Switchgrass has substantial variation in many phenotypic and phenological traits that allow it to be adapted to a large portion of the U. S. (Casler et al. 2004; Casler et al. 2007). There are two distinct groups of switchgrass ecotypes: the lowland ecotypes, consisting of solely octaploids, and the upland ecotypes, consisting of both octaploids and tetraploids (Hultquist et al. 1996). Lowland ecotypes are generally adapted to more southern latitudes and upland ecotypes adapted to more northern latitudes (Casler et al. 2004). These ecotypes are also phenotypically distinct with lowland ecotypes often thriving in warmer climates, being taller, and having later anthesis occurrence resulting in longer growth periods (Cornelius and Johnston 1941; McMillan 1959). Whereas, upland ecotypes are generally more capable of surviving harsh late winter freezes (Vogel 2005). Within these two groups of ecotypes there are two subtypes, northern and southern, based on latitude of origin within each region (Casler et al. 2004).

Understanding the adaptation of switchgrass to variable growing conditions is key to determining its potential as a biofuel crop. The growth and development of switchgrass varies with climatic (i. e., temperature and precipitation) and environmental conditions (i. e., soil type, slope, nutrients) (Casler et al. 2004; Casler et al. 2007; Wullschleger et al. 2010). Field trials at relatively small scales (< 5 m2) and large scales (3-9 ha) reveal high biomass production, greater than 12 Mg ha-1 (Schmer et al. 2008; Wullschleger et al. 2010). Northern upland ecotypes flower earlier than southern upland ecotypes, causing reduced biomass production when moved south (Casler et al. 2004). On the other hand, southern lowland ecotypes flower later, which extends the growing season, resulting in increased biomass production when moved northward. However, survival of the lowland type is reduced in the North, which is thought to be due to cold winter temperatures (Casler and Boe 2003). The regional adaptation of switchgrass is complex but thought to be primarily reliant on genetic diversity for heat resistance, cold resistance, photoperiod, and drought tolerance (Casler et al. 2007). There is substantial genetic variation for many key adaptive traits, which may make it possible to enhance biomass production and decrease mortality.

The phenotypic diversity of switchgrass also makes the management practices that optimize yearly biomass yields vary by ecotype and location (Fike et al. 2006). Field trials have focused on identifying management practices (i. e., planting date, fertilizer application, irrigation, seeding rate, harvesting) required for establishment and optimum biomass production (Parrish and Fike 2005). As biofuel production expands globally, it is critical to also understand the environmental consequences of cultivating biofuel crops (Renewable Fuels Agency 2008). Large-scale production of biofuel crops, such as, switchgrass may bring both positive and negative environmental impacts. Some environmental benefits over traditional row crops include reduced water and wind erosion (Paine et al. 1996; McLaughlin and Walsh 1998; Jensen et al. 2007; Blanco-Canqui 2010). Additionally, decreased water runoff reduces the loss of agricultural chemicals, nutrients, and sediment into nearby waters, enhances nutrient cycling and storage, and recharges groundwater supply (Paine et al. 1996; Jensen et al. 2007; Blanco-Canqui 2010). Soils under long-term switchgrass production can also improve over time with increased soil organic matter production and the sequestration of SOC (Paine et al. 1996; McLaughlin and Walsh 1998; Tilman et al. 2006; Jensen et al. 2007; Blanco-Canqui 2010).

The environmental impact of bioenergy feedstock production systems will need to be closely monitored to ensure sustainability. Field scale monitoring is however limited in scope, temporally and spatially, and in most cases fails to fully account for the effects of site-specific conditions of management, land type, soil texture-hydrologic group interactions, slope, and climate. Therefore, many open-ended questions remain about the large — scale production potential of switchgrass and its long-term environmental impacts. For example: (1) Can switchgrass produce enough biomass to support local refineries and what regions of the U. S. will be used for large-scale feedstock production? (2) What are the optimal management practices for all potential field locations? (3) What is the long-term (i. e., 25 or 50 years) effect on soil quality and erosion? (4) How big of an impact will climate change have on switchgrass biomass production? Will lands that currently produce high levels of biomass be able to sustain these levels in the future?

