Methane production potential from Miscanthus sp.: effect of harvesting time, genotypes and plant fractions.

The perennial C 4 grass miscanthus was evaluated for use as an energy crop for methane production when harvested green in the autumn. Miscanthus  ×  giganteus ( M.  ×  giganteus ) and Miscanthus sinensis ( M. sinensis ) were harvested on five occasions, from August to November 2012. Methane yields from stems and leaves were analysed using batch assay after 90 d digestion. Estimated dry matter yields were highest on 1st October for M.  ×  giganteus and 13th September for M. sinensis. Cellulose and lignin contents were greater with M.  ×  giganteus than M. sinensis and low lignin content in leaves led to rapid degradation during the early periods of anaerobic batch assay. After 90 d of anaerobic digestion, cumulative specific methane yields for M.  ×  giganteus varied for stem and leaf from 285 to 333 and 286 to 314 Nl (normalised litre) kg −1 [VS] and 291 to 312 and 298 to 320 Nl kg −1 [VS] for M. sinensis . Estimated methane yields per ha were positively correlated with the dry matter yields of miscanthus ( r  = 0.92) and the optimal harvesting time was between September–October. Methane yield at optimal harvest time was estimated as 3.824 × 10 6 Nl ha −1 (stem) and 1.605 × 10 6 Nl ha −1 (leaf) for M.  ×  giganteus and 3.507 × 10 6 Nl ha −1 (stem) and 2.957 × 10 6 Nl ha −1 (leaf) for M. sinensis . There was a discrepancy between the estimating dry matter yield by sampling single shoots and whole plot harvesting. This needs to be further investigated.