Waterhemp is increasingly difficult to manage as it increases resistance to postemergence herbicides. Dr. Debalin Sarangi, University of Minnesota Extension weed specialist, sampled waterhemp populations from 47 Minnesota counties and screened them against eight postemergence herbicides to define the issue.
Nearly all of the populations were resistant to the herbicides Raptor and glyphosate. Between 22 and 47 % of the populations were resistant to Callisto, Flexstar, and atrazine. A small number were resistant to dicamba and 2,4-D, and none were resistant to Liberty. However, nearly half of the waterhemp populations were less sensitive to several herbicides, so the potential for resistance development exists.
In this survey, 91% of the populations displayed multiple resistance, where a population is resistant to at least two herbicides. Most concerning were the populations that were resistant to the majority of the screened herbicides, leaving few remaining postemergence options.
Producers need a variety of tools and strategies to preserve herbicide effectiveness. Using preemergence (PRE) herbicides to start clean and stay clean paves the way for successful postemergence applications and higher yields.
PREs are an important tool, but weather can complicate applications. If conditions are too dry, weed control may be delayed because of a lack of moisture to activate the herbicide. On the flip side, excessive rainfall increases the risk for surface runoff or leaching.
Use cultural practices to compete with weeds. Planting soybeans early and in narrower row spacings can lead to earlier canopy closure, better competition with weeds, and less late-season waterhemp germination.
“Confuse the weeds,” urges Dr. Peters, University of Minnesota and North Dakota State University Extension sugarbeet agronomist. Consider a rotation of corn, soybeans, and wheat. The differences in planting dates, row spacings, tillage practices, how and when fertilizer is applied changes the environment for the weeds and makes it less predictable. An expanded crop rotation also means more diverse herbicide sites of action are available.
Cover crops provide benefits including weed suppresion. Dr. Eric Yu, University of Minnesota Extension educator, studied how well cereal rye (CR) suppressed waterhemp at two soybean planting dates. Weed suppression largely depended on biomass production. As termination time was delayed, particularly in the late-planted soybeans, biomass production and weed suppression increased. However, soybean yields decreased.
Even though CR produced half the biomass in the earlyplanted soybeans, 80% of the weeds were suppressed with at-plant CR termination and that increased to 90% when a PRE was added. Yields were highest in the early-planted early when CR was terminated at or after planting.
Electrocuting weeds has been around since 1979. Fast forward to today and The Weed Zapper, which generates electricity and transfers it through a bar to weeds. Plants immediately wilt, but a weed like waterhemp may also eventually regrow from the lower part of the plant. Despite the regrowth potential, seed viability is significantly reduced. Electrocution may be a good fit for eliminating escapes, although control is inconsistent and less effective on grasses and kochia.
Don’t forget tillage. Your strategy will vary based on whether your objectives are to prepare a seedbed or control weeds. Finally, interrow cultivation is still be a viable weed management tool.
Group 15 herbicides, such as Dual, Warrant, Surpass, Outlook, and Zidua, are important for waterhemp control, particularly when it is resistant to many postemergence herbicides. However, we have been relying heavily on this group heavily and that selection pressure could result in herbicide resistance, as it has in Indiana and Illinois.
Use herbicides from other groups to add diversity and slow resistance development. To demonstrate how frequently Group 15 herbicides are used, Peters urges growers to map out pigweed control programs across the rotation. How many times are the herbicides from different groups being used? Mix it up and take advantage of the chemical diversity that’s available to maintain Group 15 effectivenss.
Current ag economics and uncertain world events make it a difficult time for budget decision-making. While it may be tempting to back off on weed management in 2025, that decision could affect yields not only this year but for 5 or 10 years or longer. That’s because weed seeds remain viable in the soil, some for more than 10 years. Controlling the weed seed bank now pays off with better yields and fewer weed management headaches in the future.
For more information from University of Minnesota Extension, visit extension.umn.edu/ crop-production.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.