Though government policies can provide the impetus to promote the production and use of lignocellulosic biomass feedstocks, market interests will undoubtedly increase as demonstrated uses go beyond exclusive ethanol production. Prior to its use as a bioenergy crop, switchgrass was utilized as a forage crop noted for its high forage yields (Parrish and Fike 2005). Before widespread efforts at improving bioenergy-related traits, some of the initial improvement efforts in switchgrass were aimed at forage digestibility by cattle (Vogel 2004). Improvements here, as illustrated in a number of biological and economic metrics, led to the development of cv. Trailblazer (Vogel et al. 1991). Furthermore, switchgrass cv. Shawnee was the result of simultaneous improvements in yields and digestibility (Vogel et al. 1996). Such efforts are germane to those being pursued today relating to higher biomass and ethanol yields, as they are related to strategies for targeted traits of switchgrass and subsequent ethanol processing steps. While the future of switchgrass as a bioenergy crop may well be tied to its ability to remain a forage option, we argue that sustainable approaches along these lines will be complex, and ultimately should stray from switchgrass becoming embedded in a "food vs. fuel" dilemma. Multiple within-year harvests that include an early-season forage harvest may attract farmers concerned with ensuring profits. Early indications point towards subsequent late-season harvests of switchgrass produce admirable amounts of biomass (Mcintosh et al. 2012), though not as much as with a single harvest. How multiple harvests and management for multiple services (see Anderson et al. 2012) will impact N losses, carbon sequestration, and landscape-wide water quality and quantity is unknown.

Promoting other uses for switchgrass will increase its stock in the bioenergy and chemical product arena. Currently, switchgrass is noted primarily for its contribution to biofuels. Advances in technologies that can promote other energy and co-product uses are gaining attention. The co-firing of pelletized switchgrass with coal for electricity (Qin et al. 2006; Aravindhakshan et al. 2010; Khanna et al. 2011) is one area worth further exploration. In addition, high-value commodities that may be produced in or from the ethanol-making process in the future, such as bioplastics, extractives, and other metabolic coproducts, will increase the likelihood of a sustainable bioenergy market that promotes the use of switchgrass (Joyce and Stewart 2012). For bioenergy crops like switchgrass, quick success on these fronts could well dictate its immediate and future prominence.

Acknowledgments

We thank numerous colleagues for their input, and for their valuable contributions to the field of switchgrass biology. These people include, among many others, F. Allen, C. Auer, G. Bates, H. Bhandari, S. Bobzin, V. Dale, R. Efroymson, S. Jackson, Y. Jager, K. Kline, P. Keyser, D. Simberloff, A. Snow. This project was supported by the NIFA Biotechnology Risk Assessments Grant program and funds from the University of Tennessee.

933-948.

Data from fertility studies generally suggest that little added P is needed to achieve high switchgrass yields in bioenergy cropping systems (Hall et al.1982; Muir et al. 2001). This may be different in forage production settings, as in the case of a study by (Rehm 1990), who tested P amendments from 0 to 90 kg ha-1 in Nebraska over 4 years. Rehm (1990) reported a curvilinear response to P with production gains at rates up to 45 kg ha1. Others have reported increased establishment-year production with added P, although effects of P were not observed in subsequent seasons (McKenna and Wolf 1990).

Several studies have reported little to no response to P (Brejda 2000), despite low soil P status. In studies with once-per-year harvest, no response to P was observed after several (3 or 7) years of biomass removal (Muir et al. 2001). Switchgrass grown on low-P soils in Iowa did not respond to P (Hall et al. 1982) and in the southern Great Plains, switchgrass response to P was only benefited at one of two sites (Kering et al. 2012a) over three seasons. In the Kering et al. (2012a) study, P applications of 45 kg ha-1 yr-1 increased yields on a low-P (3.4 mg P kg-1) soil, but no response to any P rate (0, 15, 30, or 45 kg P ha-1) was observed at a second location with soil P concentration of 3.1 mg P kg-1.

Switchgrass’ relationship with the soil microbial community may be a common denominator in the oft-observed variable and limited responses to P and N. In the case of P uptake, switchgrass’ role as host to vesicular arbuscular mycorrhizae greatly improves the grass’ ability to extract and uptake P. Several studies have shown that these root colonizing fungi can greatly improve P acquisition in conditions of high soil acidity, high aluminum and low P (Koslowsky and Boerner 1989; Boerner 1992a, b; Clark et al. 1999; Clark 2002). Adding these fungi to sterilized, low-P soils can eliminate a response to P (Brejda 2000). Conversely, eradicating mycorrhize in low-P soils can reduce switchgrass production if fertilizer P is not added to the system (Bentivenga and Hetrick 1991).

Without returning nutrients to the system, repeated harvests will reduce soil P concentrations in switchgrass biomass production systems (Schmer et al. 2011). With modest yields (5.8 Mg ha1) of switchgrass harvested at anthesis, annual losses of 1.5 kg1 y-1 P ha were reported in production fields in the Great Plains (Schmer et al. 2011). Although greater losses would be expected with greater biomass yields, this factor must be weighed against the stage of plant development at the time of harvest, as the effects of higher yield would be offset by lower P concentrations with plant maturity and senescence (Parrish and Fike 2005; Lemus and Parrish 2009).

