Theses and Dissertations

ORCID

https://orcid.org/0000-0001-6232-2388

Advisor

Dodds, Darrin M.

Committee Member

Avila, Luis

Committee Member

Reynolds, Daniel B.

Committee Member

Krutz, Larry J.

Committee Member

Spencer, Dave G.; Brown-Johnson, Ashli

Date of Degree

5-16-2025

Original embargo terms

Immediate Worldwide Access

Document Type

Dissertation - Open Access

Major

Plant and Soil Sciences (Weed Science)

Degree Name

Doctor of Philosophy (Ph.D.)

College

College of Agriculture and Life Sciences

Department

Department of Plant and Soil Sciences

Abstract

Auxin and grass control herbicides are often applied in tank mixtures to provide multiple modes of action and to achieve broad-spectrum weed control. Literature indicates that auxin herbicides, when mixed in solution, may antagonize grass control herbicides, resulting in decreased grass weed control compared to the grass herbicide applied alone. Field and laboratory experiments were conducted in 2022, 2023, and 2024 at multiple locations in Mississippi. These experiments had three objectives: 1) evaluate grass weed control with glyphosate and clethodim in response to the simultaneous application with either dicamba or 2,4-D, using a commercially available direct-injection system compared to tank-mix combinations of the herbicides, 2) evaluate grass weed control with glyphosate and clethodim in response to the simultaneous sequential application of either dicamba or 2,4-D, utilizing a separate tank and boom sprayer system, compared to tank mix combinations of the herbicides, and 3) use non-radioactive absorption and translocation techniques to quantify and compare clethodim concentration differences in barnyardgrass (Echinochloa crus-galli (L.) P. Beauv.) plants, growing points, and the leaf surface following the application of clethodim alone, tank-mixed with dicamba, or the two herbicides applied sequentially. In general, results from field experiments suggest that grass control can be improved, compared to tank mixing, by using a direct-injection system or by applying auxin and grass control herbicides with one-pass sequential applications using separate tanks and booms. However, this conclusion is not all-encompassing for all auxin and grass control herbicide combinations. Results from the laboratory experiment indicate that clethodim and dicamba applied sequentially does not affect clethodim translocation to barnyardgrass growing points, compared to the two herbicides applied in tank mixture or clethodim applied alone when applied at 136 g ai ha-1 to barnyardgrass at the 3-4 leaf stage. However, the presence of dicamba, regardless of whether it was tank-mixed and/or applied sequentially with clethodim, elicited a delay in absorption of clethodim. These results suggest that the reduced grass activity associated with tank mix applications of clethodim and dicamba may not be due to biochemical antagonism within the plant. Rather, a delay in clethodim absorption could partly explain dicamba's antagonistic impact on clethodim.

Sponsorship (Optional)

United States Department of Agriculture – National Institute of Food and Agriculture (Project # MIS-531020)

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