Projecting Yield and Utilization Potential of Switchgrass as an Energy Crop

The potential utilization of switchgrass (Panicum virgatum L.) as a cellulosic energy crop was evaluated as a component of a projected future national network of biorefineries designed to increase national reliance on renewable energy from American farms. Empirical data on yields of switchgrass from a network of experimental plots were coupled with data on switchgrass physiology and switchgrass breeding progress to provide reasonable expectations for rates of improvement over current yields. Historical breeding success with maize (Zea mays L.) was found to provide a reasonable model for projected linear rates of yield improvement of switchgrass based on documented progress to date. A physiologically based crop production model, ALMANAC, and an econometric model, POLYSYS, were utilized to estimate variability in switchgrass yield and resource utilization across the eastern two‐thirds of the United States. ALMANAC provided yield estimates across 27 regional soil types and 13 years of weather data to estimate variability in relative rates of production and water use between switchgrass and maize. Current and future yield projections were used with POLYSYS to forecast rates of adaptation and economic impacts on regional agricultural markets. Significant positive impacts on US markets, including significant increases in farm income and significant reduction in the need for government subsidies, were projected. This was based on expected technological progress in developing biorefineries that will significantly increase national energy self‐sufficiency by producing feed protein, transportation fuel, and electrical power from cellulosic feedstocks.

Introduction

While ethanol from maize is the dominant means by which renewable energy is channeled from sunlight to the transportation industry (Shapouri et al., 1995), switchgrass has become another strong candidate for production of bioenergy. Switchgrass is a native perennial, warm‐season grass species within which selection has been practiced for forage and conservation uses over the past half‐century (Vogel et al., 1985). In 1991 it was selected as a candidate for utilization in production of bioenergy and bioproducts (McLaughlin and Kszos, 2005). Its strongest attributes include high biomass production capability and energy recovery capacity with low energy and material inputs, and excellent compatibility with existing agricultural practices. These qualities, combined with strong soil and water conservation values, and a high capacity to reduce emissions of greenhouse gases have led to switchgrass being considered as a potentially important component of a national energy strategy (McLaughlin et al., 2002).

Despite criticism of ethanol production from maize based on low energy efficiency and adverse environmental impacts (Pimmentel et al., 2002), maize‐based ethanol production has made an important beginning in the reduction of reliance of the United States on imported oil. Maize‐based ethanol does displace significantly more oil than is used in its production (Shapouri et al., 1995). However, McLaughlin and Walsh (1998) suggested that the efficiency of energy conversion and reduction of greenhouse emissions through production of cellulosic ethanol from switchgrass could exceed that from maize ethanol by more than an order of magnitude. Yet maize remains the standard biofuel feedstock, which provides a base that can ultimately be supplemented by other feedstocks, providing greater economic and environmental efficiencies.

If switchgrass is to provide a viable supplement to ethanol from maize, biomass production levels of switchgrass must be determined as input for a national renewable energy strategy. The Role of Biomass in America's Energy Future (RBAEF) project was initiated to help formulate such a strategy. The RBAEF project represents the most comprehensive effort in the United States to date that has focused on analysis of mature technology for production of energy from biomass. It has involved experts in bioenergy analysis from government and university coupled with active involvement of both conservation (Natural Resources Defense Council) and policy (Office of Energy Policy) organizations. The RBAEF project has considered over 20 mature process technology scenarios for production of a broad range of fuels and electrical power from cellosic biomass. Reasonably optimistic forecasts for both biomass production and bioenergy conversion were evaluated for a projected national network of biorefineries that could contribute to national energy self‐sufficiency (Greene, 2004). Switchgrass was selected as the model crop for this study. In that context the research described herein has formed a basis for considering what role switchgrass could play in a national energy supply system. Such a system would incorporate the best foreseeable technology to produce energy and value‐added products such as animal feed protein from cellulosic feedstocks. Yield levels will play a key role in the economics of such production and utilization systems as well as in determining the demographics of production.

When discussing methods for increasing plant biomass yield, some terms describing yield must be defined. Two such terms commonly used are “yield potential” and “potential yield.” As used in this study, yield potential is the maximum yield (biomass or grain) levels that have been attained at any time for a specific genotype of a crop or grass under field conditions. In contrast, potential yield is the maximum predicted yield based on simulations founded in plausible physics, biochemistry, and physiology of the crop in its normal growing environment (Fischer and Evans, 1999). This yield is considered theoretically and physiologically possible based on maximum light interception and biochemical conversion of solar radiation into dry matter accumulation.

Because maize production is a cornerstone of agricultural economics in North America, the historical improvement of maize yields represents an important standard from which to project future yield gains of other species with comparable production characteristics. Maize yield records for North America extend back more than 100 years and provide a template for both defining and understanding yield improvement through breeding and crop physiological studies (Duvick 1997, Tollenaar 1994). Maize and switchgrass not only share the common trait of being useful bioenergy crops but are also similar in that both are warm‐season, C₄ species. However, maize is an annual with only the grain used for ethanol production while switchgrass is a perennial with the entire aboveground biomass used when energy is the endpoint. Maize is a good standard of comparison because of the extensive breeding for increased yields and the extensive physiological research on processes contributing to yield. Investigation of the physiology and breeding history of these two plant species, as related to increased yields, becomes important for studies of yield potential as a theoretical upper limit of yield increases achievable through breeding.

In this chapter, we examine the past record of yield improvement in maize and the basis of those gains to provide a framework for projecting gains in yield of switchgrass. A necessary component of these analyses has been comparisons of the agronomic characteristics, breeding history, and underlying physiology of maize and switchgrass. We had three objectives in initiating this study. First, we wanted to evaluate potential yield improvement in switchgrass using maize breeding advances as a model. Second, we wanted to test and apply a physiologically based crop production model, ALMANAC (Kiniry et al., 1992), parametrized to switchgrass physiology to estimate both potential yield and yield potential of switchgrass. Finally, we wanted to describe links between productivity and production costs for regional projections of switchgrass utilization that would require widespread participation of the agricultural community of the United States in supporting renewable energy production. Such participation must be based on switchgrass providing attractive economic alternatives to conventional crops. For these analyses we have used the econometric model POLYSYS (Ugarte and Ray, 2000).