The compositional changes of Napier grass (Pennisetum purpureum) with respect to maturity (namely, 2, 4, 6, and 8 months age), and its effect on anaerobic digestion was examined under three sieving regimes (6, 10, and 20 mm). Significant changes in plant composition were observed with crop maturity. The highest methane yields of 219 +/- 4.9 NmL/g VS added was found for biomass harvested at 2 months old compared to 189 +/- 7.3, 131 +/- 4.7, and 104 +/- 2.3 NmL of methane/g VS added, respectively, for 4, 6, and 8 months old biomass. For all ages, feedstock passed through a 6 mm sieve resulted in significantly higher methane yields compared to biomass passed through 10 and 20 mm sieves. Additionally, 2 months old biomass exhibited the highest digestibility of cellulose and hemicellulose, whereas digestibility of cellulose and hemicellulose were lowest for the biomass harvested at 8 months of maturity. (C) 2014 Elsevier Ltd. All rights reserved.