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Neutral detergent insoluble nitrogen (NDIN) was used as a direct estimate of UIP. Forage samples collected from upland range and subirrigated meadow sites over the summer were incubated in situ for a time equivalent to a mean retention time estimated from the digestibility of the forage plus 10 hour to account for a lag in passage of particles from the rumen. Samples also were incubated for 75% of the estimated total mean retention time. The UIP values obtained from the fractional rates of degradation and passage were highly correlated with those estimated from samples incubated for 75% of total mean retention time while incubating the samples longer tended to underestimate the UIP fraction.