Napiergrass (Cenchrus purpureus (Schumach.) Morrone) and energycane (Saccharum hyb.) are perennial grasses that are well-suited for biomass production in the southeastern USA. The purpose of this study was to determine the effects of delayed winter harvest on biomass yield and quality of these grasses. The study was conducted on two adjacent sites near Midville, GA. Each site used a split-plot design with four replications, with species as the main plot, and harvest times (December, January, or February) as subplots. Dry matter (DM) yields were measured by mechanical harvesting, and a sample of biomass was taken from each harvest for determination of ethanol production by simultaneous saccharification and fermentation (SSF). Biomass moisture, N, P, K, and ash mass fractions were also measured. Energycane DM yields were stable from December (46.8 Mg ha(-1)) to January (42.9 Mg ha(-1)), but then declined (36.8 Mg ha(-1)), while napiergrass yields declined sharply from December (47.0 Mg ha(-1)) to January (35.0 Mg ha(-1)). Napiergrass moisture mass fraction was reduced by an average of 18% in February harvests compared to December. Mass fractions of N, K, and ash tended to decrease with later harvesting, but sometimes increased due to changes in biomass composition. Delaying harvest of napiergrass from December to January reduced N removal by an average of 144 kg ha(-1), while delaying harvest of energycane to February reduced N removal by an average of 54 kg ha(-1). In SSF, later-harvested energycane produced less ethanol per unit of DM while napiergrass was less affected by harvest date. Published by Elsevier Ltd.