The methane potential of six varieties of fresh maize (whole plant) harvested at three different times and of maize silage (whole plant) in two particle size distributions was experimentally determined in batch assays. Fresh maize gave the highest methane yield/hectare at late harvest (6270 m(3) CH(4) (10(4) m(2))(-1)). The methane yield/wet weight (WW) increased from 80 (early harvest) to 137 m(3) CH(4) (t WW)(-1) (late harvest). Maize harvested at different times, or different varieties of maize had similar specific methane production/volatile solids content (m(3) CH(4) (kg VS)(-1)). The measured yield m(3) CH(4) (kg VS)(-1) was 84% of the theoretical methane potential. The estimated ethanol yield was between 2.5 and 3.5 t ethanol (10(4) m(2))(-1). The energy yield was 62 and 19-22 MWh (10(4) m(2))(-1) if fresh maize (whole plant) is used for methane or ethanol production respectively. Reducing the particle size of maize silage to an average size of approximately 2 mm increased the methane yield m(3) CH(4) (kg vs)(-1) by approximately 10%. (C) 2010 Elsevier Ltd. All rights reserved.