There is little research to suggest that switchgrass is particularly responsive to K, whether in field or greenhouse studies (Friedrich et al. 1977; Smith and Greenfield 1979; Hall et al. 1982). Typical recommendations are to maintain K at a medium level based on typical soil test ranges (Teel et al. 2003; George et al. 2008; Douglas et al. 2009). This apparent lack of response may in part be a function of K being recycled to the soil through leaf leaching when switchgrass is harvested after senescence (Parrish and Fike 2005).

As with the other nutrients, response to limestone applications can be variable. This may be less a function of pH change than of the availability (or lack of availability) of other mineral nutrients or toxins (Parrish and Fike 2005). Switchgrass strains display differences in terms of tolerance to soil acidity, with some lines being productive—as opposed to merely tolerant—at pH 4.9 (Bona and Belesky 1992). These differences also may play a role in the variable yield responses reported. Screening for such traits may prove useful if truly marginal sites such as reclaimed mine sites are to be utilized for a future bioenergy industry.

Endophytic bacteria that live freely in the internal tissues of plants and cause no apparent harm have a diverse range of growth promotion mechanisms including nitrogen fixation. Although 78% of the earth’s atmosphere is nitrogen, nitrogen is often a limiting factor in agriculture since it is not readily available to plants. Bacteria and Archea are the only organisms that can fix atmospheric di-nitrogen, thereby making it available for plant growth. This activity is termed biological nitrogen fixation (BNF) and is catalyzed by the oxygen sensitive nitrogenase enzyme to convert N2 to bio-available NH3. Nitrogenases are complex metalloenzymes with highly conserved structural and mechanistic features (reviewed in Alberty 1994; Burgess and Lowe 1996; Rees and Howard 2000). The enzyme is oxygen sensitive, which imposes physiological constraints on the organism. Additionally, the enzyme has a relatively slow turnover time (Thorneley and Lowe 1985), which requires the microbe to synthesize large quantities of the protein, up to twenty percent of protein in the cell (reviewed in Dixon and Khan 2004). Also, the conversion of atmospheric di-nitrogen to a form that can be used by plants requires 16 ATP to reduce one molecule of N2, making it one of the most energy demanding reactions identified in bacterial organisms (Thorneley and Lowe 1985). Together, the amount of energy, the low oxygen requirement, and the amount of protein required to create the nitrogenase enzyme, place a large burden on a nitrogen fixing endophyte. As a result, the synthesis of the nitrogenase complex is stringently regulated at the genetic level (Dixon and Khan 2004). It has been suggested that bacterial endophytes are placed in a more favorable environment compared to rhizospheric bacteria because they are less vulnerable to competition from native soil bacteria and are shielded from various biotic and abiotic stresses (Reinhold-Hurek and Hurek 1998). Perhaps the most-studied grass inoculated with free living nitrogen-fixing endophytes is sugarcane. Burkholderia MG43 inoculated sugarcane plantlets produced a 20% increase in yield over un-inoculated control (Govindarajan et al. 2006), and it was demonstrated that 60 to 80% of nitrogen accumulated in sugarcane came from atmospheric nitrogen fixation (Boddey et al. 1995). The authors also noted that farmers in Brazil have observed some varieties of sugarcane grown in fields for decades, even up to a century without showing any decline in soil N reserve or yield, despite the supply deficit of nitrogen (Boddey et al. 1995). Rice has also been studied in the context of its relationship with free-living nitrogen-fixing Burkholderia spp. In one field experiment, 31% of plant nitrogen was derived from BNF and inoculation resulted in as high as a 69% increase in biomass compared to the un-inoculated control (Baldani et al. 2000). Researchers also found Burkholderia vietnamiensis inoculated rice seedlings increased yield by 5.6 to 12.16%, and 42% of nitrogen found in the inoculated plants came from atmospheric nitrogen fixation (Govindarajan et al. 2008). In addition to rice, Burkholderia were found to be among the most common nitrogen-fixing isolates from maize plants cultivated in Mexico, and many were reported to be new species (Estrada et al. 2002). These findings support the use of free-living nitrogen-fixing endophytes in the effort to reduce the use of synthetic nitrogen fertilizer and offer hope in creating high-yielding, low — input agricultural production systems.

