The aim of this laboratory-scale study was to investigate the long-term anaerobic fermentation of an extremely sour substrate, an energy crop, for continuous production of methane (CH(4)) as a source of renewable energy. The sugar beet silage was used as the mono-substrate, which had a low pH of around 3.3-3.4, without the addition of manure. The mesophilic biogas digester was operated in a hydraulic retention time (HRT) range between 15 and 9.5 days, and an organic loading rate (OLR) range of between 6.33 and 10 g VS l(-1) d(-1). The highest specific gas production rate (spec. GPR) and CH(4) content were 0.671 g VS(-1) d(-1) and 74%, respectively, obtained at an HRT of 9.5 days and OLR of 6.35 g VS l(-1) d(-1). The digester worked within the neutral pH range as well. Since this substrate lacked the availability of macro and micro nutrients, and the buffering capacity as well, external supplementation was definitely required to provide a stable and efficient operation, as provided using NH(4)Cl and KHCO(3) in this case. The findings of this ongoing long-term fermentation of an extremely acidic biomass substrate without manure addition have reflected crucial information about how to appropriately maintain the operational and particularly the environmental parameters in an agricultural biogas plant. (C) 2009 Elsevier Ltd. All rights reserved.