Date of Graduation

12-2023

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

Butts, Thomas R.

Second Committee Member

Bateman, Nick R.

Third Committee Member

Roberts, Trenton L.

Keywords

Barnyardgrass, Oxyfluorfen, Weedy rice

Abstract

Mid-southern U.S. rice (Oryza sativa L.) producers are challenged every year to control problematic barnyardgrass [Echinochloa crus-galli (L.) Beauv.] and weedy rice (Oryza sativa L.) because they have evolved resistance to most labeled herbicides. Oxyfluorfen is a Herbicide Resistance Action Committee (HRAC)/Weed Science Society of America (WSSA) Group 14 herbicide that inhibits protoporphyrinogen oxidase within the plant. Currently, there are no HRAC/WSSA Group 14 herbicides labeled for barnyardgrass or weedy rice control in midsouthern rice production. Therefore, oxyfluorfen is being evaluated for barnyardgrass and weedy rice control. Plant breeders at the University of California – Davis were able to breed a rice line that confers resistance to oxyfluorfen, allowing the herbicide to be sprayed in-season. In 2021 and 2022, field trials were conducted at the Rice Research and Extension Center near Stuttgart, AR, the Pine Tree Research Station near Colt, AR, the Northeast Research and Extension Center in Keiser, AR, and the University of Pine Bluff Small Farm Research Center near Lonoke, AR. All field trials were planted with oxyfluorfen-resistant rice and evaluated for crop tolerance and barnyardgrass or weedy rice control following applications of oxyfluorfen. Oxyfluorfen at 1,680g ha-1 provided 81% and 72% weedy rice control in 2021 and 2022 when averaged over application timings of 1-leaf, 2-leaf, 3-leaf, and tillering 7 days after treatment. When tested in the greenhouse, barnyardgrass and weedy rice control averaged over the rate of oxyfluorfen was ≥85% and ≥70%, respectfully, for the 1-, 2-, and 3-leaf growth stages at 7 days after treatment. The findings indicate that oxyfluorfen should be used on plants 3-leaf in size or smaller. Following a preemergence application of oxyfluorfen, saturated soil conditions led to 75% visible injury to oxyfluorfen-resistant rice. Combinations of clomazone and oxyfluorfen provided ≥73% barnyardgrass control in three of four site-years at 5 weeks after emergence. Adding oxyfluorfen to clomazone improved barnyardgrass control in two of four site-years. The findings from this research suggest that oxyfluorfen and clomazone mixtures could be used to control herbicide-resistant barnyardgrass plants by utilizing multiple modes of action. Preemergence applications of oxyfluorfen followed by postemergence applications of the herbicide resulted in ≥94% barnyardgrass control in one of two site-years at 35 days after the postemergence application. Oxyfluorfen applied preemergence on a silt loam soil resulted in barnyardgrass control comparable to applications of clomazone or quinclorac alone at 14 days after emergence. Based on this data, oxyfluorfen could be used as a preemergence herbicide for barnyardgrass control in rice production. Oxyfluorfen would be best suited as a preemergence herbicide due to the amount of injury caused by postemergence applications compared to herbicides currently used. Oxyfluorfen should not be used for weedy rice control due to the lack of complete control observed. While oxyfluorfen likely should not be used for weedy rice control, the herbicide could provide rice producers with an alternative mode of action to control barnyardgrass. Using oxyfluorfen would allow producers to have a larger portfolio to control problematic barnyardgrass.

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