Investigation of inquiries on weed control efficacy of XtendiMax® herbicide with VaporGrip® technology

Abstract Herbicide resistance in weeds significantly threatens crop production in the United States. The introduction of dicamba-resistant soybean and cotton stacked with other herbicide tolerance traits has provided farmers with the flexibility to use multiple herbicide options to diversify their weed management practices and delay resistance evolution. XtendiMax ® herbicide with VaporGrip ® Technology is a dicamba formulation registered for use in dicamba-resistant soybean and cotton by the United States Environmental Protection Agency (US-EPA). One of the terms of its registration includes evaluating inquiries on reduced weed control efficacy by growers or users of XtendiMax for suspected weed resistance. A total of 3555 product performance inquiries (PPIs) were received from 2018 to 2021 regarding reduced weed control efficacy by dicamba. Following Norsworthy criteria recommended by US-EPA, for screening of suspected resistance in the field, a total of 103 weed accessions from sixty-three counties in 13 states were collected for greenhouse testing over those 4 years. Collection of weed accession(s) for greenhouse testing was made only in states where resistance to dicamba was not yet confirmed in the weed species under investigation. The accessions, which consisted primarily of waterhemp and Palmer amaranth, were treated with dicamba at 560g ae ha -1 and 1120g ae ha -1 rates. All weed accessions except for an accession each of Palmer amaranth and waterhemp, were controlled ≥90% by dicamba at 21 days after treatment in the greenhouse.