The objective of this study was to determine the effect on methane production of adding different proportions of fresh vegetable waste to manure from pigs or buffaloes as substrate in plug-flow biodigesters. The treatments in a completely randomized design with four replicates were ratios of fresh vegetable waste to manure (pig or buffalo) of 0, 25, 50, 75 and 100%. Measurements were made of gas production, methane content of the gas and pH of the effluent over a period of 30 days from startup of the biodigesters until the end of the experiment after 30 days. Methane percentage in the gas and total methane production were increased threefold by including 25% of fresh vegetable waste in the substrate. Higher ratios of vegetable waste led to linear decreases in total gas and methane production. Methane production was tripled when the substrate was pig manure and vegetable waste compared with buffalo manure and vegetable waste. The ratio of vegetable waste to manure was a critical factor in determining the biogas yield and composition of methane in the gas, with the optimum being recorded at 25% of vegetable waste followed by subsequent rapid decline to zero values when the substrate was only vegetable waste.