Date of Graduation

12-2018

Document Type

Dissertation

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Jason K. Norsworthy

Committee Member

Ed Gbur

Second Committee Member

Bob Scott

Third Committee Member

Robert Wiedenmann

Fourth Committee Member

Trent Roberts

Keywords

Glufosinate, Herbicide Interactions, Weed Control

Abstract

While glufosinate-resistant weeds have not yet been identified in U.S. row crops, rapid evolution of glyphosate-resistance, and resulting cost to U.S. farmers, demonstrates the need to responsibly steward the limited number of herbicides available in agricultural systems. Field and laboratory experiments were conducted to: 1) Identify herbicide interactions that can occur in Enlist™ and Roundup Ready® Xtend® systems; 2) Identify herbicide interactions that occur when glufosinate is mixed with fomesafen, clethodim, and glyphosate; 3) Determine the optimum rate structure and application timings when multiple POST applications of glufosinate are applied to LibertyLink soybean; 4) Use 14C techniques to determine why herbicide interactions (e.g., antagonism) occur in barnyardgrass and Palmer amaranth; and 5) Use data obtained from field experiments to refine herbicide-resistance simulation models for Palmer amaranth and barnyardgrass. Glufosinate + glyphosate, glufosinate + clethodim, glyphosate + 2,4-D, and glyphosate + dicamba were all antagonistic when applied to barnyardgrass. Few antagonistic interactions were observed for Palmer amaranth control. Results from various experiments show that nozzle selection (i.e., droplet size and spray volume) is important for maximizing efficacy of glufosinate plus 2,4-D, clethodim, dicamba, fomesafen, or glyphosate. When large weeds (≥10-cm) were present in the field, two applications of glufosinate at 882 g ai ha-1 made 7-10 days apart maximized weed control and soybean yield. When glufosinate was mixed with 14C-glyphosate, reduced uptake and transport were observed in barnyardgrass and Palmer amaranth. Dicamba also reduced uptake of 14C-glyphosate in barnyardgrass, and potentially explains antagonism observed in field experiments. Glyphosate-resistance simulation models for barnyardgrass demonstrated antagonism of glyphosate by synthetic auxin herbicides increased the risk of evolving resistance 17-fold over a 30 yr period. Although glufosinate + glyphosate was also antagonistic in the field, the use of the mixture resulted in minimal risk of resistance in barnyardgrass. The Palmer amaranth resistance model suggests that intense management focused on depleting the soil seedbank is needed to mitigate the risk of evolving glufosinate-resistance, as all herbicide management programs evaluated in the model resulted in some level glufosinate-resistance after 30 yr.

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