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Focus on Soybean Webcast--Off-Target Dicamba Movement

The current production environment of herbicide-resistant (HR) weeds, HR crops, and new herbicide technologies has resulted in increased incidence of damaging offsite herbicide movement and the subsequent need to mitigate this negative aspect of herbicide use and application. The mitigation of this offsite drift is necessary to ensure the longevity of current and forthcoming new weed control tools.

The May 2020 issue of PMN Update (Issue 207) provides the link to a webcast that presents timely information about this subject for soybean producers.

The “Focus on Soybean” webcast, titled “Weather and Off-Target Dicamba Movement”, is presented by Dr. Mandy Bish, Extension Weed Specialist at the University of Missouri.

In this presentation, Dr. Bish hones in on environmental factors that are known to enhance drift and volatilization of auxin herbicides. She spends considerable time on temperature inversions, their causal factors, and how they can be predicted or detected. Highlights from the presentation follow.

•   Why inversions are important when applying auxin herbicides

•   Time of day when inversions are most likely to occur, and why this is important

•   How inversions contribute to particle suspension in the air

•   How weather, wind, topography, and nearby vegetation may affect inversion formation

•   Frequency of inversion occurrence during the growing season at Midsouth locations

•   Sunset time is not a good indicator of inversion potential/formation

•   Tools such as smoke bombs that can be used to detect inversions

•   How air temperature and relative humidity affect herbicide volatility

•   The need to measure wind speed at the site of application of auxin herbicides

This presentation is very timely since it coincides with the POST applications of dicamba herbicides that are being made or soon will be made to soybeans in the Midsouth. Even though producers are already keenly aware of the need to control offsite movement of all herbicides, this narrated presentation provides a very concise video summary of the factors contributing to this process, and the tools they can use to reduce the likelihood of offsite drift of herbicides during and after application.

Click here for a tool that can be accessed to estimate conditions that are suitable for spraying at a designated location. Remember, this and similar tools should only be used for planning purposes or to estimate spraying conditions at a location–i.e., they do not replace checking the weather in a field just prior to an actual spray application.

Composed by Larry G. Heatherly, May 2020,