Date of Graduation

12-2015

Document Type

Thesis

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor

Nilda R. Burgos

Committee Member

Jason K. Norsworthy

Second Committee Member

Robert C. Scott

Third Committee Member

Kristofor Brye

Fourth Committee Member

Andronikos Mauromoustakos

Fifth Committee Member

Alan Hopkins

Keywords

Biological sciences; Arkansas; Hppd-inhibitors; Palmer amaranth; Resistance

Abstract

Herbicide-resistant weeds in Arkansas cause problems for growers. Up-to-date information and new technologies can help plan mitigation strategies to slow resistant weeds. The objectives of this research were to provide a ‘snapshot’ of herbicide-resistant Echinochloa spp. in rice producing counties, determine how much resistance has spread across the state, and understand the effectiveness of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides for control of glyphosate-resistant (GR) Palmer amaranth relative to commercial standards currently labeled in soybean. To assess the prevalence of Echinochloa spp. resistance, 82 samples were collected from 23 rice producing counties in 2010. The samples were tested for resistance to commonly used rice herbicides: propanil, quinclorac, imazethapyr, fenoxaprop, clomazone, and glyphosate. Of the 82 samples collected, 29 were resistant to propanil, 13 were resistant to quinclorac and 9 samples were resistant to both propanil and quinclorac. Accessions were also treated with 0.5x the labeled field rate for glufosinate and isoxaflutole to determine background variation in sensitivity among populations to these herbicides as Echinochloa is among the major weeds in crops where these herbicides are used. No resistance to imazethapyr, clomazone, fenoxaprop, or glyphosate was observed; likewise all accessions were sensitive to glufosinate or isoxaflutole. One strategy for controlling herbicide-resistant weeds is the use of transgenic crops. The expected release of soybean in 2016 and cotton in 2020 with resistance to HPPD-inhibiting herbicides provide alternative mechanisms-of-action to control weeds. Experiments were conducted in 2010 and 2011 to determine the efficacy of HPPD-inhibiting herbicides as a preemergence (PRE) option for Echinochloa spp. and Palmer amaranth control and as a postemergence (POST) option with and without glyphosate or glufosinate. The PRE applied HPPD-inhibiting herbicides do not carry the residual control as the current industry standards; however they are still capable of providing 4 weeks of control of Palmer amaranth and Echinochloa spp.. For both years in the POST trials, all treatments, except glyphosate alone, provided >90% control of 2.5- to 10-cm tall GR-Palmer amaranth at 3 wk after treatment. When herbicides were applied to larger Palmer amaranth, 15- to 25-cm tall, control with isoxaflutole + glyphosate, tembotrione + glufosinate, and tembotrione + glyphosate were greater than 90%. Applications made to Palmer amaranth larger than 25 cm was not effective (< 80% control).

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