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Two experiments were conducted to evaluate the use of neutral detergent insoluble nitrogen (NDIN) at a single in situ incubation time point to estimate the undegradable intake protein (UIP) in forages as well as to compare rates of NDIN degradation. Forage samples in Exp. 1 comprised diet samples collected from range and meadow pastures monthly from May through September. In Exp. 2, clipped samples of alfalfa, birdsfoot trefoil, kura clover, and smooth bromegrass, and diet samples of the mixed legume-grass and smooth bromegrass were evaluated. Forage samples were incubated in situ for their mean retention time (MRT) estimated from IVDMD plus a 10-h passage lag to yield the total MRT (TMRT). Samples were also incubated for 0 h, 10 h, 75% TMRT, and 96 h. Undegradable intake protein was measured at 75% TMRT and TMRT, and calculated using fractional rates of degradation and passage with a 10-h passage lag. Rates of ruminal NDIN degradation were calculated using the slope of the regression of the natural logarithm of the potentially degradable NDIN remaining (96-h undegradable fraction subtracted) against time. The estimated UIP values obtained using 75% TMRT were highly correlated with those obtained using fractional rates of degradation and passage plus accounting for a 10-h passage lag in Exp. 1 (R2 = 0.95) and Exp. 2 (R2 = 0.98). Rates of NDIN degradation of range and meadow samples in Exp. 1 were slower (P < 0.05) from 0 to 10 h in May and June compared with rates from 10 h to 75% TMRT, but rates of degradation were not different (P = 0.34 to 0.71) for the rest of the collection periods. Rates of degradation were not different from 0 to 10 h and 10 h to 75% TMRT in Exp. 2 for diet (P = 0.82) or clipped samples (P = 0.86). The UIP of the forages in these experiments was accurately estimated using NDIN at a single in situ incubation time point equivalent to 75% of the TMRT, and rates of protein degradation can be obtained at this time point when 0- and 96-h incubations are included. Published in Journal of Animal Science 2006. 84:651–659. Copyright © 2006 American Society of Animal Science. Used by permission.