This study examined the effect of co-ensiling of wheat straw and sugar beet leaves on the biochemical methane potential (BMP) by both lab-scale and pilot-scale co-ensiling. BMP was increased by co-ensiling, and the increase ranged from 19 to 34% after 9 months of co-ensiling in lab-scale and from 18 to 32% after 6 months of co-ensiling in pilot-scale. No effluent run-off was found through pilot-scale co-ensiling and there was a mass loss of only 0.1%. The study demonstrates that co-ensiling of straw and green biomass has potential as biological pretreatment and for avoiding effluent run-off from pure beet leave silage.