Rob Mitchell1, D. K. Lee2, and Michael Casler3

1 Grain, Forage, and Bioenergy Research Unit, USDA Agricultural Research Service, U. S.A.
2Department of Crop Sciences, University of Illinois, U. S.A.

3 U. S. Dairy Forage Research Center, USDA Agricultural Research Service, U. S.A.

5.1 Overview

Switchgrass (Panicum virgatum L.) is aperennial warm-season grass native to the grasslands of North America, is a model perennial grass for bioenergy, and is the most advanced herbaceous perennial bioenergy feedstock. Best management practices have been developed for switchgrass bioenergy production for the agroecoregions to which it is adapted. Field production of switchgrass likely will occur on cropland that is marginally productive for row crops, similar to land that was enrolled in the Conservation Reserve Program. Long-term, field-scale research demonstrates that switchgrass for bioenergy is productive, profitable for the farmer, and protective of the environment.

Switchgrass was selected by the Bioenergy Feedstock Development Program (BFDP) at the U. S. Department of Energy (DoE) as a model herbaceous species because of its potential to simultaneously meet energy demands and address global climate change [1]. It is a perennial, warm-season (C4) grass native to North America that is broadly adapted throughout the United States and is found in every state east of the Rocky Mountains [2]. Like many perennial C4 grasses, switchgrass is highly tolerant to abiotic stresses such as drought, temperature extremes, and salinity. For that reason, it is being recommended for biomass production on marginally productive cropland where it would have minimal land use competition with commercial food crops [3].

5.2 Phylogeny, Growth, Yield and Chemical Composition

Switchgrass is a highly polymorphic species with considerable morphological and physio-
logical variation. Much of this variation can be explained by ecotype, the main taxonomic

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

subdivision named largely for phenotypic differentiation based on habitat [4]. The two ecotypes (upland and lowland) were initially distinguished by phenotypes but now can be separated by cytotypes and gene cluster, using numerous genetic markers [5]. Common upland cultivars are “Shawnee” and “Summer”, whereas common lowland cultivars are “Alamo” and “Kanlow”.

Lowland ecotypes are mostly tetraploids (2n = 4x = 36), whereas upland ecotypes are commonly tetraploid (2n = 4x =36) or octoploid (2n = 8x =72) with hexaploid (2n = 6x = 54) reported rarely [6,7]. Aneuploids appear to be common in switchgrass, particularly at higher ploidy levels [8]. Many molecular methods have been developed and used for studying the genetic relationship between upland and lowland cytotypes. The genetic relationship among 14 populations of upland and lowland switchgrass ecotypes has been characterized by using 92 polymorphic RAPD markers [9]. Hultquist et al. [10] used chloroplast DNA restriction fragment length polymorphisms to show that these upland and lowland ecotypes are genetically different in chloroplast DNA. A deletion of 49 nucleotides in trnL-UAA introns was identified in lowland cp genome [11]. Several recent studies have investigated nuclear polymorphisms using simple sequence repeats derived from expressed sequences tags (EST-SSRs) and identified several lowland and upland subpopulations [7,12-15].

Lowland ecotypes generally are taller, coarser, and more caespitose in growth form than upland ecotypes. Generally, they are better adapted to wetter and warmer environments, whereas upland ecotypes are best adapted to drier and colder environments [4,16]. In general, lowland ecotypes have greater biomass and better disease resistance than upland ecotypes [4,16]. Both ecotypes are largely self-incompatible and plants are cross-pollinated by wind [16].

Switchgrass has the typical anatomical and physiological characteristics of a C4 grass [16]. Seedling development has three phases: germination, emergence, and adventitious root development [17]. Optimum temperature for switchgrass seed germination and seedling growth is between 20 and 30°C, while germination and seedling growth are significantly reduced at soil temperature <20°C [18,19]. Seed germination is initiated with the radicle protrusion and the coleoptile emergence from the seed coat. Once the coleoptile emerges, it is pushed to the soil surface by elongation of the subcoleoptile internode, typical of the panicoid seedling development [17]. When the coleoptile reaches the soil surface, the subcoleoptile internode elongation stops, adventitious roots form, and water uptake and photosynthesis begin for plant growth. This is why proper seeding depth is critical for successful switchgrass establishment. Seeds planted deeper than 1 cm can result in poor establishment because seedling energy reserves are used for subcoleoptile elongation and adventitious root development is delayed [17]. Several tillers may be produced within six weeks of emergence.

