Co-digesting swine manure with three agricultural residues, i.e., corn stalks, oat straw, and wheat straw, to enhance biogas productivity was investigated in this study. A 3 x 3 experimental design with duplicates was adopted (3 crop residues x 3 carbon/nitrogen ratios) to examine the improvement of batch digestion in terms of biogas volume produced, CH4 content in the biogas, and net CH4 volume. The crop residues were first cut into small sections and then ground into fine particles smaller than 40 mesh size (0.422 mm) before being added to digesters. All the digesters were run simultaneously under controlled temperature at 37 +/- 0.1 degrees C. The length of experiment was 25 days. The results showed that all crop residues significantly increased biogas production and net CH4 volume at all C/N ratios, among which corn stalks performed the best with increase in daily maximum biogas volume by 11.4-fold as compared to the control, followed by oat straw (8.45-fold) and wheat straw (6.12-fold) at the C/N ratio of 20/1, which was found to be the optimal C/N ratio for co-digestion in the present study. In addition, corn stalks achieved the highest CH4 content in the biogas (similar to 68%), which was about 11% higher than that of oat straw (similar to 57%), whereas wheat straw and the control both had produced biogas with similar to 47% CH4 content. Wheat straw demonstrated a lower biogas productivity than corn stalks and oat straw even it had a higher carbon content (46%) than the latter two residues (39%). (C) 2010 Elsevier Ltd. All rights reserved.