Yield development and nutrient offtake in contrasting Miscanthus genotypes 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 genotypes subjected to green and brown harvests over a period
of seven years at one site in Southern Germany. The standard commercial
genotype Miscanthus x giganteus (Mxg) was compared to a novel
late-ripening Miscanthus sinensis genotype: 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 t ha-1 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. Mxg was shown to be tolerant of green harvests, producing
consistently high yields when harvested in October. Early signs of yield
decline were detected in both genotypes when harvested green, likely due
to incomplete carbohydrate relocation. Alternating green and brown
harvests are recommended to allow stands to replenish carbohydrate stores
and to form a litter layer.