In situ neutral detergent insoluble nitrogen as a method for measuring forage protein degradability

A method of estimating the undegraded intake protein (UIP) concentration of forages was developed and validated with a series of in situ experiments. The hypothesis was that UIP calculated from in situ neutral detergent insol uble N (NDIN) is equal to total in situ N minus the microbial N that is est imated from purines (MN). The in situ disappearance rates of total in situ N (TN), MN, and NDIN were measured for six hay samples and two range mastic ate samples. Hypothetical rates of passage (2 or 5%/h) were used to calcula te UIP (% of DM) for each N pool. Estimates of UIP from TN were higher (P = .0001) than those from either MN or NDIN, and MN estimates of UIP were sim ilar (P = .48) to NDIN estimates. A low-N fiber source (solka floc) was inc ubated in situ for 8 h. Analysis of the residue detected purines before, bu t not after, neutral detergent extraction. Several in situ incubation (i.e. , Dacron bag size and number of Dacron bags in a mesh bag) and neutral dete rgent extraction conditions were tested. None of the factors tested affecte d in situ NDIN disappearance (P > .05). The hypothesis that NDIN is complet ely digestible in the rumen was tested. Estimates of the extent of NDIN dig estion were made using 96-h in situ incubations, and UIP was recalculated f or the test samples. Mean in situ UIP concentration decreased upon recalcul ation(P = .05). In situ NDIN provides estimates of forage UIP that are equa l to estimates from MN. Forage UIP estimates are less when extent of N degr adation is estimated and included in the calculation.