Maximizing biomass and lignocellulose content is the goal of most switchgrass bioenergy harvests, but the conversion platform likely will determine the optimal switchgrass harvest practices (Vogel et al. 2011). Most research supports a single annual harvest to reach these goals, for optimizing energy inputs, and for maintaining stands (Sanderson et al. 1999; Vogel et al. 2002). Maximum first-cut yields and long-term stand maintenance can be achieved by harvesting switchgrass once during the growing season to a 10-cm stubble height when panicles are fully emerged to the post-anthesis stage (Vogel et al. 2002; Mitchell et al. 2008, 2010). Harvesting after a killing frost often reduces both biomass and nutrient removal, but can provide stable biomass yields and be beneficial for long-term stand maintenance, as well as meeting feedstock characteristics suitable for thermo-chemical conversion (Mitchell and Schmer 2012).

Upland and lowland ecotypes enter dormancy at different rates when grown in the same environment (Mitchell and Schmer 2012). In central and northern latitudes, upland ecotypes senesce rapidly and are completely dormant within 7 days after a killing frost. Lowland ecotypes, however, enter dormancy slowly and have maintained green stem bases for at least 27 days after the first killing frost when exposed to low temperatures of less than 0°C on 17 of the 27 days (Mitchell and Schmer 2012). This delayed dormancy may be one explanation for the winter injury susceptibility of lowland ecotypes in central latitudes (Mitchell and Schmer 2012).

Some research suggests that upland and lowland switchgrass ecotypes may respond differently to harvest timing (Fike et al. 2006a, b), but limited research has been conducted on this topic (Mitchell et al. 2010). Research in the upper South (USA) found that in a twice-per-season cutting system (with the first harvest at near anthesis stage), biomass yield gains were modest for lowland cultivars but increased 30 to 40% with some upland cultivars (Fike et al. 2006a, b). However, the suitability of such management, particularly for improved logistics considered below; see also (Fike et al. 2007; Cundiff et al. 2009), must be weighed against the costs of added harvest, nutrient removal and process efficiency.

Proper harvest timing and cutting height and maintaining adequate N fertility are important management practices required to maximize yield and ensure persistent switchgrass stands (Mitchell et al. 2010; Vogel et al.

2011) . As mentioned previously, research generally indicates a single, post — anthesis harvest during the growing season maximizes yield, but harvesting after a killing frost ensures stand persistence and productivity, especially during drought (Mitchell et al. 2010; Vogel et al. 2011). Vogel et al. (2002) reported switchgrass biomass in the Great Plains and Midwest increases up to anthesis, then decreases by 10 to 20% until killed by frost. This fits well with recommendations by Mitchell et al. (2010) who recommended switchgrass should not be harvested within 6 weeks of killing frost or below a 10-cm stubble height. This management ensures carbohydrate translocation to the plant crowns for setting new tiller buds and maintains stand productivity. With good harvest and fertility management, productive stands can be maintained indefinitely and certainly for more than 10 years (Mitchell et al. 2010).

Switchgrass biomass yield is affected by variables such as ecotype, cultivar, harvest date, fertility, and climate. Recently, a database of switchgrass biomass production studies was compiled from research conducted at 39 field sites in 17 states which supported the single harvest for bioenergy (Wullschleger et al. 2010). Switchgrass yield averaged 8.7 ± 4.2 Mg ha1 for upland cultivars and 12.9 ± 5.9 Mg ha1 for lowland cultivars. Switchgrass harvested once at anthesis in Nebraska and Iowa had greater biomass yields than when harvested twice; yields ranged from 10.5 to 12.6 Mg ha1 yr-1 with no stand reduction (Vogel et al. 2002). In general, harvesting after frost reduces yield, but this practice ensures stand productivity and persistence, especially during drought. Such management also reduces N fertilizer requirements for the following year by about 30% (Mitchell et al. 2010; Vogel et al. 2011). Post-frost harvests allow nutrients, especially N, to be mobilized into roots for storage during winter and to support new growth the following spring. In colder climates, this management practice may have consequences for available moisture in the next growing season as it will reduce the amount of snow captured during winter; these fall harvests also will limit winter wildlife habitat value (Mitchell et al. 2010).

Harvesting after a killing frost is a logical management decision for thermo-chemical conversion platforms and biopower because N, Ca, and other plant nutrients that function as contaminants in the thermo-chemical process are minimized in the plant tissue (Vogel et al. 2002; Fike et al.

2006a, b; Guretzky et al. 2011). Although delaying harvest to after frost may reduce recoverable biomass, it can optimize yields relative to input costs. An analysis by Aravindhakshan et al. (2011) indicated that the economic optimum for switchgrass management would include annual inputs of about 69 kg N ha1 with a single end-of-season harvest.

Some have explored leaving switchgrass standing in the field over winter and harvesting the following spring (Adler et al. 2006). Deferring harvests can reduce yields by 20 to 40% compared with autumn harvests after a killing frost, this loss had no effect on gasification energy yield per unit dry matter but did reduce energy yield per land area (Adler et al. 2006). Yield losses associated with delaying harvest until spring may be acceptable if wildlife cover during winter is critical (Adler et al. 2006), but this is not likely to be a primary driver in most biomass-to-bioenergy systems.