The 1992 Annual Progress Report of the DOE’s Biofuels Feedstock Development Program (Wright et al. 1993) supported the selection of switchgrass as a model bioenergy crop by stating, "the examination of data on yield potential, production economics, and regional site potential, led in 1991 to the selection of a perennial forage grass switchgrass as a model species for further research", recognizing that "more than one species will certainly be required ultimately, switchgrass was seen as an excellent beginning with the available programmatic resources". Several important characteristics such as being a widely adapted native species, a demonstrated capacity for high yields on relative poor quality sites, a significant capacity to improve soil quality by sequestering carbon, improved erosion control, reduced fertilizer and pesticide requirements, a capacity for providing wildlife cover, and a strong potential appeal to landowners supported this decision (Wright et al. 1993). Extensive research has continued to support the feasibility of switchgrass for bioenergy (Mitchell et al. 2012b).

Bioenergy efficiency and sustainability is held to a different standard than energy produced from petroleum since renewable fuels must have lower greenhouse gas (GHG) emissions and higher net energy values (NEV) than petroleum based transportation fuels (Mitchell et al. 2010a). The NEV, net energy yield (NEY), and the ratio of the biofuel output to petroleum input [petroleum energy ratio (PER)] have been used to quantify the energy efficiency and sustainability of ethanol produced from switchgrass (Schmer et al. 2008). Farrell et al. (2006) developed an energy model using estimated agricultural inputs and simulated yields and predicted switchgrass produced 700% more output than input energy. Schmer et al. (2008) validated the modeled results with actual inputs from switchgrass grown at the field scale on 10 farms in Nebraska, South Dakota, and North Dakota. They concluded that switchgrass produced 540% more renewable energy than non-renewable energy consumed over a 5-year period, had a PER of 13.1, and that average GHG emissions from switchgrass-based ethanol was 94% lower than estimated GHG emissions for gasoline (Schmer et al. 2008).

To sustain an agricultural production system, carbon inputs must equal or exceed the carbon outputs or soil organic carbon (SOC) will decline and overall system productivity will decline (Mitchell et al. 2010a). Historically, about half of the SOC present in pre-agricultural grasslands was presumed lost in the conversion of perennial grasslands to annual cropland that occurred after European settlement (Mitchell et al. 2010a). Consequently, SOC trend is an excellent indicator of the long-term sustainability of a production system. SOC increases rapidly when annual cropland is converted to switchgrass (Mitchell et al. 2010a). In just 5 years, growing and managing switchgrass for bioenergy on three marginally productive cropland sites in the Central Plains resulted in an average SOC increase of 2.9 Mg C ha1 yr-1 in the top 1.2 m of soil (Liebig et al. 2008). Growing switchgrass increased SOC at rates ranging from 1.7 to 10.1 Mg C ha1 yr-1 throughout North America (Garten and Wullschleger 2000, Zan et al. 2001, Frank et al. 2004; Lee et al. 2007). In irrigated switchgrass in the arid regions of the Pacific Northwest, 5-years of switchgrass cropping resulted in a 1.2 Mg ha-1 increase in SOC in the 0 to 15-cm depth, with no change below 15 cm (Collins et al. 2010).

Modeling efforts and numerous field studies have demonstrated that growing and managing switchgrass for bioenergy on sites formerly in row crop production rapidly and significantly increases SOC, improves soil quality, and promotes long-term sustainability (Liebig et al. 2005, 2008; Schmer et al. 2011; Follett et al. 2012). A limitation with many modeling efforts is that SOC accumulation is usually only predicted for sampling depths of 30 to 40 cm (Follett et al. 2012) and may be underestimating actual SOC accumulation. For example, a 9-year study on rainfed switchgrass and maize had average annual increases in SOC that exceeded 2 Mg C ha1 year-1 for the 0 to 150 soil depth and over 50% of the SOC increase occurred below 30-cm (Follett et al. 2012). The SOC for switchgrass was 2 to 4 times greater in this study than that modeled in life-cycle assessments to date. They concluded that sampling soil to only 30 to 40 cm is inadequate and

future analyses and modeling should include deep soil sampling to fully account for SOC accumulations in both switchgrass and maize (Follett et al. 2012). Chapter 12 addresses specific approaches to modeling switchgrass biomass production.

