Development of switchgrass (Panicum virgatum) as a bioenergy feedstock in the United States
Introduction
Approximately 25 years ago, it was recognized that dedicated bioenergy feedstocks would ultimately be important contributors to any substantive national renewable energy strategy that was based on biomass supply. The Bioenergy Feedstock Development Program (BFDP) that was initiated at Oak Ridge National Laboratory (ORNL) in 1978 under the sponsorship of the US Department of Energy (DOE) was developed to evaluate a wide variety of potential feedstocks that could be grown specifically for bioenergy or bioproduct supply. The focus of this program since its inception has been to select the most promising species based on actual and potential productivity levels, intensity and type of management requirements, environmental attributes, and the potential economic returns to the producers upon whom production would ultimately depend. Most of the early emphasis was on short-rotation forest systems, and this research has led to major advances in forest genetics that have significantly benefited industrial efforts to maximize fiber and energy production in short-rotation forestry [1], [2].
Approximately 12 years ago, BFDP recognized that expanded emphasis was needed on the development of herbaceous bioenergy crops that could combine close compatibility of crop management strategies with existing farming practices, generate cash flow from annual returns from harvested biomass, and have positive environmental impacts on American farmlands. This paper reports the progress that resulted from the ensuing 10-year effort to develop switchgrass (Panicum virgatum) as a bioenergy crop. Many factors suggested that switchgrass was worthy of increased emphasis as a feedstock for energy production. Foremost, levels of productivity attained in test plots were high compared with other herbaceous species evaluated [3]. Other considerations included the perennial nature of switchgrass that reduces management intensity and consumption of both energy and agrochemicals and widespread recognition of the soil and wildlife enhancing potential of switchgrass and other native grasses that have led to their playing a dominant role in the plantings of the Conservation Reserve Program (CRP) [4]. Finally, the general familiarity of the farming community with grass production, harvesting strategies, and equipment was especially advantageous. The adaptability of switchgrass to poor soil also made it an excellent choice as a model species for further research, since dedicated bioenergy crops were envisioned to be produced principally on lands not used for primary food and/or cash crops.
Although switchgrass has been recognized as a valuable forage species for almost two decades [5], [6], [7], [8] substantial new information was needed to evaluate its potential for bioenergy production. Areas of primary need included best management practices (e.g., establishment, fertilizer requirements, cutting frequency), breeding and selection potential (particularly for biomass yield), physiological characteristics (utilization of nutrients, response to water and nutrient stress, etc.), and genetics and propagation techniques. We report here on BFDP research and gains made in each of these areas through a network of research sites and projects at universities, DOE laboratories, and United States Department of Agriculture (USDA) facilities. A list of institutions, primary investigators, and areas of focus by the interdisciplinary team selected for this work is included in Table 1. This group embodied and promoted an interdisciplinary and intermural approach to programmatic research goals that to-date has led to more than 160 publications, which include 75 journal articles and 20 graduate dissertations and theses [9]. Technical progress in the intermediate stages of this program was reviewed earlier [10], [11]. We evaluate here the progress of this team by evaluating the anticipated gains to the bioenergy industry, and to the nation, that could accrue from the technical advances made within this program over its 10-year lifespan. Finally, we discuss some promising areas of research that could further amplify these gains.
Early field studies were conducted to screen and select promising lignocellulosic herbaceous crop species (annual and perennial grasses and perennial legumes) [12]. This ultimately led to a choice of switchgrass as a model species in 1991 and a focus on the interrelated aspects of breeding, management, and biological attributes necessary for biomass production. Our research strategy proceeded on two time horizons. The near-term goal was to identify the most promising existing cultivars, such that adoption of biomass production for bioenergy would be facilitated should the demand arise. The longer-term aim was to improve understanding of basic biological processes in switchgrass. These included both factors affecting above- and below-ground processes and breeding characteristics. Both perspectives were considered essential to improving switchgrass and to assessing the potential to realize longer-term economic and ecological gains from this species. Principal accomplishments in each of these areas are discussed below.
Section snippets
Establishment
One of the most persistent issues in producing switchgrass as an energy crop has been delineation of management regimes that will enable growers to rapidly and consistently establish strong stands of switchgrass. As a small-seeded species that initially allocates a large amount of energy to developing a strong root system, switchgrass will typically attain only 33–66% of its maximum production capacity during the initial and second years before reaching its full capacity during the third year
Physiological studies
Switchgrass in mature stands grows as a closed canopy and allocates a large fraction of the photosynthetic products to maintenance of a large, active root system. Thus, whole plant function is largely determined by canopy architecture and whole plant allocation processes as much as by maximum, leaf-level physiological rates. Yet rates of single-leaf photosynthesis and transpiration were an early interest in the program as potential tools in breeding research and model simulations of maximum
Breeding biology and yield improvement
In 1991, research into yield improvement of switchgrass began with a single breeding project at Oklahoma State University. This research effort in the Central US was augmented in 1996 by a second major breeding project at the University of Georgia designed to address the soils and climate of the Southeastern US. Additional, smaller-scale breeding projects were carried out in the Northern Great Plains, coordinated through the USDA, Agricultural Research Service, Lincoln, Nebraska, which
Scale-up issues
The production and basic research previously described was focused around endpoint applications with developing bioenergy industry. Researchers have contributed significantly in a variety of ways to efforts to address issues of scaling up of switchgrass production/utilization for industrial applications. Breeding studies have led to recommendations for commercialization of cultivars developed within the program and discussion of the logistics of seed production of superior cultivars [51].
Synthesis and integration
Research findings during the 10-year program to evaluate switchgrass have contributed significantly to both our near-term and long-term expectations of the role that switchgrass could play as a source of renewable energy from American farms. These gains have come from improved productivity and profitability of projected switchgrass management regimes as well as from providing a scientific basis for estimates of secondary market benefits to be derived from increased reliance on switchgrass to
Acknowledgement
This work was funded by the US Department of Energy's Office of Biomass. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725. We thank the many participants in the Bioenergy Feedstock Development Program for their years of excellent research and dedication.
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