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Title: Glyphosate-resistant horseweed (conyza canadensis) control with dicamba in Alabama

Author
item FLESSNER, MICHAEL - Auburn University
item MCELROY, SCOTT - Auburn University
item MCCURDY, JAMES - Auburn University
item TOOMBS, JORDAN - Auburn University
item WEHTJE, GLENN - Auburn University
item BURMESTER, CHARLES - Auburn University
item Price, Andrew
item DUCAR, JOYCE - Auburn University

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/27/2015
Publication Date: 10/1/2015
Publication URL: https://handle.nal.usda.gov/10113/62374
Citation: Flessner, M.L., Mcelroy, S., Mccurdy, J.D., Toombs, J.N., Wehtje, G.R., Burmester, C.H., Price, A.J., Ducar, J.T. 2015. Glyphosate-resistant horseweed (conyza canadensis) control with dicamba in Alabama. Weed Technology. 29(4):633-640.

Interpretive Summary: Widespread horseweed resistance to glyphosate has resulted in the use of dicamba as an alternative treatment. Horseweed populations in Cherokee and DeKalb counties in northern Alabama were not well controlled following glyphosate and dicamba treatments. This research evaluates horseweed populations for glyphosate and dicamba resistance and tank-mixing thereof for enhanced control efficacy. Data confirmed that Cherokee and DeKalb horseweed populations were 3.0 to 38 times more resistant to glyphosate than the susceptible population, according to population, data type, and growth stage at treatment. All populations had similar tolerance to dicamba, with the exception of Cherokee treated at the rosette growth stage as indicated by visual data, which did result in slightly higher tolerance (~2 fold greater) than the susceptible population.

Technical Abstract: Widespread horseweed resistance to glyphosate has resulted in the use of dicamba as an alternative treatment. Horseweed populations in Cherokee and DeKalb counties in northern Alabama were not well controlled following glyphosate and dicamba treatments. This research evaluates horseweed populations for glyphosate and dicamba resistance and tank-mixing thereof for enhanced control efficacy. Populations used for resistance determination were tested at rosette and bolt growth stages. GR evaluation treatments ranged from 0 to 36.0 kg ae ha-1. Dicamba tolerance determination treatments ranged from 0 to 1.12 kg ai ha-1 without and with glyphosate at 1.12 kg ae ha-1. Data confirmed that Cherokee and DeKalb horseweed populations were 3.0 to 38 times more resistant to glyphosate than the susceptible population, according to population, data type, and growth stage at treatment. All populations had similar tolerance to dicamba, with the exception of Cherokee treated at the rosette growth stage as indicated by visual data, which did result in slightly higher tolerance (~2 fold greater) than the susceptible population. When glyphosate was tank-mixed with dicamba, the response of GR populations was similar to that of dicamba alone. Therefore, any potential herbicide resistance management benefit of the tank-mixing dicamba with glyphosate is likely negated when attempting to control GR horseweed. Conversely, adding glyphosate to dicamba drastically enhanced control of the susceptible population at both growth stages.