Article originally appeared on Illinois farmdoc.

The continual evolution of weed species and populations resistant to herbicides from one or more site-of-action groups represents one of the most daunting challenges facing Illinois soybean producers. Waterhemp has evolved resistance to herbicides from more site-of-action groups than any other Illinois weed species, including resistance to Group 15 herbicides (products such as Dual II Magnum, Zidua, Warrant, Outlook, etc.). Soil-residual herbicides are components of an integrated weed management program that provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides. However, the relatively recent discovery of resistance to Group 15 herbicides is yet another example of how waterhemp continues to challenge herbicide-only management programs.

Compared with resistance to foliar-applied herbicides, resistance to soil-applied herbicides generally is more difficult to detect in the field. Resistance to foliar-applied herbicides manifests as treated plants (assuming appropriate application rate and timing) that are not controlled, whereas resistance to soil-applied herbicides manifests as a reduced duration of residual control. Because the duration of residual control can also vary substantially from field to field and year to year due to climatic and soil factors, it is difficult for farmers to “observe” resistance to preemergence herbicides. Consequently, we hypothesize that waterhemp resistance to Group 15 herbicides is more extensive in the state than Illinois soybean producers realize.

Our weed science program has characterized two Illinois waterhemp populations resistant to Group 15 herbicides and, as stated previously, we suspect this type of resistance is more widespread. Group 15 herbicides are commonly premixed with other herbicides; a search of Group 15 products labeled for use in Illinois revealed thousands of commercially available products that contain one or more of these active ingredients. Additionally, soybean farmers who rely on a “layered residual” to extend weed control following the postemergence herbicide application have only Group 15 herbicides from which to select. A more comprehensive assessment of Group 15 resistance across the soybean producing areas in Illinois would provide the data necessary to refine herbicide recommendations to help slow the evolution of additional Group 15-resistant populations.

Financial support from the Illinois Soybean Association provides us with the opportunity to survey Illinois waterhemp populations for resistance to Group 15 herbicides. Personnel within our weed science program will make random waterhemp seed collections from fields across the state, but we invite you to become an active participant in our research project. If you would like to submit waterhemp samples for our upcoming greenhouse research project, please download this sample submission form before you go to the field. If you are unsure how to collect the samples, please watch this informative video from our group: https://go.illinois.edu/waterhempsurvey

Hager, A. “Survey for Resistance to Group 15 Herbicides in Illinois Waterhemp.” Department of Crop Sciences, University of Illinois, September 8, 2023.