Date of Graduation

8-2019

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

L. Tom Barber

Committee Member

Jason K. Norsworthy

Second Committee Member

Trenton L. Roberts

Third Committee Member

Edward E. Gbur, Jr.

Keywords

Herbicide, Palmer amaranth, PPO-inhibitors, Resistance

Abstract

Already one of the most troublesome weeds in row crop production in the southern U.S., protoporphyrinogen oxidase (PPO)-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.] was first documented in Arkansas in 2015. Since this confirmation, PPO-resistant Palmer amaranth has been identified throughout the Midsouth. The following research evaluated both current and future herbicide programs for controlling PPO-resistant Palmer amaranth and quantified field-level resistance to PPO-inhibiting herbicides. On-farm research, located in fields with confirmed PPO-resistant Palmer amaranth, was conducted in 2016 and 2017. In preemergence (PRE) herbicide experiments, PPO-inhibiting herbicides still proved useful when combined with herbicides such as metribuzin and/or pyroxasulfone. Interestingly, a decline in control from S-metolachlor (<78%) was observed in PRE experiments, suggesting heavy reliance on this herbicide alone may lead to control failures. In fact, no PRE herbicide program utilizing only one site of action (SOA) provided effective, sustained control of PPO-resistant Palmer amaranth, regardless of their chemistry. No PPO-inhibiting herbicide applied postemergence (POST) provided effective control of PPO-resistant Palmer amaranth (<40%). Postemergence experiments also highlighted the lack of achievable control in glyphosate-resistant soybean [Glycine max (L.) Merr.] vastly in contrast to control provided by options in glufosinate-, 2,4-D-, and dicamba-resistant soybean. Control of PPO-resistant Palmer amaranth in soybean was possible and achieved by multiple effective SOAs PRE followed by a timely POST program containing glufosinate, 2,4-D, or dicamba.

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