Date of Graduation

8-2024

Document Type

Thesis

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Norsworthy, Jason K.

Committee Member

Butts, Thomas R.

Second Committee Member

Roberts, Trenton L.

Third Committee Member

Bateman, Nicholas R.

Keywords

Barnyardgrass; Fenclorim; Italian ryegrass; Rice; S-metolachlor; Wheat

Abstract

The continued evolution of herbicide resistance is one of the primary concerns facing Arkansas growers today. Problematic weeds such as barnyardgrass [Echinochloa crus-galli (L.) Beauv.] and Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] have evolved resistance to some of the most used herbicides in rice (Oryza sativa L.) and wheat (Triticum aestivum L.). To counteract this resistance, alternative herbicide sites of action should be explored. S-metolachlor is a very-long-chain fatty acid (VLCFA)-inhibiting herbicide [Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Group 15] commonly used in U.S. row crops to control grasses and small-seeded broadleaf weeds. S-metolachlor is not currently labeled for use in rice nor wheat, but recent studies describe the potential use of Group 15 herbicides in these crops. Using a herbicide safener is one strategy to improve herbicide selectivity and reduce injury to crop plants. Fenclorim is one such safener previously shown to effectively safen acetochlor applications in rice. Experiments were conducted from 2021 to 2023 to investigate the potential use and efficacy of S-metolachlor applied with fenclorim as a seed treatment for barnyardgrass and Italian ryegrass control in rice and wheat, respectively. Experiments in wheat evaluated S-metolachlor at two application timings, preemergence (PRE) and delayed-preemergence (DPRE), and two rates of fenclorim, 0.5 and 2 g ai kg seed-1. Results from these experiments indicated visual wheat injury from S-metolachlor at 1.12 kg ai ha-1 applied PRE was reduced from 34 to 18% with nontreated seed and fenclorim-treated wheat at 2.0 g ai kg-1 of seed, respectively. However, a minimal effect of the herbicide safener was observed during the DPRE application. Wheat yield followed a similar pattern to injury, with increased yield (2,500 vs. 1,500 kg ha-1) occurring when fenclorim-treated wheat at 2.0 g ai kg-1 of seed was used compared to the nontreated with a PRE application of the highest rate of S-metolachlor, respectively. Experiments in rice were conducted using three commonly planted Midsouth varieties, ‘Diamond,’ ‘DG263L’, and ‘PVL03’; however, varieties were not directly compared. In rice, when S-metolachlor was applied DPRE, fenclorim increased rice tolerance for both ‘Diamond’ and ‘DG263L’; however, commercial tolerance was not achieved with S-metolachlor rates higher than 0.68 kg ai ha-1. When applied as an early-post application, S-metolachlor at 1.12 kg ai ha-1 resulted in up to 50% injury in ‘Diamond’ rice, but yield did not differ from the nontreated control. S-metolachlor at 0.68 kg ai ha-1 provided 90% barnyardgrass control at 28 days after treatment on a clay soil, while less than 20% rice injury was observed with fenclorim. A similar experiment was conducted on a silt loam soil evaluating both a DPRE and 1-leaf application timing and indicated that when applied DPRE, only S-metolachlor at 0.28 kg ai ha-1 in combination with a fenclorim seed treatment provided a level of tolerance similar to a microencapsulated acetochlor. Barnyardgrass control was unsatisfactory at this rate and thus would not be recommended. While it is crucial to continue to explore alternative herbicides to delay the development of resistant weed species, this formulation of S-metolachlor with fenclorim would not provide adequate and consistent crop safety to rice and wheat with the necessary rates for effective weed control.

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