In the context of bioenergy production, sorghum and miscanthus are relevant candidates for biogas production. As for many lignocellulosic biomasses, pretreatments can improve the accessibility of carbohydrates for microorganisms during anaerobic digestion. The objectives of this study were (1) to assess environmental impacts of lime and soda pretreatments of both biomasses in co-digestion with manure and (2) to compare the heat produced from natural gas with heat produced from biomethane generated from whole plants of sorghum and miscanthus under different studied scenarios. A comprehensive attributional life cycle analysis (LCA) was performed on 21 sorghum scenarios and 12 miscanthus scenarios. Certain scenarios explored direct and indirect land use change (dLUC and iLUC). An environmental evaluation highlighted that most of the impacts are generated by crop production and by the purification and injection step for both sorghum and miscanthus. Compared to natural gas, the study emphasized that, unlike lime treatment, soda treatment does not provide an added value. Although most impacts are favourable towards natural gas, sorghum-based methane presents very good results (below 0) for six impact categories. The reduction of climate change ranges from - 90 to - 105%. Miscanthus can reduce climate change by - 60 to - 80%, but almost all other impact categories are in favour of natural gas. Lime pretreatment always presents best results. For both sorghum and miscanthus, it is crucial that crops are not cultivated on a land in competition with food production.