Conventional, molecular, and transgenic breeding efforts will be required to increase biomass and ethanol yields necessary to reach government — mandated fuel benchmark (Gressel 2008; Jakob et al. 2009). Conventional breeding, and to a lesser extent, molecular breeding, has provided modern — day switchgrass cultivars via selection of typically high biomass individuals from native populations. In reality, the domestication process in switchgrass

Box 1. Improvement efforts in the bioenergy crop switchgrass on multiple fronts present numerous challenges. This diagram attempts to integrate the primary areas for improvement (ovals) with efforts currently underway or in need of occurring (rectangles) via solid-lined arrows, and how these efforts directly relate to socio-economic and environmental concerns via dashed-lined arrows. Many of the relationships between the efforts and concerns are unknown and are in need of further research efforts.

is in its infancy, and challenges to improve the species will be hampered by its life cycle. In light of this, molecular efforts (Bouton 2007) and transgenic efforts hold promise to attain quick positive results.

Since biofuel output may rightly be considered the most important metric, a logical area to continue research efforts entails overcoming recalcitrance (i. e., to enable cellulose in cell walls to be more readily available for breakdown). There are a number of steps in the biofuel-making process, and improvements targeting recalcitrance can take place at a number of those steps (Fig. 3). With regard specifically to switchgrass, success has been achieved transgenically by downregulation of lignin formation (Fu et al. 2011). It is important to note that continued improvements along these lines will need to abide by and possibly incorporate gene flow prevention, and work in this direction shows promise (see Chuck et al.

2011) . Simultaneously, continued efforts aimed at better understanding of switchgrass gene flow biology is necessary for regulatory effectiveness (Kwit and Stewart 2012; Nageswara-Rao et al. 2013). Other than pollen longevity (Ge et al. 2011), little is known about effective pollination distances that agronomic switchgrass is capable of, and which, if any, wild relatives are

Biomass is harvested and delivered to

the biorefinery

Figure 3. Depiction of the steps involved in lignocellulosic ethanol production. Switchgrass breeding improvements to increase ethanol yields constitutes part of the first of several steps in the process. Image used by permission of Bioenergy Science Center.

Color image of this figure appears in the color plate section at the end of the book.

amenable to hybridization (Kwit and Stewart 2012). It is imperative that we increase our existing knowledge on the drivers of transgene spread to satisfy imminent regulatory and conservation concerns regarding switchgrass and other potential transgenic bioenergy crops. This includes garnering a better understanding on gene flow and the fitness of F1 hybrids and their offspring. While efforts to overcome recalcitrance may not appear to have fitness consequences promoting invasiveness in escaped or introgressed populations, improvements in other areas may (see below).