Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

Amit Jhala

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Humberto Blanco-Canqui, Saleh Taghvaeian, Sam Wortman

Department

Agronomy

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Agronomy

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

Herbicide-resistant Palmer amaranth (Amaranthus palmeri S. Watson) pose a severe threat to agronomic crop productivity in the United States, including corn and soybean production in Nebraska. Therefore, there is an urgent need for diversified and sustainable management approaches. Integrating cover crops (CCs) with herbicide programs represents a promising strategy to achieve Palmer amaranth suppression, sustain crop yields, and contribute to long-term soil health in irrigated corn–soybean production systems in Nebraska.

A meta-analysis of 41 studies and 595 paired observations demonstrated that CCs reduced Amaranthus spp. density and biomass by 37–59% depending on the growing season, with grasses and mixtures providing the most consistent and reliable suppression. Building on these findings, multi-year field trials in Nebraska evaluated early spring–planted oat and barley CCs in soybean production. Oat consistently produced greater biomass than barley and, when combined with herbicide applications, reduced Palmer amaranth density and biomass by more than 90%, while maintaining soybean yields comparable to those in no-cover crop systems.

Complementary studies assessed root biomass production by spring-planted CCs and their effect on soil properties. Total root biomass production across years in the 0–30 cm soil depth was 1,402 kg ha–1 for oat, 1,166 kg ha–1 for barley, and 270 kg ha–1 for no CC (NCC). Despite the significant amount of biomass input, neither oat nor barely CC affected soil properties such as bulk density, soil sorptivity, particulate organic matter, soil organic C, nutrients, and microbial properties relative to NCC after 3 year of the study. However, CCs had 7–8% higher proportion of 0.25–0.5 mm aggregates, and 8–10% lower proportion of <0.25 mm aggregates in year 3. This suggests that spring-planted CCs can improve soil aggregation but only in medium-term. Long-term (>15 yr) no-till management history, fine-textured soil, and high soil organic matter (3.7%) at the site may explain the limited CC effects.

Research on planting-green strategies in corn further demonstrated that delaying cereal rye termination from planting corn to the V3 growth stage increased biomass up to 10,888 kg ha−1 and reduced Palmer amaranth density, biomass, and seed production by more than 99% compared with no cover crop. High cereal rye biomass provided weed suppression comparable to herbicide programs, while low-biomass conditions required herbicide inclusion for effective Palmer amaranth control. Corn yield was unaffected by delayed termination, likely due to irrigation and split nitrogen management.

Together, this body of research highlights the potential of cover crops as a cornerstone of integrated Palmer amaranth management systems. By strategically selecting cover crop species, optimizing termination timing, and integrating with herbicides, producers can effectively suppress Palmer amaranth, sustain crop productivity, and build a foundation for improved soil health.

Advisor: Amit Jhala

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