The canes of Miscanthus are harvested once each year, from the second year after estab­lishment using a self-propelled forage harvester, like that used for harvesting silage maize or by hay harvesting equipment, including cutters, conditioners and balers [92, 120]. Hay harvesting equipment is used more frequently as it generates a denser feedstock, thus decreasing transport volume [120].

distributions for the invasive species of agronomical origins are similar to the bioenergy crops.

M. sinensis — Large climate niche

(broad climatic tolerance) of bioenergy crops positively correlated with invasiveness.

M. sacchariflorus This does not indicate

invasiveness but broad climatic tolerance have to be considered in evaluation of invasiveness of bioenergy crops

Miscanthus is generally harvested in late winter when biomass quality is at its peak for combustion processes and before crop growth increases again from early spring. By late winter, the composition of biomass is more suited for bioenergy such as combustion (see section genetic diversity for biomass composition). Moisture content is also lower in late winter, enabling a higher production of net energy. In one study, moisture content was shown to decrease by more than half between early and late winter, from 47 to 52% in December to 16-20% in March [43].

A disadvantage of a late winter harvest is that there is less harvestable biomass at this time due to the loss of senescent leaves and the remobilization of nutrients from aboveground to belowground biomass [28]. More than a third of the aboveground biomass can be lost in M. x giganteus between the early harvest period of September or October and the late winter harvest time of February to March [28, 37, 38]. Similar yield losses have been observed for M. sinensis and M. sacchariflorus [24]. An early harvest might better suit cellulosic ethanol production because there should be more lignocellulose sugar available at this time [121]. However, it is likely that early harvested crops will subsequently require more nitrogen and other fertilizer, as considerably more nutrients are exported from the system with an early rather than a late harvest. For example, Strullu et al. [28] showed that M. x giganteus crops harvested in late winter mobilized 71% of their peak nitrogen content to belowground biomass compared with just 42% of the peak nitrogen content of early harvested plants. In the same trial, the nitrogen content of senescent leaves collected over winter from the soil of 2-3 year-old crops amounted to 15.5 ± 3.5 kgN ha-1 yr-1 in late harvested treatments but was negligible in early harvested treatments [50]. An early harvest is, therefore, likely to increase the crop’s requirement for added nitrogen and other fertilizer.

In addition, leaf fall during winter provides carbon to the soil and the litter layer by senescent leaves can also help control weeds. In the same field trial, senescent leaves correspond to about 3 tDM ha-1 yr-1, which correspond to about 1.3 tC ha-1 yr-1 [50]. This was smaller than the amount measured by Kahle et al. [122] for older plants (4.5 tDM ha-1 yr-1, corresponding to about 2 tC ha-1 yr-1). The weight of the litter formed by senescent leaves of Miscanthus x giganteus accumulated at the soil surface was measured every year during nine years of cultivation by Christian et al. [46]. It increased during the four first years and was then fairly constant in time (with about 6 tDM ha-1 on average), probably because the fall of new senescent leaves each year was compensated by leaves decomposition.

In summary, a winter harvest has advantages over an earlier harvest. It allows a remo­bilization of nutrient from aboveground to belowground biomass and it can contribute to enhance carbon sequestration in soil. Nevertheless, the quality of the canes at that date might reduce accessibility of the cellulose and hemicellulose, which would require appropriate processes for some end-uses such as biofuel.