Changing Landscape of Waterhemp Herbicide Resistance in Wisconsin: A Comparative Analysis Over Time

A comprehensive assessment of herbicide resistance in waterhemp accessions to commonly used corn and soybean herbicides over time is crucial for documenting shifts in herbicide resistance patterns and identifying effective management options. Therefore, a greenhouse study was conducted to evaluate the response of statewide waterhemp accessions collected in 2023 to commonly used preemergence (PRE) and postemergence (POST) herbicides and to assess changes in resistance over a five-year period by comparing these results with previous statewide data from 2018. One hundred percent, 88%, 83%, 41%, and 24% of 2023 accessions exhibited ≥50% survival following the POST application of atrazine, 2,4-D, dicamba, fomesafen, and mesotrione at the 1× rate, respectively. Glufosinate provided complete control, with 0% survival and ≥97% biomass reduction across all accessions. The 1× application of mesotrione, metribuzin, and S -metolachlor applied PRE resulted in >95% density reduction in all accessions. The 1× application of atrazine and fomesafen resulted in <90% density reduction in 50% and 82% of accessions, respectively. The 2023 accessions showed a significant increase in survival (%) at the 1× rate for several POST herbicides, including 2,4-D (from 11% to 52%), dicamba (from 8% to 49%), atrazine (from 11% to 81%), fomesafen (from 3% to 21%), and mesotrione (from 2% to 11%), compared with 2018. Similarly, a change in waterhemp density reduction (%) was observed between 2018 and 2023 accessions for PRE herbicides, including atrazine (from 84% to 91%), fomesafen (from 97% to 81%), and S -metolachlor (from 98% to 95%). No changes were observed in the density reduction of the 2018 and 2023 accessions in response to mesotrione and metribuzin at the 1× and 3× rates. The rapid adaptation of waterhemp herbicide resistance within five years highlights the urgent need to incorporate effective integrated weed management strategies in Wisconsin cropping systems.