Date of Award:

8-2025

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Committee Chair(s)

J. Earl Creech

Committee

J. Earl Creech

Committee

Mirella Ortiz

Committee

Corey Ransom

Committee

Eric Westra

Abstract

Hexazinone + diuron is an herbicide mix commonly used to control weeds in established alfalfa in the late fall or early spring when alfalfa is dormant. If applied outside full dormancy, this mix can cause significant crop injury. The Intermountain West’s cold weather leaves a narrow time frame for applicators to apply herbicide. To help extend the application period to when alfalfa has not yet entered complete dormancy, the burndown herbicide paraquat can be added to the tank mix. Paraquat burns back existing plant matter to prevent hexazinone + diuron from spreading throughout the plant and causing long-term crop injury. However, the EPA has instituted strict regulatory measures for paraquat use owing to its extreme toxicity, which has led growers to seek alternative burndown herbicides to pair with hexazinone + diuron. The objective of this study was two-fold, to determine if carfentrazone and/or saflufenacil can be used instead of paraquat and evaluate if fall or spring is a better application timing to maximize yield and weed control while limiting crop injury. This study evaluated hexazinone + diuron alone and in combination with paraquat, carfentrazone, and saflufenacil. Field studies were conducted in Utah in 2023 and 2024 to assess dry matter yield (DM yield), alfalfa growth rate, forage quality, and weed density. Results indicate that fall applied hexazinone + diuron alone provided equal or greater DM yield, growth rate, forage quality, and weed control compared to combination with contact herbicides. This challenges previously held beliefs that a contact herbicide like paraquat is required to prevent crop injury. When herbicides are applied in the spring the addition carfentrazone or saflufenacil improved weed control and forage quality while not impacting alfalfa growth rate at harvest compared to the standalone application of hexazinone + diuron. However, DM yield remained similar. Carfentrazone and saflufenacil are both excellent alternatives to paraquat, performing similarly or better for all response variables collected in this study. Spring application is likely to increase crop injury, but in most cases complete recovery was observed by harvest and by applying in the spring, residual weed control will be extended through the growth period.

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