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Four experiments including vegetative and dormant forages tested modifications of the in situ neutral detergent fiber nitrogen (NDFN) method of estimating forage undegraded intake protein (UIP). Experiments 1, 2 and 3 tested bag size, closure, rinsing, density, and reflux conditions. None of the modifications affected in situ NDFN content. Experiment 4 compared rates of in situ NDFN digestion calculated with or without correction for undegradability. A close relationship exists between rates calculated by the two methods. Modifications make the improved in situ NDFN method a more desirable means of estimating forage UIP than the standard method.