Switchgrass, a perennial, warm season prairie grass and the leading candidate grass energy crop, could be grown in all rural areas in the continental United States. east of the Rocky Mountains, from North Dakota to Alabama, with the exception of southern Texas, southern Florida, and Maine.20 Until the mid-1990s, switchgrass was primar­ily known in scientific agricultural publications as a forage crop for livestock. Shortly after the turn of the millennium, the species had been tested as direct energy source, co-combusted with coal at 7-10% of the energy production levels.21 Simply burning switchgrass operates at 32% energy efficiency but using pellet grass in space-heating stoves can achieve 85% conversion efficiency. The environmental and agronomic advantages of switchgrass as a direct energy crop are severalfold:21

• Like all biomass crops, emissions are low in sulfur and mercury (especially when compared with coal).

• Switchgrass requires modest amounts of fertilizer for optimum growth, much lower application rates than with corn.

• Switchgrass stands are perennial, needing no recurrent soil preparation and so greatly reducing soil erosion and runoff caused by annual tillage.

• An acre of switchgrass could be the energy equivalent of 2-6 tons of coal, the high variability being associated with fertilizer application, climate variation, and others.

In hard economic terms, however, recycling alternative fuels such as municipal solid waste and used tires has been calculated to be preferable to either switchgrass or any form of biomass, independent of the scale of use in mass burn boilers.22 This analysis is clearly restricted to what can be “acquired” for recycling, and has very differ­ent likely outcomes if lower-wastage, non-Western economies and societies were to be similarly analyzed. Using the criterion of bulk burnable material resulting from biomass drying, herbaceous plants have been advocated as the best choice for flexibly harvestable materials intended for power production via steam boilers, this choice being over that of corn stover, tree seedlings such as fast-growing willow, tree trimmings, by-products of lumber production, or switchgrass.23

Upland and lowland cultivars of switchgrass differ appreciably in their biomass yield, tolerance to drought, and response to nitrogen fertilizer application; even between both upland and lowland variants, the differences were found to be suffi­ciently great to merit recommendations for specific types of growth habitat if energy cropping were to be practiced.24 In the northern prairies, nitrogen fertilizer use results in only variable and inconsistent increases in biomass production; a single annual harvest after the first frost is optimal for polymeric material but with reduced total nitrogen and ash as well as coinciding with low “infestation” by grass weed spe­cies; a mixture of switchgrass and big blue stem grass (Andropogon gerardii Vitman) has been recommended over dependence on a monoculture approach.25 Nitrogen application was also found to be of little benefit in a 50-year trial in southern Eng­land, where five varieties of switchgrass and one of panic grass (Panicum amarum A. S. Hitchin & Chase) were compared; delaying harvest until the dead-stem stage allowed more mineral nutrients to return to the soil.26

Like all grasses, switchgrass suffers — as a substrate for bioethanol — from its low polymeric sugar content but elevated contributions of low-molecular-weight material to its dry mass; the lignin component of the insoluble material is reduced when compared with other major lignocellulosic materials (table 1.5). This subop­timal chemistry has spurred attempts to discover means to produce industrially or commercially important biomaterials from switchgrass, in particular, high-value and nutritional antioxidants.27 28 Soluble phenolics are a potential industrial resource for fine chemicals and are present in the highest concentrations in the top internodes of the grass, whereas lower internodes contained greater amounts of cell wall-linked pheno- lics such as coumaric and ferulic acids.28 Steroidal sapogenins, starting points for the synthesis of pharmacologically active compounds, are possible hepatoxins for grazing animals.29 Grass fibers can also be used as raw material for biocomposites, packagings, and thermoplastics, and switchgrass could be a large-scale substrate for fermentations to biomanufacture biodegradable polyhydroxyalkanoate polymers.30 Pulps prepared from switchgrass also show promise as reinforcement components in newsprint.31

As with the Iogen process for bioethanol, dilute acid hydrolysis has been explored as a pretreatment methodology for switchgrass; in a batch reactor, the optimum condi­tions were 1.2% sulfuric acid at 180°C for 0.5 minute; subsequent cellulase digestion released 91.4% of the cellulose as glucose and cellobiose.32 Cellulose and lignin in switchgrass pretreated with dilute acid appeared not to interact when cellulase was added to degrade the insoluble polyglucan, acid-extracted lignin having little or no effect on the rate or extent of cellulose reactivity and saccharification.33 Mixing switch — grass with aqueous ammonia and heating under pressure at 120°C for 20 minutes aided the subsequent digestion with cellulase and xylanase.34 Milder conditions were, however, developed for the ammonia fiber explosion technology — 100°C for 5 min­utes — and resulted in a 93% solubilization of the polyglucan content of the grass.35

Switchgrass has also been included as a test biomass substrate in experimental studies of simultaneous saccharification and fermentation (SSF) (section 4.5).