Yield development and nutrient offtake in contrasting miscanthus hybrids under green and brown harvest regimes

Harvest time is an important variable that determines the yield of miscanthus biomass, its possible end uses, and the nutrient offtake from the field. Green harvests result in a higher yield and greater nutrient removal from the field. Brown miscanthus harvests, carried out in late winter or early spring, result in lower yields and a lower nutrient offtake, whereby the harvested biomass is better suited to use in combustion. To look at the long-term impact of green harvests on miscanthus, this experiment followed the yield development of two miscanthus hybrids subjected to green and brown harvests over a period of seven years at one site in Southern Germany. The standard commercial hybrid Miscanthus × giganteus (Mxg) was compared with a novel late-ripening Miscanthus sinensis hybrid: Syn55. Average yields of Mxg were 19.9 t ha⁻¹ for green harvests and 13.2 t ha⁻¹ for brown harvests compared to 13.9 and 12.9 t ha⁻¹ for green and brown harvested Syn55, respectively. Yields of Mxg were very different for green and brown harvests; green harvested Mxg had very high nutrient offtake, while brown harvested Mxg had the lowest nutrient offtakes of all treatments. Syn55 showed a less marked difference between green and brown harvests likely due to its tendency to retain its leaves over winter. Syn55 was however not tolerant of a green harvest, with yields of brown harvested stands surpassing the yield of green harvested stands in several years. Although Mxg demonstrated consistently high yields when harvested in October, some signs of yield decline were detected in both hybrids when harvested green, which was due to insufficient carbohydrate relocation. Alternating green and brown harvests are recommended to allow stands to replenish carbohydrate stores and to form a litter layer.