Date of Graduation

5-2022

Document Type

Thesis

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Jason K. Norsworthy

Committee Member

Thomas R. Butts

Second Committee Member

Trenton L. Roberts

Third Committee Member

Nick R. Bateman

Keywords

Acetochlor, Barnyardgrass, Crop safety, Pyroxasulfone, Weed control, Weedy rice

Abstract

The development of herbicide resistance and the lack of effective herbicides to control problematic weeds has caused Arkansas rice (Oryza sativa L.) production to pursue alternative sites of action. Currently, very long-chain fatty acid elongase inhibitors are not labeled for U.S. rice production but have been widely used for Asian rice production systems. Previous research has demonstrated the utility of acetochlor and pyroxasulfone to provide in-season weed control for Arkansas rice production, but variable crop tolerance has been observed. Additionally, acetochlor at 1,260 g ai ha-1 elicited less rice injury when seeds were treated with a herbicide safener seed treatment of fenclorim at 2.5 g ai kg-1 of seed relative to without the fenclorim seed treatment. Therefore, trials were conducted in 2020 and 2021 to evaluate rice tolerance and weed control with pyroxasulfone, microencapsulated acetochlor, and a fenclorim seed treatment.. In-season applications of acetochlor provided better control of weedy rice and barnyardgrass with earlier application timings and increasing rates. The fenclorim seed treatment enhanced crop tolerance to acetochlor applied delayed-preemergence (DPRE) averaged over acetochlor rate. Also, rice demonstrated good tolerance to acetochlor applied DPRE at 1,260 g ai ha-1 with a fenclorim seed treatment at 2.5 g ai kg-1 of seed, which led to ≤ 19% rice injury, ≥ 88% barnyardgrass control, and ≥ 45% weedy rice control 28 days after treatment. Other studies evaluated the fenclorim seed treatment dose for acetochlor applied DPRE at 1,260 g ai ha-1. The fenclorim seed treatment rate of 2.5 g ai kg-1 of seed reduced rice injury from acetochlor relative to no fenclorim and provided comparable heights and number of shoots to the nontreated check at each evaluation. Increasing from 2.5 to 5 g kg-1 of seed provided no additional improvements for tolerance, and rice tolerance to acetochlor from fenclorim at < 2.5 g kg-1 of seed was inconsistent and not commercially viable due to variable tolerance. Across 16 common Arkansas rice cultivars, DPRE acetochlor at 1,260 g ai ha-1 caused ≤ 24% injury, and rice planted under adverse growing conditions exhibited < 20% injury with acetochlor and fenclorim. Regardless of the fenclorim seed treatment, rice demonstrated good tolerance to fall applications of acetochlor but not pyroxasulfone. Pyroxasulfone at the low and high rate, respectively, caused 39 and 47% injury 28 days after emergence and averaged over the fenclorim seed treatment. Additionally, weedy rice control ranged from 48 to 0% with acetochlor, and the fenclorim seed treatment did not influence weed control. Based on the results of these experiments, the fenclorim seed treatment will provide adequate crop tolerance to microencapsulated acetochlor but not to pyroxasulfone. Additionally, in both studies evaluating weed control, the fenclorim seed treatment did not influence weed control, indicating that the safening response for cultivated rice is not reciprocated to adjacent weeds. Should microencapsulated acetochlor be registered for use in U.S. rice production with the addition of the fenclorim seed treatment, rice producers would have a new, effective site of action to control problematic weeds without compromising crop tolerance.

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