The appropriate nutritional management of grazing dairy cows requires an accurate estimation of the content of total digestible nutrients (TDN) in fresh herbage from grazing pastures. The present study aimed to confirm the accuracy of existing estimations of TDN developed for fresh meadow fescue for perennial ryegrass (PR) – white clover mixed herbage, and to develop a new TDN estimation for PR – WC mixed herbage. The herbage was harvested from a PR – WC mixed pasture from April through October. A total of 16 Suffolk wethers were used for this study to evaluate the digestibility of the herbage. The TDN content of the herbage ranged from 68.9% to 77.7% dry matter (DM). The TDN estimated using the existing lignin-based model were similar to the actual TDN, whereas the existing organic b fraction (Ob)-based models underestimated the actual TDN. This may have been caused by high Ob digestibility due to high Ob content and the same level of lignin content in the herbage, as compared with those of meadow fescue. There were significant negative correlations between TDN content and acid detergent fiber (P < 0.01), lignin (P < 0.001), and Ob (P < 0.001) contents. These results demonstrated that the TDN content of PR – WC mixed herbage could be estimated accurately using the following estimations: TDN (% DM) = –4.33 lignin – 0.39 cellulose + 94.64 (R2 = 0.83, P < 0.001) or TDN (% DM) = –0.57 Ob + 93.39 (R2 = 0.87, P < 0.001).