Plant Crops Bioenergy Research UK
Miscanthus
Miscanthus (Miscanthus spp.) is a genus of perennial, woody, tufted or rhizomatous grasses, related to sugarcane and native to South-east Asia. Like Switchgrass this genus has a C4 photosynthetic pathway. The solid pith stems (approximately 10 mm in diameter, and usually unbranched) grow rapidly, about 4 m yr-1 in Europe and 7-10 m yr-1 in China. This rapid growth rate is not attained until approximately 2-3 years after establishment, during which time stems grow a little over 2 m yr-1. In the UK the most common genotype planted as an energy crop is M. x giganteus, a pollen sterile natural hybrid, probably of M. sinensis (diploid) x M. sacchariflorus (tetrapoloid). Shots emerge from rhizomes in late spring once daytime temperatures exceed approximately 10C, and continue to grow until October; maximum growth rate is through May-July. Senescence results in nutrients being translocated back to the rhizomes, and loss of leaves, full senescence occurs with the first autumn frosts. The stems dry through the winter and free standing almost leafless canes are left by February of the next year.
Management characteristics
In the UK rhizome division is favored over micro-propagation as the technique for obtaining planting material. This involves splitting two - three year old plants with a rotary cultivator while the rhizomes are dormant. Rhizome pieces are collected for re-planting; they must contain 2-3 buds and be moist on planting. Rhizomes can be cold stored for up to 1 year (< 4C), and once planted have a low frost tolerance. Optimal planting density is 20, 000 plants ha-1 and a planting depth of 5 -10 cm, conventional seed potato planters provide a very good establishment success (95 %) although bespoke planters increase planting rate (Defra, 2001). Miscanthus is productive on soils ranging from sands to those high in organic matter and can tolerate a range of soil pH but the optimum is between pH 5.5 - 7.5. This crop has good nutrient-use-efficiency resulting from efficient translocation of nutrients to the rhizomes during senescence, and retention of leaf litter in the field, harvesting only the stems results in little nutrient removal from the soil. If required particularly during the establishment phase, liquid or granular fertilizer can be applied. Using a C4 photosynthetic pathway this crop is also water-use-efficient. Once established the crop can produce an annual harvest for up to 15 years. Competition during establishment can influence yield, the predominant weeds are wild oat (Avena spp.), creeping thistle (Cirsium arvense), and couch grass (Elymus repens). Sites should be cleared of perennial weeds before planting but leaf litter produced by the second year of establishment generally suppresses weed competition. Following the establishment year a spring application of a broad spectrum herbicide may be needed. No pests and diseases to which Miscanthus is susceptible in its native Asia have been reported in the UK, and no common European insect pests are known to significantly affect yield. However disease such as barley yellow dwarf virus (BYDV) is known to occur and may limit yield.
Harvest and yield
Harvest is once a year and carried out between January and March, and for energy cropping this is commonly carried out with a mower-conditioner, although a forage harvester may be used. The ridged stems are broken up by the conditioner accelerating moisture loss, and the windrows can be further left to dry in the field. In the UK average stem moisture content has been around 50 % but can be as low as 20 % conditioning and field drying help to substantially reduce stem moisture content, the product is then baled. Depending on moisture content and end-use the biomass may also be processed into pellets or briquettes in the field or at a processing facility. In the UK second year harvestable yield range from 4-10 t ha-1, and from the third year onwards average yields range between 10-13 t ha-1 yr-1 and can reach 20 t ha-1 yr-1 (Defra, 2001). Following seven annual harvests of M x giganteus from multiple non-irrigated sites across England and Wales mean yield (stems + leaf litter) was 18. 5 ODT ha-1 yr-1 for a site in Devon (South England) to 7.3 ODT ha-1 yr-1 for a site in North Yorkshire (North England). Of all sites the site in Devon had the largest number of days in the growing season and the highest mean incident solar radiation (Price et al., 2004).
Broad chemical composition and energetics
The broad chemical characteristics of Miscanthus on a percentage basis are: Cellulose 44 %, Hemicellulose 24 % and lignin 17 %. This crop has a net calorific value of 17 GJ t-1 dry weight. The energy inputs (MJ) and the biomass energy output (MJ) were modeled for Miscanthus 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 35.9 (Bullard and Metcalf, 2001). Compared with Reed canarygrass and Switchgrass the establishment phase of Miscanthus production is more energy demanding but the high yields give this crop the highest long-term energy return of the other dedicated energy grasses.
Production
Between 2001 and 2006 the cumulative planting of Miscanthus had reached 3356 hectares. This area will nearly triple in 2007 with approved planting for 2007 making a cumulative total of 12627 hectares producing Miscanthus (DTI, 2006).
References
Price L, Bullard M, Lyons H, Anthony S, and Nixon P. (2004). Identifying the yield potential of Miscanthus x giganteus: an assessment of the spatial and temporal variability of M. x giganteus biomass productivity across England and Wales. Biomass and Bioenergy. 26 pg 3 - 13.
