Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Jason Norsworthy

Committee Member

Thomas Butts

Second Committee Member

Michael Popp

Third Committee Member

Trent Roberts

Fourth Committee Member

Andy Mauromoustakos

Keywords

dicamba, glufosinate, herbicide programs, sequential herbicide application, weed management

Abstract

The commercial launch of XtendFlex® crops enabled the use of dicamba, glufosinate, and glyphosate in-season. Utilizing herbicides that target different sites of action within troublesome weeds has been a tactic proposed to mitigate the likelihood of target-site resistance evolving; however, if interactions of the herbicides are detrimental to control of weedy species the likelihood of metabolic resistance increases. The objective of this research was to: 1) optimize efficacy and economic benefits of dicamba, glufosinate, and glyphosate; 2) characterize any interactions that were observed; 3) understand the mechanisms responsible for the reductions in weed control; 4) attempt to overcome interactions that were detrimental to weed control; 5) identify if any Palmer amaranth populations were resistant to dicamba or glufosinate in Arkansas and identify alternative control methods. Label restrictions do not allow for mixtures of dicamba and glufosinate to be applied; therefore, evaluation of sequential application intervals and sequences were evaluated. When glufosinate was applied prior to dicamba from 6 hours to 7 days often a reduction in control was observed when compared to dicamba followed by (fb) dicamba or dicamba fb glufosinate at the 14-day interval. Utilizing 14C-herbicides a reduction in dicamba translocation occurred when a prior glufosinate application was made and thus a reduction in dicamba translocation was attributed to reduction in Palmer amaranth control. When dicamba was applied prior to glufosinate a reduction in control was often observed when applications were made at intervals less than 7 days. The reduction in control was attributed to rapid reduction of Palmer amaranth groundcover following a dicamba application, thus allowing for less surface area for the later applied glufosinate to come in contact with. Generally, from field experiments, the use of dicamba fb dicamba at a 14- to 21-day interval or dicamba fb glufosinate at the 14-day interval provided the highest level of Palmer amaranth control and highest net benefit to producers. Palmer amaranth populations in Arkansas were also found to harbor resistance to glufosinate and auxin herbicides. Alternative integrated weed management strategies (e.g. crop rotation, harvest weed seed control, cover crops, etc.) should be implemented to mitigate the spread of these biotypes as well to mitigate resistance evolving in other geographies.

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