Lignin is a complex macromolecule; its cross-linked structure renders it the most recalcitrant substance for biochemical conversion to biofuels (Vanholme et al. 2008; Ralph 2010; Carpita 2012; Vanholme et al. 2012). As with polysaccharides, lignin cleavage requires synergistic action of diverse ligninolytic enzymes, including high redox potential ligninolytic peroxidases, laccases and oxidases (Leonowicz et al. 2001; ten Have et al.

2001) .

Peroxidases, which cleave C-C and C-O bonds, are classified into heme-dependent lignin peroxidases (LiPs), manganese peroxidases (MnPs), and versatile peroxidases (VPs). Phanerochaete chrysosporium and Trametes versicolor represent efficient lignin-degrading white rot fungi; both produce LiPs and MnPs (Gold et al. 1993; Johansson et al. 1996). LiPs can directly oxidize a variety of phenolic and nonphenolic aromatic compounds via long-range electron transfer. MnPs cleave phenolic substrates depending on oxidation of Mn2+ to Mn3+ by H2O2 (Harvey et al. 1992; Wariishi et al. 1992). A LiP-like MnP enzyme, called a variable peroxidase is found in Pleurotus enryngii and Bjerkandera spp. and possesses a hybrid molecular architecture that combines different oxidation sites connected to a heme cofactor (Moreira et al. 2005; Ruiz-Duenas et al. 2007). Evolutionary analysis of peroxidases in 31 fungal genomes revealed lignin-degrading peroxidases are possessed by diverse white rot, brown rot, and mycorrhizal species in the Agaricomycetes (Floudas et al. 2012). Interestingly, molecular clock analyses places the timing of the evolution of lignin degrading peroxidases at the end of Paleozoic Era, which coincides with the end of accumulation of the vast terrestrial carbon deposits that created coal.

Laccases, which also cleave C-C and C-O bonds, are widespread, four-copper containing metalloenzymes, able to catalyze the oxidation of a variety of phenolic and lower-redox potential compounds in the presence of redox mediators (Leonowicz et al. 2001). Wood-rot fungi are the main producers of laccases, especially fungi in the class of Basidiomycetes, even though some bacteria and plants also excrete these enzymes (Leonowicz et al. 2001; Sirim et al. 2011). The laccase from Pycnoporus cinnabarinus is essential for that fungi’s lignin depolymerization ability (Eggert et al. 1997). Two laccase isozymes of T. versicolor were able to depolymerize hardwood pulp lignin in the presence of 2, 2′-azinobis (Bourbonnais et al. 1995).

Apart from peroxidases and laccases, several oxidases involved in H2O2 production and aldehyde-alcohol transformation, are indispensable in lignin decomposition (Leonowicz et al. 2001). A glucose 1-oxidase mutant of Phanerochaete chrysosporium exhibited little or no ability to degrade lignin (Ramasamy et al. 1985). Aryl alcohol oxidases (AAOs) and aryl alcohol dehydrogenase (AAD) are responsible for aromatic aldehyde-alcohol transformation during ligninolysis (Leonowicz et al. 2001). Bacterial enzymes, such as ring-fission enzymes, demethylases and p-etherases, specifically degrade the oligolignols with low molecular mass that are liberated by diverse peroxidases and/or laccase (Masai et al. 1999).

Besides fungi, a number of bacteria are able to breakdown lignin (Bugg et al. 2011). Streptomyces viridosporus T7A secretes a lignin peroxidase to depolymerize lignin. Pseudomonas putida mt-2 and Rhodococcus jostii RHA, possess comparable lignin-degrading activities. Soil bacteria such as Nocardia and Rhodococcus also mineralize lignin.

Among these lignin-degrading enzymes, laccases have been developed for larger-scale applications, such as removal of polyphenols in wine and beverages, conversion of toxic compounds and textile dyes in wastewaters, and bleaching and removal of lignin from wood and non-wood fibres (Rodriguez Couto et al. 2006). Laccases have also been used to reduce phenolic inhibitors that form during biomass pretreatment and inhibit biological fermenters (Alvira et al. 2012). Organisms that produce ligninolytic enzymes can effectively function as pretreatment agents, as much as doubling subsequent saccharification yields (Akin et al. 1995). However, these organisms tend to grow relatively slowly, with treatments taking place over days or even weeks. It appears that ligninolytic enzymes remain a relatively poorly tapped resource for facilitating biofuel production.

