Switchgrass, along with big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), and indiangrass (Sorghastrum nutans), were the grasses that accounted for nearly all of the aboveground primary production of the tallgrass prairie. Tallgrass prairie once covered 56 million ha of the central USA, but today less than 4% remains in native vegetation (Rahmig et al. 2008). Switchgrass has a long history of grazing as a component of the tallgrass prairie and grazing switchgrass monocultures has occurred for more than 40 years (Kreuger and Curtis 1979). The first switchgrass cultivar, ‘Nebraska 28’, was released jointly by the USDA and the University of Nebraska in 1949. It was developed from native seed collections in Holt County, Nebraska for livestock forage production and conservation purposes. Nebraska 28 produced steer average daily gains (ADG) of 0.93 kg hd-1 d-1 and body weight gains of 147 kg ha-1 (Kreuger and Curtis 1979). Switchgrass ADG was less than that for indiangrass, but greater than that for big bluestem. Grazing switchgrass with monogastrics is not recommended due to potentially lethal concentrations of saponins, especially for horses (Lee et al. 2009). Commercially available switchgrass cultivars bred specifically for livestock forage production includes ‘Trailblazer’ (Vogel et al. 1991) and ‘Shawnee’ (Vogel et al. 1996). Trailblazer had a 23% increase in body weight gain per hectare when compared to the cultivar ‘Pathfinder’ (Vogel et al. 1991) and Shawnee had greater in vitro dry matter digestibility (IVDMD) than ‘Cave-in-Rock’ and greater forage yield than Trailblazer (Vogel et al. 1996).

Switchgrass biofuel development began in 1984 when the U. S. Department of Energy (DOE) funded field evaluations for about 34 herbaceous species at 31 sites in seven states for their suitability for biomass production (Wright 2007; Vogel et al. 2011; Parrish et al. 2012). Switchgrass was one of the top biomass producers at most of the sites and was selected as a model biofuel feedstock by DOE in 1991 (Wright 2007). Switchgrass is a broadly-adapted native with cultivars available for most US regions, it can be grown from seed, there is an existing seed industry, and it can be grown and harvested with available hay equipment (Vogel et al. 2011). The DOE funded switchgrass production and breeding research through the Biofuels Feedstock Development Program from 1992 to 2002 (McLaughlin and Kszos 2005; Wright 2007; Vogel et al. 2011). In 2002, DOE discontinued the Feedstock Development Program (McLaughlin and Kszos 2005; Sanderson et al. 2006) and focused on crop residues like maize stover for bioenergy because of the assumed availability and low cost (Vogel et al. 2011). USDA-ARS expanded funding for bioenergy in 2002 to include switchgrass genetics, breeding, and management, focusing on its potential use on marginal croplands similar to land that is currently held out of production in CRP (Vogel et al. 2011). Vogel et al. (2011) reported new research programs were initiated on perennial energy crops like switchgrass in 2006 in the USA and estimated that over $1 billion has been invested in the USA on biomass energy research since 2006 by both government and commercial companies.

The increased scientific importance of switchgrass is demonstrated by the number of publications focusing on switchgrass. Parrish et al. (2012) reported that the number of switchgrass publications has increased exponentially since 1990, but the volume of switchgrass research is very small compared to other crops. Reports on switchgrass averaged 8 per year from 1990 through 1994, but increased to 16 articles per year in the second half of the decade (Parrish et al. 2012). In 2010, 165 publications were reported for switchgrass, whereas 5,610 publications were reported for maize (Parrish et al. 2012). Research efforts are not limited to the USA, but have been reported in more than 20 countries including Australia, Canada, China, Colombia, Germany, Greece, Ireland, Italy, the Netherlands, Spain, and the United Kingdom, as well as other areas throughout Europe and Asia (Parrish et al. 2012).

