The increase was predicted of the methane (CH4) yield between untreated and pretreated weeds prior to co-digestion with cow dung. The mixed substrate slurry of weed was prepared using a ratio of weed to cow dung to water of 10:10:80, respectively. The total volatile solids (TVS) and total solids (TS) of the mixed substrate slurry of the untreated weeds were the two parameters input in the developed equation. Four types of weeds were used in the test: French weed, para grass, water lettuce, and sedge. The results showed that the % increase in CH4 yield depended on the ratio of TVS/TS of the mixed substrate slurry of the untreated weeds. The proposed developed equation was Y = 221.05X - 152.46, where X is the initial ratio of TVS/TS of mixed substrate slurry of untreated weed and cow dung, and Y is the % increase in CH4 yield obtained when the weed is pretreated using 1% sodium hydroxide for 24 hr prior to co-digestion. Water hyacinth was used as the substrate to verify the equation, and the results indicated that the equation performed well. Verification confirmed that the proposed equation successfully provided an accurate (within 10%) prediction of the % increase in CH4 yield. The equation could be useful in developing pretreatment options for weed co-digestion and environmental management.