Anaerobic co-digestion of contrasting substrates can result in synergistic or antagonistic effects on methano-genesis. Biogas and methane yields of the mixtures of cattle slurry (CS1 and CS2) or pig slurry with grass silages (GS1 and GS2) were measured using in vitro anaerobic batch digesters, and synergistic and antagonistic effects were investigated. Slurries and silages were incubated as individual substrates or as part of binary mixtures (slurry: silage mass ratios of volatile solids (VS) of 1:0, 0.75:0.25, 0.5:0.5, 0.75:0.25 and 0:1). The biogas yields of CS1, CS2, pig slurry, GS1 and GS2 were 405.9, 380.4, 550.8, 673.7 and 610.6 L kg(-1) of VS, respectively while the corresponding methane yields were 269.1, 246.4, 380.1, 427.7 and 359.0 L kg(-1) of VS. The sequential replacement of either cattle slurry by either grass silage caused a progressive increase in biogas and methane yields, but there was not as clear-cut increase when pig slurry was replaced by grass silages. The methane yield for slurry and silage mixtures displayed non-linear blending and the maximum effect, which was always antagonistic, was at a 0.5:0.5 mass ratio of VS, and ranged from 5.7-7.6 % below the yields predicted from mono-digestion of individual substrates.