Evaluation of acid detergent insoluble nitrogen as an indicator of protein quality in nonforage proteins.

Two trials were conducted to determine the adequacy of ADIN as an indicator of N digestibility. In Trial 1, eight nonforage plant protein sources were heated at 150 degrees C for 90 min in the presence of xylose (12.8% of CP, DM basis) to produce heat damage. Thirty-four wethers (average BW 40 kg +/- 9.9) were used to determine the effect of heat on N digestibility. Differences in ADIN were evident among the protein sources, and no significant heat x protein source interaction was observed for N digestibility. Apparent N digestibilities were reduced (P < .001) for all protein sources by heat treatment. Acid detergent insoluble N and N digestibility were correlated (r2 = .66). However, the assumption that ADIN was completely indigestible led to underestimation of N digestibility: approximately 58% of the ADIN was digestible in these feeds. In Trial 2, seven dried distillers grains from different distilling plants were tested for N digestibility using 24 wethers (average BW 35 kg +/- 3.6). Visual differences in color indicated possible differences in degree of heating in these feeds. The ADIN contents were quite variable among these feeds; however, there were no differences in N digestibility. The correlation between ADIN and N digestibility was weak (r2 = .24). These results indicate that ADIN values in nonforage protein sources predicted more protein damage than that estimated by in vivo N digestibility values.