The extranuclear DNA containing organelles (e. g., chloroplast and mitochondria) of any particular plant species are extremely useful tools in assessing genetic variation, resolving phylogenies (Timothy et al. 1979; Gielly and Taberlet 1994; Aizawa et al. 2007) and serving as vectors for transgene expression (Nakahira and Shiina 2005; Lu et al. 2006; Remacle et al. 2006; Farre et al. 2007; Hanson et al. 2012). In particular with plant species that have tetraploid genomes and higher (many of the grasses), using nuclear markers to compare across ploidy levels are difficult in these populations because gene copy number and allele frequencies are affected under polysomic inheritance (Young et al. 2011). As an alternative in these complex grassland ecosystems, chloroplast genome sequences can possibly provide a greater understanding of the evolutionary processes that have taken place during establishment from a comparative approach of isolated subpopulations (Young et al. 2011). As noted, switchgrass accessions primarily consist of lowland and upland ecotypes (Porter 1966) where lowland accessions are predominantly tetraploids (2n=4x=36), while upland accessions are octaploids (2n=8x=72) (Bouton 2007) making population genetic comparisons difficult because of a variance of ploidy levels affecting orthologous loci. In most angiosperms, the chloroplast genome consists of a quadripartite structure that includes a large single copy region (LSC) and a small single copy region (SSC) flanked by two inverted repeats, and maintains a pattern of maternal inheritance (Soltis et al. 1990; Faure et al. 1994; Vivek et al. 1999) making it an ideal genetic marker for phylogenetic studies (Chung et al. 2003; Liu et al. 2012). Previous studies aimed at discriminating ecotypes was focused on an RFLP marker in the rbcL gene (Hultquist et al. 1996), but the lack of comprehensive data points represents a need for more robust analysis. At present, two switchgrass chloroplast genomes from individuals representative of the lowland (Kanlow) and upland (Summer) ecotypes (Young et al. 2011) have been sequenced and compared to identify a 21 bp insertion in the Summer ecotype at the C-terminal region of rpoC2 gene that is a reproducible marker for resolving ecotypes (Young et al. 2011).

Along with large-scale, intensive production of switchgrass, agronomic trait improvement, such as disease and insect resistance, will become more and more important (Gressel 2008). Native switchgrass has extensive genetic diversity with fair resistance to the majority of potential pathogens (Bouton 2007). However, without knowledge of the genetic basis of disease resistance in switchgrass and the structure of pathogen populations, current and future switchgrass breeding programs that target high biomass yield and improved feedstock quality are likely to reduce the genetic diversity of disease resistance (Tanksley and McCouch 1997). Airborne foliar fungal pathogens like rust have a great potential to cause nationwide epidemics on switchgrass, resulting in significant biomass yield losses (Gustafson et al. 2003). Foliar diseases, in addition to reducing yields, can reduce the availability of saccharifiable cellulose due to increased lignification of host cell walls (Moerschbacher 1989; Parrish and Fike 2009; Shen et al. 2009). Among all potential switchgrass diseases that could negatively impact the commercial production of switchgrass, rust caused by the fungus Puccinia emaculata Schwein is the most destructive and widespread disease problem (Zeiders 1984; Gravert and Munkvold 2002; Gustafson et al. 2003; Krupinsky et al. 2004; Parrish and Fike 2005; Carris et al. 2008; Zale et al. 2008; Crouch et al. 2009; Hirsch et al. 2010; Tomaso-Peterson and Balbalian 2010).

Additionally, switchgrass seedheads can be heavily infected by smut and bunt caused by Tilletia maclagani (Berk.) G. P. Clinton and T. pulcherrima Syd. & P. Syd., respectively. While the impact of bunt infection on switchgrass production is not clear beyond plant inspection issues (Carris et al. 2008), smut has been shown to severely reduce seed and biomass yields in Iowa (Gravert et al. 2000; Thomsen et al. 2008) and has heavily infected Nebraska switchgrass accessions in Oklahoma (S. Marek, personal communications). In general, these seedborne diseases should be remediated by treating seeds with fungicides (Taylor and Harman 1990). Switchgrass is also affected by numerous fungal leaf spot diseases (Roane and Roane 1997; Gravert and Munkvold 2002; Farr and Rossman 2010), including anthracnose caused by Colletotrichumgraminicola (Ces.) G. W. Wilson and C. navitas J. A. Crouch, B. B. Clarke & B. I. Hillman (Crouch et al. 2009; Li et al. 2009), Helminthosporium leaf spot, caused by Bipolaris sorokiana (Sacc.) Shoemaker and B. oryzae (Breda de Haan) Shoemaker (Zeiders 1984, Artigiano and Bedendo 1995; Krupinsky et al. 2004; Tomaso-Peterson and Balbalian 2010), and to a minor extent tar spot, caused by Phyllachora graminis (Pers.) Fuckel, as well as undocumented diseases caused by Pyrenophora sp. and Phaeosphaeria sp. (Farr and Rossman 2010; and S. Marek, unpublished observations), could potentially impact biomass yields. In addition to these fungal diseases, at least two viral diseases, Panicum mosaic and barley yellow dwarf, affect switchgrass, with the former disease sometimes causing the death of tillers and plants (Sill and Pickett 1957; Garrett et al. 2004). Host resistance is the most effective, economical, and environmentally friendly way to control plant disease. Screening germplasm to identify resistance resources to various switchgrass diseases and developing durable and broad spectrum disease resistance will be one of the key breeding objectives in the future. Other than traditional breeding selection, genetic engineering may also have great contributions for disease control in switchgrass (Punja 2001; Stuiver and Custers 2001; Venter 2007; Collinge et al. 2008).

