Semester of Graduation

Fall 2025

Degree

Master of Science (MS)

Department

School of Plant, Environmental, and Soil Sciences

Document Type

Thesis

Abstract

Glyphosate-resistant (GR) Italian ryegrass (Lolium perenne ssp. multiflorum) has become an economically significant weed management challenge in Louisiana row crop production systems. The problem is further exacerbated by the occurrence of biotypes with resistance to acetyl-CoA carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors—herbicide sites of action (SOAs) commonly used for ryegrass control in soybean (Glycine max) and cotton (Gossypium). Field, greenhouse, and laboratory experiments were conducted during the 2023–2024 and 2024–2025 growing seasons at the LSU AgCenter Northeast Research Station in St. Joseph, LA, and at the LSU AgCenter Weed Science facilities in Baton Rouge, LA, to develop integrated management strategies for GR Italian ryegrass. The first study evaluated the combined effects of fall-applied residual herbicides and a cereal rye (Secale cereale L.) cover crop on ryegrass control and seedbank reduction prior to soybean planting. S-metolachlor (1420 g ai ha⁻¹) and clomazone (631 g ai ha⁻¹) were applied in the fall, and cereal rye was terminated using glyphosate (1120 g ae ha⁻¹) either 8 or 2 weeks before planting. S-metolachlor provided 64% control at 17 weeks after treatment (WAT) and 57% at 23 WAT, whereas clomazone was less effective and caused 86% cover crop injury at 17 WAT. Cover crop alone, terminated 2 weeks prior to planting, achieved up to 82% suppression of GR Italian ryegrass. When paired with S-metolachlor, control exceeded 90%, indicating a highly effective integrated approach. A second study assessed the impact of herbicide timing on GR Italian ryegrass control, seed traits, and progeny fitness. Nine herbicide SOAs commonly used in spring burndown programs preceding soybean, corn, or cotton were applied at three reproductive stages: anthesis, soft dough, and late grain filling. Paraquat (1103 kg ai ha⁻¹), glufosinate (656 g ai ha⁻¹), and clethodim (280 g ai ha⁻¹) applied at anthesis significantly reduced seed viability by up to 48 percentage points compared to the non-treated control. These treatments also reduced 100-seed weight, germination, tiller count, biomass, and progeny seedling vigor. In conclusion, fall-applied residual herbicides combined with a cereal rye cover crop and early reproductive-stage herbicide applications offer a sustainable, multi-pronged approach for managing GR Italian ryegrass.

Date

11-3-2025

Committee Chair

Jha, Prashant; Miller, Donnie; Webster, Lucas

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