Co-digestion of forage silage with cattle slurry can greatly extend the stability of methanogenesis as compared to mono-digestion of the silage. Biogas and methane yields of the mixtures of perennial ryegrass silage (grass harvested at two growth stages i.e. stem elongation vs. floral development) with cattle slurry and of red clover silage (clover harvested at two growth stages i.e. mid-vegetative vs. early seed-pod development) with cattle slurry were measured, and synergistic effects were investigated. Silage and slurry were incubated as sole substrates or as part of binary mixtures (forage silage:cattle slurry ratios of 1:0, 0.75:0.25, 0.5:0.5, 0.25:0.75 and 0:1 on a volatile solid basis). The maximum measured synergistic effects for perennial ryegrass silages with cattle slurry and red clover silages with cattle slurry were observed at 0.75:0.25 and 0.5:0.5 (forage silge:cattle slurry), respectively. The forage silage:cattle slurry ratio to produce the maximum synergistic effects differed with the forage species ensiled and its growth stage when harvested. (C) 2018 Elsevier Ltd. All rights reserved.