As indicated previously, switchgrass is productive on sites that are poorly suited to annual crop production. The perennial nature of switchgrass will make stands productive for at least 10 years with good management (Mitchell et al. 2012a). However, the economic feasibility of switchgrass for bioenergy hinges on establishing stands with a harvestable yield in the planting year (Perrin et al. 2008). In the planting year, it is feasible to produce and harvest 50% of the yield potential of the cultivar after a killing frost and produce and harvest 75% to 100% of the yield potential of the cultivar in the first full growing season after planting (Vogel et al. 2011; Mitchell et al. 2012a, b). Adequate weed control and uniform stands in the planting year allows for full biomass potential one to two years after planting (Schmer et al. 2006). With upland cultivars, 4 to 5 Mg ha1 after a killing frost is typical during the planting year if precipitation is near the long-term average (Mitchell et al. 2012a, b). In the first year after seeding, fields can be near full production, producing 8 to 13 Mg ha1 after a killing frost in the central Great Plains (Mitchell et al. 2010). A switchgrass stand is considered mature and at full production in the second full production year (third growing season). Lowland cultivars like ‘Kanlow’ and ‘Alamo’ originated in southern latitudes and are typically adapted to areas south of 40° N. latitude and have not been evaluated at the field scale in the Great Plains and Midwest (Vogel et al. 2011). Alamo average biomass production fertilized at 168 kg N ha-1 yr-1 was 14.5 and 10.7 Mg ha-1 yr-1 at two Texas locations (Muir et al. 2001). Without applied N, biomass production declined over the years. Small plot trials of lowland ecotypes in Nebraska produced an average of 10.1 Mg ha1 in the year after seeding, with Kanlow producing 11.7 Mg ha1 in the year after seeding (Mitchell et al. 2010). Wullschleger et al. (2010) compiled a switchgrass biomass production database of 39 field sites in 17 states which supported the single harvest for bioenergy. Mean biomass yield across all locations was 8.7 ± 4.2 Mg ha1 for upland cultivars and 12.9 ± 5.9 Mg ha1 for lowland cultivars and the yield difference between ecotypes was significant. Yield trials in Nebraska indicate new material developed specifically for biomass provides a 2.2 Mg ha1 yield increase. Upland x lowland hybrids are promising for biomass energy, with hybrids increasing yield by 32 to 54% compared with parental lines (Vogel and Mitchell 2008). Deploying hybrid switchgrass to the field scale will result in potential harvestable yields of greater than 20 Mg ha1 in the Great Plains and Midwest (USDOE 2011).

Harvesting biomass removes large quantities of nutrients from the system (Mitchell et al. 2008, 2012b). Since nitrogen (N) is the most limiting nutrient for switchgrass production and is the most expensive annual production input, reducing N removal from the switchgrass production system has a positive effect on the economic and environmental sustainability of the system (Mitchell and Schmer 2012). Nitrogen removal is a function of N concentration and biomass yield, with biomass N concentration increasing as N fertilizer rates increase (Vogel et al. 2002). Biomass has been optimized when switchgrass is harvested at the boot to post-anthesis stage and fertilized with 120 kg N ha-1, with N removed similar to the N applied (Vogel et al. 2002). The interaction of N rate and harvest date must be considered to replace only the N needed to prevent over-fertilization. Harvesting 10 Mg ha-1 of switchgrass DM with whole — plant N of 1% removes 100 kg of N ha-1, but delaying harvest until after frost reduces whole-plant N to 0.6%, resulting in the removal of only 60 kg of N ha-1 (Mitchell et al. 2012b). Few studies have quantified the nutrient removal associated with growing switchgrass for bioenergy. In the Pacific Northwest under irrigation, Collins et al. (2008) reported that each kg of N produced 83 kg of biomass and the macronutrient export averaged 214 kg N ha-1, 40 kg P ha-1, 350 kg K ha-1, 15 kg S ha-1, 60 kg Ca ha-1, 38 kg Mg ha-1, and 6 kg Fe ha-1. Averaged across cultivars, switchgrass removed less than 1 kg ha-1 of B, Mn, Cu, and Zn. In southern Oklahoma, biomass yields of switchgrass averaged 17.8 Mg ha-1 and removed 40 to 75 kg N ha-1, 5 to 12 kg P ha-1, and 44 to 110 kg K ha-1, an indication of its utility as a low-input bioenergy crop (Kering et al. 2012). In the Northeast USA, delaying harvest until spring reduced ash content and leached nutrients from the vegetation (Adler et al. 2006). An evaluation of switchgrass harvest and storage management was published recently and covers this topic in more detail (Mitchell and Schmer 2012). Additional research is needed to match harvest date, nutrient removal, nutrient composition, and conversion platform to optimize nutrient management and limit over-fertilization and unnecessary nutrient contaminants in the feedstock production stream. Chapter 2 contains a more detailed discussion on switchgrass agronomics.