Biogas Production from Maize: Current State, Challenges, and Prospects. 1. Methane Yield Potential

Growing interest in converting biomass to renewable energies has led to a considerable expansion of maize cultivation in Germany to provide substrate for anaerobic digestion, producing methane for heat and electricity generation. For decades, maize has been bred for human and livestock nutrition as well as industrial purposes, but not for biomethanization. This review addresses the optimization potential for enhancing maize methane yield, especially open issues pertaining to biogas maize breeding objectives. A great challenge to be faced is the precise quantification of maize-specific methane yield (SMY), i.e., the methane yield per unit biomass. Methodological aspects covered in this review include the impact of the fermentation test procedure as well as of substrate conservation and pretreatment. The contribution of genotypic variation to methane hectare yield (MHY) and SMY are discussed and changes in SMY and MHY during maturation are assessed with respect to harvest timing. The review concludes with a systematic overview of research findings on the relation between SMY and chemical composition, approaches to SMY estimation, and their validation. There is still considerable controversy concerning a biogas maize ideotype; recent research, however, suggests that it differs from the forage maize ideotype, and that a high methane yield can be achieved by different breeding strategies.