In addition to biotic stress, abiotic stress tolerance, such as tolerance to salinity and drought, will also be very useful. Towards that direction, Ceres has introduced a salinity-tolerance gene into switchgrass, which allows the plants to grow in sea water. The company implied that the unprecedented salt tolerance level could help in growing switchgrass (and other crops) on the 15 million acres of salt-affected soils in the U. S., as well as growing switchgrass in over a billion acres of abandoned cropland all over the world.

Concluding Remarks

Switchgrass is an important biomass/biofuel crop which would contribute substantially to our renewable energy in the future. Although molecular and genetic engineering studies just started several years ago, exciting results on quality improvement of switchgrass as a biofuel feedstock have been obtained. In addition, value-added engineering has emerged, which could be the first step towards improving the economics of biofuel production from lignocellulosic materials. With substantially improved transformation technology, many genes, which have been shown useful in model plant species, or emerge from molecular and genomic studies, could be introduced into switchgrass for its improvement via biotechnology.

As in other outcrossing transgenic plants, transgene escape, mainly through pollen grains, will be a concern. Kausch et al. (2009) has a detailed discussion to address the issue and potential solutions. Interested readers are referred to that review article.

Steam explosion is a physio-chemical pretreatment, performed in a pressurized vessel at 190-270°C for 1-10 minutes at 200-450 psig pressure with sudden release of pressure to cause an "explosion" within the physical structure of the material. A potential drawback includes the processing of lignocellulosic biomass at elevated temperatures resulting in formation of inhibitory compounds that may further inhibit the fermentation process downstream.

Acid Pretreatment

Diluted- and concentrated-acids are used for acid pretreatment. Sulfuric acid, hydrochloric acid and phosphoric acid are the most common types of acids used. Sulfuric acid pretreatment for lignocellulosic biomass has been a popular method for many years. The use of diluted or concentrated acids is highly dependent on the properties of the lignocellulosic biomass (such as lignin content, crystallinity and available surface area). In addition, the pretreatment processing time is more dependent on the nature of the feedstock. For instance, hardwood and softwood have longer pretreatment times compared to grasses, primarily due to high lignin contents and high crystallinity indexes. The ability of concentrated-acid pretreatment to break lignin at lower temperature makes it a more suitable pretreatment than diluted acid pretreatment. The concentrated sulfuric acid pretreatment (70-77% concentration) is generally performed at 50°C. At higher temperatures, the formation of inhibitory compounds such as furfural and hydroxyl-methyl furfural takes place, inhibiting microorganisms such as

E. coli and S. cerevisiae during ethanol fermentation (Galbe and Zacchi 2002; Drapcho et al. 2008).

Francis M. Epplin,[22]‘* Andrew P. Griffith[23] and
Mohua Haque[24]

Introduction

Prior to investing hundreds of millions of dollars in a switchgrass (Panicum virgatum) biomass biorefinery, due diligence would require a business plan that encompasses the complete chain from feedstock acquisition to the sales of products produced. These issues are only of importance if technology is developed to enable companies to profit from procuring switchgrass biomass, converting it to one or more useful products, and selling these products. Technologies and systems will be required to enable processing of lignocellulosic biomass, including switchgrass biomass, into a product or a

portfolio of products. One or more of these products must either be able to fulfill a unique niche of consumer demand or compete economically with existing products. If these biobased products include substitutes for fossil fuels, the potential market for cellulosic biomass could be very large. The potential market for switchgrass depends on its delivered cost relative to alternative feedstocks such as other dedicated energy crops, crop residues, or other sources of biomass. The purpose of this chapter is to identify practical issues related to the economics of developing switchgrass as a dedicated energy crop and to provide estimates of the price for delivered switchgrass biomass that would be required to compensate for the cost of inputs used to produce and deliver it to a biorefinery.

