Leaves, stems, and whole-plants of 27 cultivars were assessed for ruminal undegradable proteins (RUP) using an inhibitor in vitro procedure and for protein fractions using the Cornell Net Carbohydrate and Protein System (CNCPS). The CNCPS divides the protein into soluble non protein nitrogen (A), soluble true protein (B1), rapidly degradable true protein (B2), slowly degradable protein (B3), and undegradable protein (C). Differences among cultivars were significant for leaf CP and stem RUP. Significant differences were observed among cultivars for A and B1 fractions in leaves and stems and for the B2 fraction in leaves. Maturity affected all protein fractions of whole-plant CP and the B3 fraction in leaves and the C fraction in stems. In both leaves and stems, RUP values were negatively correlated with the soluble fractions (A and B1) but positively correlated with degradable true protein fractions (B2 and B3).