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Title: Glyphosate-resistant horseweed also appears dicamba resistant

item FLESSNER, M - Auburn University
item MCELROY, J - Auburn University
item TOOMBS, J - Auburn University
item BURMESTER, C - Auburn University
item Price, Andrew
item DUCAR, J - Auburn University

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/28/2015
Publication Date: 2/28/2015
Citation: Flessner, M.L., Mcelroy, J.S., Toombs, J.N., Burmester, C.H., Price, A.J., Ducar, J.T. 2015. Glyphosate-resistant horseweed also appears dicamba resistant. Southern Weed Science Society. CDROM.

Interpretive Summary:

Technical Abstract: Glyphosate resistant horseweed (Conyza canadensis) was first reported in the Delaware in 2000 and has since been reported across the U.S. Horseweed with suspected glyphosate resistance was reported in north Alabama and was also suspected to be resistant to dicamba. The objective of this research was to evaluate horseweed populations collected from north Alabama for glyphosate and dicamba resistance and compare their tolerance level to a known susceptible population. Horseweed was collected from three suspected glyphosate resistant populations in north Alabama. Two maturity levels were evaluated—rosette and bolt. Two greenhouse herbicide rate response experiments were conducted to determine glyphosate and dicamba resistance, respectively. Glyphosate tolerance evaluation treatments included 0, 0.56, 1.12, 2.24, 4.5, 9.0, 18.0, and 36.0 kg ae ha-1 glyphosate applied at 280 L ha-1. Dicamba tolerance determination treatments included dicamba at 0, 0.035, 0.07, 0.14, 0.28, 0.56, and 1.12 kg ai ha-1 with and without glyphosate at 1.12 kg ae ha-1. I50 values (herbicide rate resulting in 50% visual control or fresh weight reduction) were compared between populations using 95% confidence intervals. Visual control at the bolt growth stage indicates that Cherokee, DeKalb, and TVS were 38, 3.0, and 5.2 times more tolerant than the Auburn population, respectively; however, DeKalb was not significantly different than Auburn. Fresh weight reduction at the bolt stage indicates that Cherokee, DeKalb, and TVS were 12, 14, and 5.1 times more tolerant than the Auburn population, respectively, and all populations were significantly different than Auburn. Dicamba tolerance results indicate that all populations had similar tolerance to dicamba, when comparing similar growth stages for both data types, with the exception of Cherokee at the rosette growth stage as indicated by visual data, which did result in slightly higher tolerance (~2 fold greater) than the Auburn population. When glyphosate was tank-mixed with dicamba, the response of glyphosate resistant populations (Cherokee, DeKalb, and TVS) was similar to that of dicamba alone within population at both growth stages, with the exception of Cherokee at the rosette growth stage as indicated by fresh weight reduction data. Therefore, adding glyphosate to dicamba did not enhance control of glyphosate resistant horseweed populations (Cherokee, DeKalb, and TVS). Conversely, adding glyphosate to dicamba drastically enhanced control of the Auburn population at both growth stages. It is therefore our conclusion that glyphosate-resistant horseweed populations are now so resistant to glyphosate that applying glyphosate plus dicamba is equivalent to applying dicamba alone.