Anaerobic fermentation of the slurry allows for, i) Taking advantage of waste energy and ii) Reduce the emission of harmful (NH3) and greenhouse gases (e.g. CH4 and N2O). Generation of pig slurry, in those areas with a high concentration of pig farms (e.g. Catalonia and Aragon), became an environmental problem but also constitute abundant resource for energy-recycling. The slurry potential as a substrate for anaerobic fermentation is low, however this performance can be improved by synergic combination with other substrate as agro-industrial by-products. The objective of the present assay was to study the in vitro potential of some local by-products (i.e. wheat straw, apple pulp and sugar beet pulp) as a co-substrate and ruminant faeces as co-inoculum to optimize CH4 under anaerobic fermentation. The use of referred co-substrates improved significantly biogas and CH4 production although a negative relationship between methane production and slurry dilution was detected (4 and 6% vs. 2% slurry DM, P <0.05).