Masters Theses

Date of Award

12-2025

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant Sciences

Major Professor

Bruno C. Pedreira

Committee Members

Thomas Mueller, Renata Nave Oakes, Lawrence Steckel

Abstract

Alfalfa (Medicago sativa L.) productivity and persistence are highly dependent on soil fertility and weed control. Still, resistant weeds, such as glyphosate-resistant Palmer amaranth (Amaranthus palmeri S. Watson), pose an additional challenge. This thesis assessed strategies for managing glyphosate-resistant Palmer amaranth in Roundup Ready® alfalfa. Two field studies were conducted over two years in Knoxville, TN. The first experiment assessed the interaction between boron (B) fertilization and glyphosate management during establishment in a 3×3 factorial randomized complete block design with four replicates. Treatments included three boron rates (0, 2.25, and 4.5 kg B ha⁻¹) and three herbicide programs: no herbicide, spring glyphosate (SG), and fall-plus-spring glyphosate (FSG). Forage was harvested three times per year, and responses measured included forage accumulation (FA), weed mass (WM), Palmer amaranth mass (PAM), canopy height, and nutritive value (CP, ADF, aNDF, and IVTDMD48). Across years, FSG increased total FA by 30% (13,000 kg DM ha⁻¹) and reduced WM, PAM, and population by an average of 70%. Boron fertilization increased FA during periods of low B availability but had no effect once tissue concentrations were adequate. The second experiment evaluated the efficacy of herbicide modes of action and their combinations for controlling glyphosate-resistant Palmer amaranth. The 11 treatments were control, glyphosate, flumioxazin, indaziflam, saflufenacil, paraquat, glyphosate+flumioxazin (G+F), glyphosate+indaziflam (G+I), glyphosate+saflufenacil (G+S), glyphosate + paraquat (G+P), and indaziflam+paraquat (I+P). Treatments applied before the 1st harvest (control, glyphosate, flumioxazin, indaziflam, G+F, and G+I) had their effects evaluated in visual injury (chlorosis and necrosis) rated at 3, 7, 14, and 21 days after treatment (DAT), and weed control at 21, and FA, WM, canopy height (CH), and nutritive value for three harvests. Glyphosate + flumioxazin (G+F) provided the most consistent weed control (≈90%) without affecting FA or nutritive value. Collectively, FSG and well-timed boron fertilization support alfalfa establishment and productivity, improving the crop’s competitiveness with glyphosate-resistant Palmer amaranth under low-boron conditions. Although Palmer amaranth was not observed during alfalfa harvests in the second experiment, the evaluated herbicides, appropriate alternatives to control the weed, caused only transient visual injury and did not reduce FA or nutritive value.

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