Switchgrass production for bioenergy is economically feasible (Perrin et al. 2008; Mitchell et al. 2012a, b). A large regional field scale trial was conducted in 50 production environments on 10 farms in Nebraska, South Dakota, and North Dakota (Perrin et al. 2008). Actual on-farm production costs were tracked for each farm, including land costs, which accounted for nearly half of the production costs. The cost of production for switchgrass to the farm gate averaged $66 Mg-1 (Perrin et al. 2012). Five farmers delivered switchgrass to the farm gate at an average cost of $52 Mg-1over the 5-year period. The 5-year average cost for farmers with experience growing switchgrass was $39 Mg1, and one producer grew switchgrass for $34 Mg1. Switchgrass farm-gate costs tend to decline over time with highest costs occurring on a per mass basis during the establishment year; a result of high input costs and low biomass yields (Perrin et al. 2008). When the authors projected field production for 10 years, farm-gate delivery costs were reduced to $46 Mg-1. They concluded that, with experience, farmers could achieve switchgrass production costs of $40 to $55 Mg1. Assuming a conversion rate of 0.329 liters of ethanol per kg of switchgrass, the farm-gate feedstock cost would range from $0.12 to $0.16 L1 (Perrin et al.

2008) . Land and other production costs have increased since the regional field scale study was completed. Perrin et al. (2012) estimated an updated switchgrass farm-gate price of $75 Mg1 and $60 Mg1 for biomass yields of 6.7 Mg ha1 and 13.5 Mg ha1, respectively. Farm-gate costs in growing switchgrass for bioenergy are largely variable with respect to yield, with approximately 25% of total costs being fixed (Perrin et al. 2012). Farm-gate prices are also dependent on land type being converted, regional variations in land costs, yield potential, and rotation time length. An estimated 5.1 x 106 ha to 11.8 x 106 ha could be allocated to switchgrass production in the United States by 2030 assuming a farm gate price of $44 Mg1 to $66 Mg1 (USDOE 2011). Future improvements in large-scale harvest machinery and implementation of farm telematics will likely reduce variable switchgrass harvest and delivery costs. See Chapter 13 for a more detailed discussion of the economics of switchgrass feedstock production.

Conclusions

Switchgrass is the most advanced herbaceous perennial feedstock for bioenergy. Switchgrass research has been conducted for more than 75 years, with a focus on bioenergy for more than 20 years. Mitchell et al.

(2012) reported on the feasibility of growing switchgrass for bioenergy. They reported that all practices for growing switchgrass for biofuels including establishing, managing, and delivering to the biorefinery gate have been developed, with specific management requirements for most US agroecoregions (Mitchell et al. 2012). They concluded that the research to date fully supports that switchgrass for bioenergy is productive, protective of the environment, and profitable for the farmer. Additionally, switchgrass has been seeded on millions of hectares of CRP grasslands since 1986, so it is familiar to many producers. Further research on the processes of converting switchgrass to transportation fuels at the commercial scale is needed. Additionally, field-validating some of the models for deploying switchgrass at the landscape scale are needed to demonstrate the feasibility and environmental benefits, especially for wildlife, of large-scale feedstock production. Switchgrass has high biomass production potential, wide adaptability, low fossil fuel energy requirements, and is compatible with modern agriculture practices making it an ideal herbaceous energy crop for large-scale bioenergy production. Significant research has been conducted on switchgrass genetics, agronomic management, and harvest practices which will be invaluable for an emerging cellulosic bioenergy industry.