ST. PAUL, MN, February 2, 2022 — Controlling weeds in crop production is challenging as herbicide resistance increases and effective herbicides become more limited. “Weed management is all about fundamentals,” explained Dr. Tom Peters, Extension sugarbeet agronomist and weed scientist at the University of Minnesota and North Dakota State University.
Those fundamentals include understanding weed biology. Weed identification is not easy, since what you see in the field may be very different from a photo. “You can’t control the weeds if you’re not sure what they are,” reinforced Peters.
In addition to identification, learning about the life cycles, emergence patterns and reproduction characteristics of the different weeds is also important. Together, these fundamentals will help crop producers select effective tools for their weed management programs.
After the dry 2021 growing season, will herbicide carryover be an issue in 2022? In a dry year, there will be more herbicide residual in the soil than in a wet year. Peters’ rule of thumb is to add the rainfall totals from June 1 to September 1. If the total is less than six inches, then growers should look at the herbicides applied in 2021, crops planned for 2022, and use that information to investigate the potential for carryover.
Herbicide resistance continues to be a major concern in Minnesota. Dr. Debalin Sarangi, University of Minnesota Extension weed scientist, began screening waterhemp populations for resistance to several postemergence herbicides in 2020. He is considering a population resistant when more than 40% of its individuals can survive three times the normal herbicide application rate. Results from 2020 showed significant levels of resistance to two of the eight herbicides evaluated and lower levels for three more herbicides. In addition to resistance to single herbicides, multiple resistance is on the rise, making weed management even more challenging.
With increasing herbicide resistance, using an integrated weed management approach is critical. “It’s very important to go with a strong foundation, whether you are a corn or soybean grower,” according to Sarangi. He recommended relying on preemergence (PRE) herbicides because that gives growers a clean start. Then layering PREs is a good option for controlling late emerging weeds.
“Tillage and cultural control are part of this same conversation,” added Peters. Tillage may not eliminate resistance concerns, but it’s another tool that complements diversified herbicide rotations.
For more information on weed management recommendations from the University of Minnesota Extension, visit extension.umn.edu/weed-management.
University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series, offered Wednesdays through March, features discussions with specialists that provide up-to-date, research-based information to help farmers and ag professionals optimize crop management strategies for 2022. For more information and to register, visit z.umn.edu/strategic-farming.