Abstract: The objective of this study was to evaluate the nutritional value of reed canary grass at different growth stages using in vitro gas production and the Cornell net carbohydrate and protein system (CNCPS) methods, and to compare results from these two methods. The in vitro gas production method was used to determine gas production, fermentation parameters (a, b, c), dry matter (DM) digestibility, and the degradation rates of neutral detergent fiber (NDF) and acid detergent fiber (ADF) at heading, flowering, seed filling and milk ripe growth stages of reed canary grass. CNCPS components were also determined for comparison of the two methods. The cumulative total gas production and the a value (122.68 and 4.46 mL) at the flowering stage were significantly higher than those at other stages, and they tended to decrease from the heading to milk ripe stages. Similarly, DM digestibility and degradation rates of NDF and ADF at the heading and flowering stages were significantly higher than those at the seed filling and milk ripe stages. As reed canary grass matured, the crude protein (CP) and soluble protein (SOLP) contents tended to decrease, and were significantly lower at the seed filling and milk ripe stages than at the heading and flowering stages NDF, ADF and acid detergent lignin (ADL) components increased while the NDF insoluble protein (NDFIP) and ADF insoluble protein (ADFIP) generally decreased. The carbohydrate (CHO) content at the filling and milk ripe stages was significantly higher than that at the heading and flowering stages, while sugar content in CHO (CA) and non-structural carbohydrate (CNSC) contents were higher at the heading and flowering stages. The unavailable fiber (CC) content was also higher at the milk ripe stage. The protein component, the non-protein nitrogen (PA), and the bound protein (PC) showed similar trends to those of CA and CC. The levels of CNSC, CA, starch+non-starch polysaccharide fractions (CB₁), CB₂, PA, and low degradability protein (PB₃) levels of reed canary grass were significantly or very significantly correlated with the gas production at different time points. Therefore, the nutritional value of reed canary grass was the highest at the flowering stage, followed by the heading stage, filling stage and milk ripe stage in that order. The results from the two evaluation methods, in vitro gas production and the CNCPS method, were closely correlated, suggesting that it was feasible to estimate CNCPS components of forages using the in vitro gas production method.