After the oil embargo of the mid 1970s, Oak Ridge National Laboratory (ORNL) added non-nuclear energy issues, including biofuels, to its research agenda (ORNL was officially incorporated into the newly established U. S. Department of Energy in 1977). At the time that ORNL was seeking an alternative source of energy, others were searching for a solution to the "farm problem". In 1978, more than 10.5 million hectares of U. S. cropland were classified as "idle" (Lubowski et al. 2006). Much of this "idle" land was diverted from crop production as a result of various federal programs including the feed grain, wheat, and cotton commodity programs (Tweeten 1970). The development of switchgrass as an energy crop was envisioned as a way to convert this "idle" land to productive use. At the same time it was seen as a way to reduce the cost of government commodity and conservation programs that were funded to entice land owners to set aside the land from the production of traditional crops. The "billion-ton update" published by the U. S. Department of Energy in 2011 projected that a switchgrass price of $66/dry Mg would be sufficient to entice land owners to convert land from current use and establish switchgrass on 21 million U. S. hectares (U. S. Department of Energy 2011).

In the U. S. the infrastructure for production, harvest, storage, transportation, and price risk management of grain is well-developed. The structure of farms used to produce grain and the infrastructure required to harvest, store, and transport grain in the U. S. has evolved over time. Relative to grain, cellulosic biomass from perennial grasses is bulky and difficult to transport. In the U. S., feedstock acquisition logistics for grains such as wheat and corn are relatively simple. Users may post a competitive price, and grain will be delivered by the existing marketing system. However, the infrastructure required to deliver a steady flow of large quantities of cellulosic biomass from fields where it could be produced and harvested to biorefineries where it would be processed remains to be developed.

One method to be considered is vertical integration. Most large U. S. firms that harvest and process trees (lignocellulosic feedstock) into wood products are vertically integrated. Through either ownership or leases, these firms have acquired the rights to millions of hectares and manage the production, harvest, and delivery of feedstock to their mills. Production characteristics and harvest cost economies could result in a structure for switchgrass production for use as a low-valued dedicated energy crop that more closely resembles the structure of integrated timber production and processing businesses. In some parts of the world, these firms harvest and process timber continuously throughout the year. If the low-cost cellulosic feedstock is a perennial with a long stand life and wide harvest window such as switchgrass, market forces could be expected to drive the structure toward vertical integration. For a mature industry, switchgrass production, harvest, and transportation could be expected to be centrally managed and coordinated, which more closely resembles a vertically integrated timber production, harvesting, and processing business than an atomistic grain system. Whether an atomistic structure such as that for U. S. grain or a vertically integrated structure, such as that for U. S. wood products would be the most economically efficient system for producing, harvesting, and delivering a flow of switchgrass biomass feedstock to biorefineries has yet to be determined.

Based on small plot research, in the years after switchgrass is established, it requires little annual maintenance (Fuentes and Taliaferro 2002). Other than harvest, most stands can be maintained with one pass over the field per year to apply fertilizer. Consequently, the relative share of harvest costs to total production costs is substantially greater for bulky biomass from switchgrass than for more dense grain from corn and wheat. Harvest costs (mowing, raking, baling, field stacking) are estimated to account for 45 to 65 percent of the total farm gate costs (including the cost of establishment, land, and fertilizer) to produce switchgrass biomass (Epplin et al. 2007). In contrast, harvest costs account for less than 15 percent of the total farm gate costs of production for corn grain. The structure is likely to be determined by the most cost efficient harvest, storage, and transportation systems.

Two related but different approaches are used to produce estimates of the switchgrass biomass price that would be required to compensate for the cost of inputs used to produce and deliver biomass. First, we present a listing of operations that may be used to establish, maintain, and harvest switchgrass. Second, this information is followed by a presentation of conventional enterprise budgets. Third, given the importance of harvest costs, assumptions regarding harvest machines are presented. Fourth, results of a mathematical programming model that is designed to estimate the costs to deliver a flow of feedstock throughout the year to a biorefinery is presented.

Maximizing biomass and lignocellulose content is the goal of most switchgrass bioenergy harvests, but the conversion platform likely will determine the optimal switchgrass harvest practices (Vogel et al. 2011). Most research supports a single annual harvest to reach these goals, for optimizing energy inputs, and for maintaining stands (Sanderson et al. 1999; Vogel et al. 2002). Maximum first-cut yields and long-term stand maintenance can be achieved by harvesting switchgrass once during the growing season to a 10-cm stubble height when panicles are fully emerged to the post-anthesis stage (Vogel et al. 2002; Mitchell et al. 2008, 2010). Harvesting after a killing frost often reduces both biomass and nutrient removal, but can provide stable biomass yields and be beneficial for long-term stand maintenance, as well as meeting feedstock characteristics suitable for thermo-chemical conversion (Mitchell and Schmer 2012).

