Date of Award

Spring 2026

Abstract

A primary challenge for perennial forage production systems is maintaining crop productivity and forage quality over time. Including legume species in these systems is an opportunity to reduce external inputs, to extend the seasonal productivity of a pasture, and to balance the nutritional characteristics of grass species. However, legume persistence in mixed stands is frequently poor. Identifying harvest practices, legume species, or species mixtures that improve legume persistence would help to maximize the ecosystem services of forages. I conducted a four-year field experiment (2019 to 2022) in southeastern New Hampshire to evaluate the productivity and persistence of four perennial legumes (alfalfa, Medicago sativa L.; birdsfoot trefoil, Lotus corniculatus L.; red clover, Trifolium pratense L.; white clover T. repens L.), each grown in biculture with orchardgrass (Dactylis glomerata L.), as well as two four-species mixtures of legumes with orchardgrass. Cropping treatments were harvested at two frequencies (three- or five-harvests per season) and two cutting heights (5 cm or 10 cm residual forage height). To supplement insights from the field experiment, and to elucidate broader relationships between management practices, yields, stability, and crop persistence, I conducted a literature review, where yields from multi-year forage management experiments were extracted, normalized, and analyzed. In Chapter 1, I report on the productivity, persistence, and stability of the four legume-grass bicultures under the four harvest management treatments. Legume species identity had a relatively large effect on total forage yields, and red clover was the most productive and persistent of the species evaluated. All legume species declined over the four-year study, but reduced harvest frequency and increased cutting height mitigated this trend. This more lenient harvest management treatment was not the most productive, and likely there are important tradeoffs between stand productivity and maintaining legumes within this forage system. Chapter 2 focuses on the performance of two legume mixtures. A variety of studies that have experimentally manipulated plant species richness have shown there is often an association between species richness, productivity, and other benefits compared to monocultures. It remains unclear, however, if simply manipulating legume species richness might provide similar benefits or improve legume persistence in a perennial legume-grass forage system. Legume mixtures overyielded the bicultures, equaled the most productive biculture, and averaged more than 30% legume over the experimental period. Three harvests per season increased legume proportions, reduced weed pressure, and led to more stable and asynchronous mixtures, although total productivity was lower. I used the land equivalent ratio as a measure of mixture efficiency, and the legume mixtures supported equal or greater legume biomass than would be expected based on legume performance in biculture, despite lower legume seeding rates. These results indicate legume mixtures are an opportunity to increase legume species richness with little risk, as they maintain the benefits and functionality of legume-grass bicultures. In Chapter 3, I evaluated how the weed community assembled over the four-year study period in response to harvest management and the composition and species richness of legume mixtures. Much of the work on weed community assembly has been conducted in annual cropping systems, while the basis of the theory is grounded in “natural” ecosystems where the nature, magnitude, and frequency of disturbance differs relative to most agricultural systems. Evaluating weed community assembly processes in perennial forage systems can shed light on the relative strength and nature of the abiotic and biotic assembly filters operating in these systems and thereby inform strategies for managing weeds. Each year had a relatively large effect on seasonal total weed abundance, weed species richness and diversity, and weed community structure. Harvest intensity influenced weed abundance and diversity most notably in years two to four. The most aggressive harvest management treatment was associated with greater weed diversity and abundance. Forage legume species and mixtures had a smaller but observable effect on weed abundance and diversity, but there was no evidence that crop selection shaped community composition. The weed community in this forage system was structured more by harvesting practices than plant competition or other biotic filtering mechanisms, a finding consistent with a hierarchical filtering framework of community assembly. The fourth chapter is a literature review summarizing the effect of forage management practices on crop performance. While yields are often reported, crop stability is usually not calculated, and stand persistence is known to be a challenge but is rarely quantified. Knowing the relationship between these three metrics, and between them and common management practices, would greatly improve our ability to clarify and address challenges in forage systems. I extracted data from previous studies and calculated the relationships between and among these three yield variables and various forage management practices. I found a generally consistent positive relationship between relative yield and yield stability, while yield and stand persistence were not consistently correlated. Management practices had variable effects on yield outcomes. For example, application of N improved yield, stability, and persistence. Similarly, increasing forage species richness led to greater annual yields; however, increasing species or cultivar diversity was not always associated with increased stability. Stronger conclusions about the relationships between forage yield and performance metrics and management practices will likely be possible if future research prioritizes the reporting of yield stability and persistence data along with yield data.

Document Type

Dissertation

First Advisor

Richard Smith

Second Advisor

Andre Brito

Third Advisor

Alexandra Contosta

Department or Program

Natural Resources and Environmental Studies

Degree Name

Doctor of Philosophy

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