Control of glyphosate-resistant horseweed (Conyza canadensis) with tiafenacil mixes in corn

Four field experiments were completed in commercial corn fields during 2019 and 2020 to determine glyphosate-resistant (GR) horseweed control in corn with tiafenacil alone or in combination with bromoxynil, dicamba, or tolpyralate applied preplant (PP). Corn planted 1 to 10 d after herbicide application was not injured with any of the herbicides tested. GR horseweed interference reduced corn grain yield 32% when left uncontrolled. Herbicides reduced GR horseweed interference and resulted in corn grain yield that was similar to the weed-free control. Glyphosate (900 g ae ha-1) + tiafenacil at 12.5, 25, and 37.5 g ha-1 controlled GR horseweed 63%, 68%, and 72% at 4 wk after treatment (WAT) and decreased GR horseweed density 64%, 43%, and 83% and dry biomass 69%, 55%, and 83%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha-1 plus bromoxynil (280 g ai ha-1) controlled GR horseweed 81%, 88%, and 87% at 4 WAT and reduced GR horseweed density 82%, 94%, and 93% and dry biomass 93%, 93%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha-1 plus dicamba (300 g ai ha-1) controlled GR horseweed 86%, 88%, and 88% at 4 WAT and decreased GR horseweed density 76%, 89%, and 86% and dry biomass 94%, 98%, and 98%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ha-1 plus tolpyralate (30 g ai ha-1) controlled GR horseweed 90%, 90%, and 91% at 4 WAT and decreased GR horseweed density 93%, 91%, and 95% and dry biomass 98%, 97%, and 97%, respectively. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p controlled GR horseweed 95% and 100% at 4, 8, and 12 WAT and caused 99% and 100% density or biomass reduction, respectively. Nomenclature: Atrazine; bromoxynil; dicamba; dimethenamid-p; glyphosate; saflufenacil; tiafenacil; tolpyralate; horseweed; Conyza canadensis (L.) Cronquist; corn; Zea mays L.