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Sunn hemp at a cutting age of 50 days was investigated for biogas production by studying the anaerobic digestion of sunn hemp alone and the co-digestion of sunn hemp with cow dung at a concentration of 20% by fresh weight but with various proportions. The single-stage, semi-continuous reactors were operated at a hydraulic retention time of 40 days. Mixed ruminal microorganisms from cows were used as inoculum. The results indicated that the CH4 content from the digestion of sunn hemp alone was 47.57% and the CH4 produced per area of cultivation was 9,231 m(3)/hectare. year. Co-digestion could increase the CH4 content to greater than 50%. Proportion of sunn hemp: cow dung: water of 10: 10: 80 by fresh weight was optimal for co-digestion. At this recommended ratio, the CH4 yield reached 412 L/kg total volatiles solids degraded. Sunn hemp could be a very good renewable energy source for biogas production.