The relationship between intestinally available protein and detergent insoluble protein of feedstuffs

A comparision was made of intestinally available rumen undegradable intake protein (IAUIP) of total mixed rations and the neutral detergent insoluble crude protein (NDIP) to test if NDIP could be used as a simple method for estimating IAUIP content. The IAUIP content of six total mixed rations were determined and the results were regressed on the NDIP content of the same rations. Prediction of IAUIP by use of the NDIP content yielded a significant linear relationship (IAUIP = 0.565 + (NDIP × 0.8987); r2 = 0.60). Correction of the NDIP to account for acid detergent insoluble crude protein (ADIP) content weakened the relationship (IAUIP = 1.5100 + [(NDIP − ADIP) × 0.9891]; r2 = 0.53). Furthermore, intestinally unavailable rumen undegradable intake protein (IUUIP) values were regressed on the ADIP content and showed a low linear relationship (IUUIP = 0.2536 + (ADIP × 0.9803); r2 = 0.16). These data suggest that NDIP content of TMRs can be used to estimate the IAUIP content of feeds and ADIP content of rations is a poor indicator of the amount of intestinally unavailable UIP in total mixed rations.