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

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 of having 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 on dicamba-resistant soybean and cotton crops by the U.S. Environmental Protection Agency (EPA). One of the terms of its registration includes an evaluation of inquiries on reduced weed control efficacy by growers or users of XtendiMax for suspected weed resistance. A total of 3,555 product performance inquiries (PPIs) were received from 2018 to 2021 regarding reduced weed control efficacy by dicamba. Following the criteria recommended by EPA for screening of suspected resistance in the field, a total of 103 weed accessions from 63 counties in 13 states were collected for greenhouse testing over those 4 yr. Weed accessions for greenhouse testing were collected 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 rates of 560 g ae ha −1 and 1,120 g ae ha −1 . All weed accessions, except for one accession each of Palmer amaranth and waterhemp, were controlled by ≥90% with dicamba at 21 d after treatment in the greenhouse.