Our objectives were to compare potentially digestible NDF (pdNDF) and pdNDF digestion rate (k_d), using in situ (IS) or traditional in vitro (TR) assays, and compare subsequent estimates of total-tract NDF digestibility (TTNDFD) to in vivo (IV) TTNDFD measurements. High and low digestible corn silages (HDCS and LDCS) and alfalfa silage (AS) previously characterized for pdNDF, k_d and TTNDFD by in vivo analysis were utilized. For the IS and TR analyses, samples were incubated for 6, 12, 24, 30, 48, 72, 96, 120, and 240h. For TR, 0.5g dried, 1mm ground feed was weighed into flasks and digested with Van Soest buffer. Rumen fluid was collected from 2 cannulated, lactating cows consuming a high forage diet. Rumen fluid was held under CO₂, processed and used to immediately inoculate samples. For IS, 0.5g dried, 2mm ground feed was weighed in Ankom F57 bags. TR samples were digested in each of 2 separate runs and IS in each of 3 cannulated, lactating cows consuming a high forage diet. The NDFD data from both methods were modeled using nonlinear option of SAS JMP (v11.0) to determine pdNDF and associated k_d for each digestion method. The k_d (%/h) and concentration of pdNDF (% of NDF), using IS were 2.39 and 88.4 for HDCS, 2.24 and 83.5 for LDCS, and 5.54 and 65.5 for AS, respectively. The k_d and pdNDF using TR were 4.64 and 74.93 for HDCS, 9.82 and 56.29 for LDCS, and 5.28 and 69.84 for AS, respectively. Total-tract NDF digestibility (TTNDFD) was predicted using the following equation: pdNDF(k_d/(k_d+k_p)). The k_p (2.67%/h) was derived from a meta analysis of in vivo passage rates for pdNDF. HDCS, LDCS, and AS TTNDFD were 46.4, 42.3, and 49.2 for IS; and 52.8, 51.5, and 49.2 for TR. Weighted averages of the feed TTNDFD values were used to predict TMR TTNDFD for comparison with in vivo observations. Resulting TMR TTNDFD were then compared across techniques using a linear model within JMP. Main effects were IS, TR and IV. Means for each technique were compared using student’s t-test. The TTNDFD determined from TR (50.7) differed (P < 0.01) from IS (46.3) and IV (43.6). Estimates of total-tract NDF digestion based on TR, k_d, and pdNDF overestimated TTNDFD measured in vivo.