Glyphosate-Resistant Common Ragweed (Ambrosia artemisiifolia) Control with Postemergence Herbicides and Glyphosate Dose Response in Soybean in Ontario

Abstract Field trials were conducted in Ontario in 2013 and 2014 in soybean to determine the efficacy of POST herbicides on common ragweed resistant to group 2 and group 9 herbicides. Glyphosate dose-response experiments were conducted in the field on two resistant common ragweed populations and one susceptible population. None of the POST herbicides evaluated provided 80% control of glyphosate-resistant (GR) common ragweed. The most effective POST herbicide mixture was glyphosate (Monsanto Canada Inc., 67 Scurfield Blvd., Winnipeg, Manitoba, Canada) plus fomesafen(Syngenta Canada Inc., 140 Research Lane, Research Park Guelph, Ontario, Canada), which provided 68 to 98% control of GR common ragweed. Chlorimuron, cloransulam, imazethapyr, and thifensulfuron provided control similar to glyphosate alone. An application of glyphosate/fomesafen reduced biomass by as much as 95%. Glyphosate plus acifluorfen reduced GR common ragweed biomass by as much as 92%. The remaining POST herbicide tank mixes evaluated reduced GR common ragweed biomass by less than 80%. Glyphosate plus bentazon, glyphosate plus chlorimuron, and glyphosate plus thifensulfuron resulted in soybean yields similar to the weedy control, with yield reductions of 70, 62, and 73%, respectively. An application of glyphosate plus fomesafen or glyphosate/fomesafen had the lowest soybean yield reductions of 29 and 34%, respectively. The resistant biotype required a 2- to 28-fold increase in glyphosate dose compared to the susceptible population to achieve 50% control. Nomenclature: Acifluorfen; bentazon; chlorimuron; cloransulam; fomesafen; glyphosate; imazethapyr; thifensulfuron; common ragweed, Ambrosia artemisiifolia L. ; soybean, Glycine max. L. Merr. Resumen En 2013 y 2014 en Ontario, se realizaron estudios de campo en soja para determinar la eficacia de herbicidas POST sobre Ambrosia artemisiifolia resistente a herbicidas de los grupos 2 y 9. Experimentos de respuesta a dosis de glyphosate fueron realizados en el campo con dos poblaciones resistentes y una población susceptible de A. artemisiifolia. Ninguno de los herbicidas POST evaluados brindó >80% de control de A. artemisiifolia resistente a glyphosate (GR). Las mezclas de herbicidas POST más efectivas fueron glyphosate más fomesafen, las cuales brindaron 68 a 98% de control de A. artemisiifolia GR. Chlorimuron, cloransulam, imazethapyr, y thifensulfuron brindaron un control similar a glyphosate solo. Una aplicación de glyphosate/fomesafen redujo la biomasa hasta 95%. Glyphosate más acifluorfen redujo la biomasa de A. artemisiifolia GR hasta 92%. Glyphosate más bentazon, glyphosate más chlorimuron, y glyphosate más thifensulfuron resultaron en rendimientos de soja similares al testigo con malezas, con reducciones en el rendimiento de 70, 63, y 73%, respectivamente. Una aplicación de glyphosate más fomesafen o glyphosate/fomesafen tuvieron las menores reducciones en el rendimiento de la soja con 29 y 34%, respectivamente. El biotipo resistente requirió un incremento de 2 a 28 veces en la dosis de glyphosate al compararse con la población susceptible para alcanzar 50% de control.