Plant Crops Bioenergy Research UK
Switchgrass
Switchgrass (Panicum virgatum L.) a perennial warm season grass is native to North America and a dominant species of the tall grass prairies. Like Miscanthus Switchgrass has a C4 photosynthetic pathway, it also outcrosses and is highly polymorphic. Two main ecotypes of switchgrass have been identified. The lowland ecotypes, generally found in flood plains, are tetraploids, growing between 0. 60-3.0 m in height. Upland ecotypes are hexaploids and octoploids, standing 0. 9-1.5 m tall. In its native environment 90 % of dry matter production occurs between June and August, growth begins between April and May; flowering begins in early June and continues until August with seed dispersal occurring mainly during August and September.
Management characteristics
Switchgrass is established from seed and seed beds are prepared using existing technology. Competition from weeds is a major source of failure during establishment. Seedbeds should be weed free as this crop is not competitive during establishment; however it is tolerant to moderate soil salinity and an acidic soil with a pH range 4.5 - 7.6, although soil neutrality is best for good establishment. Moderately deep, dry to poorly drained, sandy to clay loam soils are best. The C4 pathway gives this crop greater water use efficiency and increased productivity in hot dry environments compared with crops using the more common C3 photosynthetic pathway. Once established this crop can be productive for 10-15 years and has good pest and disease tolerance reaching full yield potential in the third year.
Harvest and yield
Harvest is once a year when the crop is most commonly cut with a mower-conditioner and baled. Harvest takes place during the winter when crop moisture content is approximately 15 -30 % which is low enough to avoid pre-storage drying. This timing of harvest also allows for nutrient translocation back to the rhizomes reducing the nutrient demand of next season's growth. In the native environment yield increase benefited from growth with N fertilization yield was increased by 15 kg biomass kg-1 N, growing with legumes, particularly Birdsfoot trefoil (Lotus corniculatus L.) also benefited yield. Across two sites in Virginia USA on marginal soils optimal yield of between 8.8-9.0 t ha-1 were obtained with an N application of 50 kg N ha-1, giving yield increases of between 1-3.5 t ha-1 over the unfertilized trials (Parrish et al., 1990). In a UK wide field trail yield increased annually for each of three years, and no nutrients were added. Maximum yield was 15.4 t ha-1 and this was obtained by a lowland ecotype averaging 9.63 t ha-1 yr-1 for four sites (DTI, 2006).
Broad chemical composition and energetics
The broad chemical characteristics of Switchgrass on a percentage basis are:Cellulose 42-50 %, Hemicellulose 44-51 % and lignin 13-20 %. This crop has a net calorific value of 18.3 GJ tonne-1 of dry matter. The energy inputs (MJ) and the biomass energy output (MJ) were modeled for Switchgrass during a 20 year cropping period. This took into account the energy demanding establishment phase, the high yield productivity phase and the final energy demanding decommissioning phase. During this 20 year period the ratio of energy output from the harvested biomass to energy input to attain the biomass was 28.9 (Bullard and Metcalf, 2001).
