Bélanger, G., Savoie, P., Parent, G., Claessens, A., Bertrand, A., Tremblay, G. F., Massé, D., Gilbert, Y. and Babineau, D. 2012. Switchgrass silage for methane production as affected by date of harvest. Can. J. Plant Sci. 92: 1187-1197. Switchgrass (Panicum virgatum L.) is a warm-season grass recognized as a potential biomass crop for energy production in North America, but little information exists on the effect of harvest date on forage and silage characteristics of switchgrass grown in eastern Canada. Our objectives were to determine how harvest date affects several forage and silage characteristics of switchgrass and to relate these to specific methane yield from anaerobically digested switchgrass silage. Switchgrass, seeded in 2002 and 2006, was harvested and ensiled as a one-cut system on three dates in 2007: late July, early September, and early October. The regrowth from the late July harvest was also harvested in early October as a two-cut system. Silage quality parameters [pH, and concentrations of N, N-NH₃, total amino acids (TAA), and volatile fatty acids (VFA)] indicated adequate fermentation of all silage samples. In a one-cut system, delaying harvest from late July to early September increased forage dry matter (DM) yield from 9.0 to 11.5 Mg ha^-1, forage soluble carbohydrate (SC) concentration from 51 to 85 g kg^-1 DM, and silage SC concentration from 13 to 25 g kg^-1 DM; delaying harvest from late July to early October decreased forage in vitro true digestibility (IVTD) from 720 to 582 g kg^-1 DM, forage in vitro digestibility of the neutral detergent fibre (dNDF) from 590 to 409 g kg^-1 DM, and silage acetate concentration from 7.7 to 2.6 g kg^-1 DM. The regrowth had higher IVTD and dNDF, lower acid detergent fibre concentration, and higher silage lactate and acetate concentrations than a single harvest taken in early September or early October. The two-cut system and the single harvest in early September produced the highest seasonal forage DM yields (11.5 and 11.9 Mg ha^-1). High specific methane yield was (i) correlated with low forage fibre concentration and high DM digestibility and (ii) more correlated to silage concentrations of lactate and acetate than to silage SC concentration.