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 6h 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.