Biomass yield and quality of 20 switchgrass populations in southern Iowa, USA☆
Introduction
Perennial, herbaceous energy crops offer a significant opportunity to improve agricultural sustainability through crop diversification, decreased erosion, and improved water quality compared with a traditional annual row crop system [1]. The perennial nature of these crops makes their cultivation desirable on highly erosive land, particularly if they can produce acceptable yields on poor quality soils. In order to be profitably grown, energy crops need to produce high yields of biomass, low concentrations of water, nitrogen, and ash, and high concentrations of lignin and cellulose [2]. Switchgrass, a warm-season (C4) grass native to much of the central, midwestern, and southeastern United States, has been proposed as the herbaceous perennial plant most suitable for biofuel production in these regions [3].
Across its wide native geographic range, switchgrass has evolved into two types: (i) lowland ecotypes, which are vigorous, tall, thick-stemmed, and adapted to wet conditions, and (ii) upland ecotypes, which are short, rhizomatous, thin-stemmed, and adapted to drier conditions [4]. Lowland ecotypes are predominantly tetraploid (2n=4×=36); upland ecotypes are typically hexaploid (2n=6×=54) or octaploid (2n=8×=72) [5], [6]. Switchgrass is photoperiod sensitive, and flowering is related to the latitude at which a particular germplasm evolved, with northern populations flowering earlier than southern populations [7]. Selection of late flowering genotypes originating at lower latitudes under northern conditions has resulted in switchgrass yield improvements for the Great Plains and Upper Midwestern US [8].
Switchgrass grown for biofuel can either be used directly to generate power by cofiring with coal or indirectly as a fuel by fermentation to ethanol [9]. The quality of switchgrass for fuel depends on the concentration of energy, primarily derived from the cell walls and particularly from lignin and cellulose. In addition, certain elements and minerals, including potassium, sodium, chlorine, silica, and others could cause corrosion, slagging, and fouling of the boilers and other components of the power plant, decreasing efficiency and increasing maintenance costs [10]. A switchgrass of ideal quality for co-firing would contain a high concentration of lignin and cellulose while minimizing total ash, chloride, and other undesirable elements.
Cultivar selection can have a major impact on the ultimate productivity, persistence, and profitability of a forage crop [11], [12]. By extension, we might expect that cultivars differ for biofuel traits, and the optimum forage cultivar may not be the same as the most desirable biofuel cultivar, given the contrasting needs of the two uses. The switchgrass cultivar Cave-In-Rock (CIR) is widely recommended for forage in Iowa and other upper Midwestern US states because of its persistence, high dry matter yield, and superior nutritive value. However, a thorough examination of CIR and other cultivars for biofuel characteristics has not been conducted in the region.
The objective of this experiment was to test the hypothesis that Cave-in-Rock was the best switchgrass cultivar for biofuel production in southern Iowa by evaluating the biomass yield and quality of a diverse assemblage of 20 cultivars and germplasms.
Section snippets
Plant materials
Twenty cultivars and experimental populations of switchgrass were included in the study. The cultivars included in the study were ‘Alamo’, ‘Blackwell’, ‘Caddo’, ‘Carthage’, Cave-in-Rock (CIR), ‘Forestburg’, ‘Kanlow’, ‘Pathfinder’, ‘Shawnee’, ‘Shelter’, ‘Sunburst’, and ‘Traiblazer’. In addition, eight experimental populations were included: two selected by Drs. E.C. Brummer and K.J. Moore at Iowa State University from CIR for grazing tolerance (IA-GT) and late maturity (IA-LM); four developed by
Biomass yield
Biomass dry matter yield averaged across all entries and years was . Substantial variability was evident among years, with the average yield ranging from in 1998 to in 1999 (Table 1). The improvement in yield between 1998 and 1999 was concurrent with an increased stand percentage (Table 1) and a higher nitrogen fertilization rate. Germplasms varied widely for biomass yield (Table 2), but no cultivar by year interaction was present. Two lowland cultivars, Kanlow and
Conclusions
Differences in yield among cultivars were greater than differences in biomass quality and mineral composition. Thus, selection of appropriate cultivars, at least for southern Iowa, can be based primarily on biomass production. Cave-in-Rock, although widely grown, does not appear to have the maximum biomass potential for southern Iowa. Several cultivars and populations developed from lowland ecotypes had better yield than CIR, but their adaptability to southern Iowa is uncertain. Cultivar
Acknowledgements
This Research was funded by the Department of Energy's Bioenergy Feedstock Development Program at Oak Ridge National Laboratory under subcontract 90X-SY510V with Iowa State University, Ames, Iowa. The authors would like to thank Mark Downing, Sandy McLaughlin, Mark Smith, Trish Patrick, Jim Secor, John Sellers, Marty Braster, Jim Cooper, and the Chariton Valley RC&D for their assistance with this project.
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Journal Paper No. J-19831 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011, Project No. 2569, supported by Hatch Act and State of Iowa Funds.