Switchgrass growth during the establishment year varies depending on region, weather, soil fertility, and competition with weeds [20], but in general it is feasible to produce and harvest 50% of the cultivar’s yield potential after a killing frost. Furthermore, in the first full growing season after seeding, it is very feasible to produce and harvest 75-100% of the cultivar’s yield potential [20-22] with many fields in the central Great Plains approaching full production of 8-13 Mg ha-1 [23].

New growth in post-establishment years starts in early spring, with new tillers being ini­tiated from axillary buds on the crown and/or rhizomes [24-26]. Moore et al. [27] presented the phenologic development of switchgrass by maturity stages: emergence, vegetative/leaf development, stem elongation, reproductive/floral development, and seed development and ripening. Although the durations of each stage are dependent on genetics, both photoperiod and temperature play a critical role on vegetative growth and reproductive development [28-30]. Mitchell et al. [29] and Castro etal. [31] indicated that photoperiod is the primary determinant of switchgrass development, but temperature or heat units can significantly modify reproductive development.

Switchgrass biomass yield is influenced by agroecoregion and management practices, such as ecotype, cultivar, fertilization, and harvest timing. Maughan [32] reported a meta­analysis of 106 sites from 45 studies covering the eastern two thirds of the United States and southeastern Canada. Switchgrass biomass yield across all regions of the study, including both lowland and upland ecotypes, averaged 6.6 ± 3.0 Mg ha-1 during the establishment year, increased to 9.1 ± 5.5 Mg ha-1 in the second year, and reached a maximum of

10.9 ± 5.2 Mg ha-1 in the third year. During the post-establishment years, biomass yield for lowland and upland ecotypes was 11.1 ± 6.1 and 6.7 ± 3.2 Mg ha-1, respectively. Among regions, the lower central region, equivalent to U. S. Plant Hardness Zones 6 and 7, had the highest biomass of 6.7 ± 3.2 Mg ha-1 and the north region, equivalent to U. S. Plant Hardness Zones 3 and 4, had the lowest biomass yield of 7.3 ± 3.1 Mg ha-1. High-yielding cultivars developed for biomass yield in the Great Plains and Midwest are in the release process for commercial availability.

Lignocellulosic biomass is composed primarily of structural carbohydrates, cellulose and hemicellulose, and lignin, polyphenols, with a lower concentration of other proteins, nutrients, acids, salts, and minerals. Structural carbohydrates, which generally comprise two-thirds of the dry biomass, can be hydrolyzed to sugars and those sugars can be fermented to ethanol or other forms of liquid fuel. Even though lignin is not converted to fuel by the fermentation process, other conversion technologies, such as gasification and fast pyrolysis, could use lignin as an energy source. Biomass yield is the most important characteristic for sustainable bioenergy production. However, feedstock chemical composition and its consistency, which directly influence conversion process yield, are also very important.

Switchgrass has a similar feedstock composition to other lignocellulosic feedstocks. Lee et al. [33] reported that switchgrass biomass has 37% cellulose, 29% hemicellulose, 19% lignin, 3% crude protein, and 6% ash when harvested in late autumn or after a killing frost. They also indicated that the chemical composition of switchgrass is relatively simi­lar to other crop residues, such as corn (Zea mays) stover and wheat (Triticum aestivum) straw. However, growth environment and genetics cause significant variation in feedstock composition [34]. Feedstock composition also has a significant impact on conversion effi­ciency, with one study demonstrating a range in potential ethanol production from 61 to 127 mg g-1 [34]. The range of composition data collected from multiples studies explained this variation, with cellulose, hemicellulose, and lignin varying from 31 to 45%, 22 to 25%, and 18 to 22%t, respectively.

Harvest timing is a major cultural practice affecting feedstock composition [33,35-39]. Delaying harvest to after a killing frost provided biomass with higher structural carbo­hydrates and lignin as well as lower protein and ash compared to biomass harvested at anthesis. Further delaying harvest to the following spring reduced ash and protein concen­trations even more [33,35,36]. Dien et al. [40] reported switchgrass mineral components were related to plant maturity (Table 5.1). Other studies indicate that either late season or

Table 5.1 Stage of maturity is the primary factor controlling switchgrass biomass composition within a cultivar.

post-frost harvest are likely to provide biomass with lower nitrogen, phosphorus, potas­sium, and chlorine [36,38,41]. Consequently, cultural practices can be used to provide a feedstock with the most desirable composition profile [21].