this stage, given that Table 1H in the “2019 MSU Weed Control Guide” from MSU Extension lists maximum weed and crop heights for postemergence applications to corn.

Hopefully, we will not have a repeat of the difficult 2019 season anytime soon and planned, robust two-pass herbicide programs with timely preemergence herbicide applications can be made that preserve corn yield and buffer against environmental uncertainty.

MSU Extension offers additional educational resources and programs to help farmers as they deal with delayed planting seasons at https://www.canr.msu.edu/agriculture/delayed-planting-resources.

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