COMBINING HARVEST DATE AND CUTTING HEIGHT TO OPTIMIZE THE SUSTAINABILITY OF MISCANTHUS PRODUCTION FOR ENERGY IN THE MEDITERRANEAN REGION

Miscanthus is a p erennial ener gy gr ass and achi eves hi gh p roduct ivit y under high su mmer temperatures, in the Mediterranean. Considering its sustainable production, the aim of this work was to evaluate the influence of harvest date and cutting height on the yields and biomass quality of Miscanthus for energy purposes. In this context, Miscanthus biomass from field studies in Portugal were sampled at three different harvest dates: September, November and January. For each harvest date, stems were separated into fractions of 50 cm. The fresh and dry weight of each fraction was recorded and the moisture, ash and nitrogen content were measured. Results indicate that higher yields are obtained in September and the higher the stubble height the lower the yields. On average, yields obtained in November and January are 20% lower than the September harvest. And increasing the stubble height to 50 cm decreases the yields by 30%. Yet, extending the harvest date to November and January improves the biomass quality for combustion. From September to November, on average, the moisture content decreases by 40% (from 50g to 30g H2O per 100g stems), the ash content of the stems decreases by 20% (from 3.5% to 2.7%, dry basis) and the nitrogen content of the stems decreases by 60% (from 0.19 % to 0.07%, dry basis). Increasing the stubble height from 0 to 50 cm does not influence the biomass quality. However, discarding the tops of the stems (fractions above 200cm), improves the biomass quality for combustion, once the ash content is reduced by 10% and the nitrogen content is reduced by 16%, while only 6% of yield loss is verified. The integrated analysis indicate that Miscanthus should be harvested in the period November-January, and that fractions above 200 cm should be left in the ground alongwith leaves, in order to improve the biomass quality and the soil nutrient status. In order to maximize the yield, the cutting height should be as low as possible given the constraints associated with the harvest machinery.