Rob Mitchell[1]‘* and Marty Schmer[2]

This chapter provides an overview of switchgrass and its development into a biomass energy crop. It is not our intent to provide an exhaustive review of a specific topic, but to introduce numerous topics that will be covered in detail in later chapters.

Switchgrass is native to the North American tallgrass prairie and to habitats east of the Rocky Mountains and south of 55°N latitude (Stubbendieck et al. 1997). Switchgrass is a warm-season perennial grass that has the characteristic C4 physiology and anatomy (Parrish et al. 2012). Switchgrass is broadly-adapted to soil types, fertility, and moisture conditions throughout North America. Switchgrass plants are generally caespitose or with short rhizomes and reproduce both sexually and asexually. The main taxonomic subdivision is at the ecotype level named largely for phenotypic differentiation based on habitat (Casler 2012). Switchgrass has two primary ecotypes (upland and lowland) and two primary ploidy levels (tetraploid and octoploid) (Vogel et al. 2011). Switchgrass genotypes are largely self-incompatible and seed production results from cross-pollination by wind (Vogel 2004). Switchgrass generally grows 1 to 3 m tall depending on environment and genetic background and can develop extensive root systems that occupy most of the soil profile to a depth of 3 m (Weaver 1954; Vogel 2004). The extensive fibrous and deep perennial root system protects soil from erosion and sequesters large quantities of carbon (C) in the soil profile (Liebig et al. 2005).

Although interest escalated after switchgrass was mentioned in the 2006 State of the Union Address by President Bush, switchgrass is not a new crop and switchgrass research is not a new undertaking. The USDA location in Lincoln, Nebraska, USA has conducted switchgrass research continually since 1936. As a result, switchgrass was seeded on thousands of hectares of marginally-productive cropland as part of the Conservation Reserve Program (CRP) as well as in pastures and vegetative filter strips throughout the eastern half of the USA. The biomass accumulation and root structure make switchgrass well-suited for both bioenergy production and conservation plantings. For example, switchgrass grown in vegetative filter strips has removed 47% to 76% of the total reactive phosphorus in surface runoff water in areas treated with manure (Sanderson et al. 2001). Although the first 50-years of research and use focused on switchgrass for livestock and conservation, the research since 1990 has emphasized bioenergy (Vogel 2004; Vogel et al. 2011; Parrish et al. 2012).

An important consideration in growing perennial energy crops such as switchgrass is the type and amount of land that will be required to grow an adequate feedstock supply. Switchgrass is well suited to marginally — productive or difficult to farm parcels (Mitchell et al. 2012a, b). Most switchgrass production for bioenergy likely will occur on marginally productive land currently planted to other crops and from areas enrolled in CRP. The ability of these marginally productive sites to provide long-term sustainable production of maize (Zea mays L.) and soybeans (Glycine max L. Merr) is in question. A 5-year study conducted on marginally productive land that qualified for CRP in Nebraska demonstrated that the potential ethanol yield of switchgrass was equal to or greater than the potential ethanol yield of no-till maize grown on similar sites (Varvel et al. 2008). Switchgrass provides environmental advantages compared to traditional annual crops such as reduced inputs, reduced erosion on marginal cropland, and enhanced wildlife habitat (Mitchell et al. 2010a). However, there are concerns about converting CRP to switchgrass. For example, the historic loss of grassland habitat has reduced many grassland nesting birds. The establishment of more than 12 million ha of perennial grasslands under CRP has mitigated this grassland habitat loss and has been a highly successful program for grassland bird species recovery in the Great Plains and Midwest. The response of grassland birds to the conversion of CRP, which is typically floristically and structurally diverse, to switchgrass, which is more uniform, is uncertain (Robertson et al. 2010). In the end, switchgrass must be productive, protective of the environment, and profitable for the farmer to be adopted on a large scale (Mitchell et al. 2012b).

Mitchell et al. (2012b) addressed supplying feedstock to a commercial scale biorefinery. They made some basic assumptions (i. e., an ethanol yield of 334 liters from each Mg of switchgrass dry matter (DM) using SSF, switchgrass yield of 11 to 22.4 Mg DM ha-1, 40 km transport distance) and reported that a 189 million liter (50 million gallon) per year biorefinery requires about 567,000 Mg of feedstock each year. Given these parameters, the total land area required in switchgrass production can range from 5 to 50% of the cropland in the 40-km radius around the biorefinery, depending on the biomass production of the feedstocks.

One viable land resource may be non-irrigated center pivot corners (Mitchell et al. 2012b). A center pivot located on a quarter section (~64 ha, 160 acres) typically irrigates only 53 ha (132 acres), leaving 11 ha (28 acres) of rainfed cropland in the four corners. Consequently, the pivot corners are marginally productive relative to the irrigated land because they receive no supplemental water. For example, a single fuelshed in eastern Nebraska that is heavily irrigated with center pivots could grow 50,500 ha of switchgrass in pivot corners alone, enough for one 189.3 million liter (50 million gallon) per year ethanol plant at 11.2 Mg ha-1 (5 tons acre-1), or two 189.3 million liter per year plants at 22.4 Mg ha-1 (10 tons acre-1) (Mitchell et al. 2012b). Managing switchgrass as a hay crop is not foreign to most farmers and the economic opportunities presented by switchgrass for small, difficult to farm, or poorly-productive fields will provide an economic incentive for many farmers to grow switchgrass (Mitchell et al. 2012b). Due to escalating interest in switchgrass for bioenergy, in-depth evaluations of switchgrass for bioenergy are increasingly available (Vogel et al. 2011; Sanderson et al. 2012).