Switchgrass (Panicum virgatum L.) is a promising biomass crop but little information exists on its potential for methane (CH₄) production. We determined the effect of harvest dates on herbage composition and methane production from switchgrass grown in eastern Canada. Switchgrass, seeded in 2002 and 2006, was harvested as a one-cut system on three dates (late July, early September, and early October) in 2007. The regrowth of plots harvested in late July was also harvested in early October as a two-cut system. Herbage non structural carbohydrate (NSC), neutral detergent fiber (NDF), and acid detergent fiber (ADF) concentrations, along with in vitro true digestibility (IVTD) of dry matter (DM), in vitro digestibility of NDF (dNDF), and DM yield were determined at each harvest. Switchgrass herbage samples (35% DM) were ensiled for 60 days and later anaerobically digested in mesophilic batch reactors to measure CH₄ production. Determinations of silage pH (< 4.4) and concentrations of NH₃-N (< 38 g kg^-1 total N) indicated good silage quality for all harvest dates. In a one-cut system, delaying the harvest date increased herbage yield (9.0 to 11.5 Mg DM ha^-1) and NSC concentration (51 to 85 g kg^-1 DM) up until early September, decreased ITVD (720 to 582 g kg^-1 DM), dNDF (590 to 409 g kg^-1 NDF), and specific methane yield (0.288 to 0.207 L CH₄ g^-1 volatile solids) up until early October, and had little effect on ADF and NDF concentrations. In a one-cut system, maximum methane yield per unit area was reached in early September (2.53 × 10⁶ L CH₄ ha^-1). The two-cut system, however, produced more methane per unit area (3.17 × 10⁶ L CH₄ ha^-1) than the one-cut system because the less mature switchgrass had greater IVTD, dNDF, and specific methane yield.