Ruminant nutrition models, such as the Cornell Net Carbohydrate and Protein System (CNCPS), have evolved over time resulting in increased precision and accuracy with which they can predict animal performance. In tandem with this evolution, more precise and accurate methodologies have been developed to describe the feed chemistry inputs required to conduct model simulations. However, these updated methods have not been applied to pasture-based systems. The objective of this study was to characterize the nutritive value of perennial ryegrass (Lolium perenne L.; PRG) dominated pastures using comprehensive feed chemistry methods. Fifty-five samples from three independent experiments were used in this study. The data generated demonstrated that the amylase- and sodium sulphite-treated NDF corrected for ash residue (aNDFom) method was significantly lower than the amylase- and sodium sulphite-treated NDF (aNDF) method; hence, due to ash contamination, aNDF consistently overestimated the true measure of cell wall content in PRG. Results from multiple time point in vitro aNDFom digestibility analysis indicated that immature PRG comprises of a large potentially digestible pool that degrades at a rapid rate. A 12-h time point combined with 30, 120, and 240 h better describes the aNDFom degradation curve of PRG. In addition, undigested aNDFom at 240 h of fermentation was affected by PRG category; whereby, spring and summer were lowest (98 and 111 g/kg of aNDFom, respectively), autumn intermediate (155 g/kg of aNDFom), and drought stressed pastures the highest (200 g/kg of aNDFom). The extent of in vitro aNDFom digestion was not reached prior to 240 h; however, a strong Pearson correlation coefficient between 120-h and 240-h time points (0.98) indicated the potential to predict undigested aNDFom from the 120-h time point. Based on the N fractionation scheme employed by the CNCPS, a large proportion of PRG N is soluble and likely highly degradable in the rumen. An assay capable of quantitatively describing the rumen degradation of PRG N is required. Using a single 21-h HCl hydrolysis, to determine amino acid concentration, underestimated the concentration of Ile and Val indicating that multiple-hydrolysis time procedures are required to predict the true amino acid concentration of PRG. In conclusion, the updated feed chemistry procedures conducted in this study, allowed a more accurate characterization of the nutritive value of PRG. This increased understanding in combination with formulation models, such as the CNCPS, can provide more robust information on how PRG interacts with the ruminant animal, in order to increase the efficiency and productivity of pasture-based systems.