How to make herbicides work better

March 04, 2024

On Feb. 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 key factor influencing coverage, but if regulations or drift concerns mandate use of a larger droplet size, you will likely need to increase the gallon per acre (GPA) rate to get the coverage needed.

With a contact herbicide like glufosinate (a.i. in Liberty), droplet size is even more important 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 and 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 when applying glufosinate to enhance control. Targeting weeds before they exceed 3 to 4 inches in height is important with any postemergence application and even more so when applying glufosinate.

Use of a residual herbicide is critical for weed control success, particularly when dealing with weeds resistant to key postemergence herbicides and for weeds with longer emergence patterns, like waterhemp and Palmer amaranth. Overlapping residuals, where you apply a residual herbicide at planting and then around 30 days after planting, is also useful. It is important to mix up herbicide groups, however, to help delay the development of resistance to these products as well.

Finally, make sure you read the label. Herbicide labels provide valuable advice on making effective 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! webinar series, offered Wednesdays through March, features discussions with specialists to provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2024. For more information and to register, visit

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program.