Grass for biogas production: The impact of silage fermentation characteristics on methane yield in two contrasting biomethane potential test systems

Grassland biomass is likely to be harvested and stored as silage to ensure a predictable quality and a constant supply of feedstock to an anaerobic digestion facility. Grass (Phleum pratense L. var. Erecta) was ensiled following the application of one of six contrasting additive treatments or a 6 h wilt treatment to investigate the effects of contrasting silage fermentation characteristics on CH4 yield. In general, silage fermentation characteristics had relatively little effect on specific CH4 yield (from 344 to 383 Nl CH4 kg−1 volatile solids). However, the high concentrations of fermentation products such as ethanol and butyric acid following clostridial and heterofermentative lactic acid bacterial fermentations resulted in a numerically higher specific CH4 yield. While the latter fermentation products of undesirable microbial activity have the potential to enhance specific CH4 yield, the numerically higher specific CH4 yield may not compensate for the associated total solids and energy losses during ensiling.