Upland and lowland ecotypes enter dormancy at different rates when grown in the same environment (Mitchell and Schmer 2012). In central and northern latitudes, upland ecotypes senesce rapidly and are completely dormant within 7 days after a killing frost. Lowland ecotypes, however, enter dormancy slowly and have maintained green stem bases for at least 27 days after the first killing frost when exposed to low temperatures of less than 0°C on 17 of the 27 days (Mitchell and Schmer 2012). This delayed dormancy may be one explanation for the winter injury susceptibility of lowland ecotypes in central latitudes (Mitchell and Schmer 2012).

Some research suggests that upland and lowland switchgrass ecotypes may respond differently to harvest timing (Fike et al. 2006a, b), but limited research has been conducted on this topic (Mitchell et al. 2010). Research in the upper South (USA) found that in a twice-per-season cutting system (with the first harvest at near anthesis stage), biomass yield gains were modest for lowland cultivars but increased 30 to 40% with some upland cultivars (Fike et al. 2006a, b). However, the suitability of such management, particularly for improved logistics considered below; see also (Fike et al. 2007; Cundiff et al. 2009), must be weighed against the costs of added harvest, nutrient removal and process efficiency.

Proper harvest timing and cutting height and maintaining adequate N fertility are important management practices required to maximize yield and ensure persistent switchgrass stands (Mitchell et al. 2010; Vogel et al.

2011) . As mentioned previously, research generally indicates a single, post — anthesis harvest during the growing season maximizes yield, but harvesting after a killing frost ensures stand persistence and productivity, especially during drought (Mitchell et al. 2010; Vogel et al. 2011). Vogel et al. (2002) reported switchgrass biomass in the Great Plains and Midwest increases up to anthesis, then decreases by 10 to 20% until killed by frost. This fits well with recommendations by Mitchell et al. (2010) who recommended switchgrass should not be harvested within 6 weeks of killing frost or below a 10-cm stubble height. This management ensures carbohydrate translocation to the plant crowns for setting new tiller buds and maintains stand productivity. With good harvest and fertility management, productive stands can be maintained indefinitely and certainly for more than 10 years (Mitchell et al. 2010).

Switchgrass biomass yield is affected by variables such as ecotype, cultivar, harvest date, fertility, and climate. Recently, a database of switchgrass biomass production studies was compiled from research conducted at 39 field sites in 17 states which supported the single harvest for bioenergy (Wullschleger et al. 2010). Switchgrass yield averaged 8.7 ± 4.2 Mg ha1 for upland cultivars and 12.9 ± 5.9 Mg ha1 for lowland cultivars. Switchgrass harvested once at anthesis in Nebraska and Iowa had greater biomass yields than when harvested twice; yields ranged from 10.5 to 12.6 Mg ha1 yr-1 with no stand reduction (Vogel et al. 2002). In general, harvesting after frost reduces yield, but this practice ensures stand productivity and persistence, especially during drought. Such management also reduces N fertilizer requirements for the following year by about 30% (Mitchell et al. 2010; Vogel et al. 2011). Post-frost harvests allow nutrients, especially N, to be mobilized into roots for storage during winter and to support new growth the following spring. In colder climates, this management practice may have consequences for available moisture in the next growing season as it will reduce the amount of snow captured during winter; these fall harvests also will limit winter wildlife habitat value (Mitchell et al. 2010).

Harvesting after a killing frost is a logical management decision for thermo-chemical conversion platforms and biopower because N, Ca, and other plant nutrients that function as contaminants in the thermo-chemical process are minimized in the plant tissue (Vogel et al. 2002; Fike et al.

2006a, b; Guretzky et al. 2011). Although delaying harvest to after frost may reduce recoverable biomass, it can optimize yields relative to input costs. An analysis by Aravindhakshan et al. (2011) indicated that the economic optimum for switchgrass management would include annual inputs of about 69 kg N ha1 with a single end-of-season harvest.

Some have explored leaving switchgrass standing in the field over winter and harvesting the following spring (Adler et al. 2006). Deferring harvests can reduce yields by 20 to 40% compared with autumn harvests after a killing frost, this loss had no effect on gasification energy yield per unit dry matter but did reduce energy yield per land area (Adler et al. 2006). Yield losses associated with delaying harvest until spring may be acceptable if wildlife cover during winter is critical (Adler et al. 2006), but this is not likely to be a primary driver in most biomass-to-bioenergy systems.