January 29, 2024

On February 7, Dr. Tommy Butts, Extension Weed Scientist with the University of Arkansas, and Tom Hoverstad, Researcher at the University of Minnesota Southern Research and Outreach Center in Waseca, joined UMN Extension Educator-Crops.

Ryan Miller for a discussion on “Making herbicides work better.” Specifically, this session focused on making choices and adjustments to ensure a successful herbicide application.

Of the many pieces that can affect weed control, nozzle selection is critically important as this can drastically influence spray coverage. Nozzles affect droplet size, which is the critical factor influencing coverage. Still, if regulations or drift concerns mandate a larger droplet size, you will likely need to increase the gallon per acre (GPA) rate to get the required coverage.

With a contact herbicide like glufosinate (a.i. in Liberty), droplet size is even more critical than spray volume, although both are key in achieving acceptable control. For example, nozzles that produce large droplet sizes are NOT recommended when applying glufosinate due to the droplet size being too large to achieve the needed level of coverage.

With systemic herbicides like dicamba or glyphosate (a.i. in Roundup), droplet size has less of an effect, but even here, if droplet size is too big and spray volume is too low, you will not have the coverage needed, and control can be reduced.

Time of day, temperature, humidity at the time of application, adjuvants, and tank-mix partners can also affect herbicide effectiveness. For example, it is recommended to apply glufosinate during the day, from around 8 or 9 a.m. to no later than 5 p.m. Ammonium sulfate should be added to the tank to enhance control. Targeting weeds before they exceed 3 to 4 inches in height is essential with any postemergence application and even more so when applying glufosinate.

Using a residual herbicide is critical for weed control success, mainly when dealing with weeds resistant to essential postemergence herbicides and for weeds with more extended emergence patterns, like water hemp and Palmer amaranth. Overlapping residuals, where you apply a residual herbicide at planting and then around 30 days after planting, is also helpful. It is essential to mix up herbicide groups to help delay the development of resistance to these products.

Vital residual programs with multiple modes of action preemergence gave the best weed control and the best return on investment in the analysis conducted by Dr. Butts. Due to increasing issues with herbicide-resistant weeds, farmers must use integrated weed management strategies to control weeds and not just herbicides. These practices include growing crop canopy development, narrow rows, earlier planting dates, cover crops, strategic tillage, cultivation, cleaning equipment, harvest weed seed control, and more.

Finally, make sure you read the label. Herbicide labels provide valuable advice on making practical herbicide applications to give you the best chance of success. Watch this episode and other recordings at University of Minnesota’s Strategic Farming: Let’s talk crops! The webinar series, which is offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and agriculture professionals optimize crop management strategies for 2024. For more information and to register, visit

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