Relationship between nitrogen solubility and in situ protein degradability in ruminant feedstuffs

The relationship between in situ protein degradability and N solubility in three solvents was investigated in a range of ruminant feedstuffs with different physical characteristics and crude protein (CP) content. In addition, relationships between N solubility in the three solvents and classical protein fractions (albumins, globulins, prolamins and glutelins) and also between protein fractions and soluble non-protein N or total true protein were examined. For all types of feedstuffs, a relatively high correlation (r = 0.52-0.75) was found between dry matter (DM) degradation in sacco and protein solubility in the three solvents, but there was no close relationship between protein degradation in sacco and protein solubility in any of the three solvents used, or between CP content of feedstuffs and ruminal protein degradation, or between the estimates of ruminal protein degradability and the rate constant for protein disappearance from the rumen. However, when the feedstuffs were categorised into groups of the same type, there was a high correlation (r = 0.64-0.91) between DM or ruminal protein degradation and solubility in the three solvents. A negative correlation (r = -0.60 to -0.71) was found between CP content of feedstuffs and solubility in any of the three solvents used for all feedstuffs. There was no relationship between 1.5 or 3 h protein disappearance from the rumen and protein solubilities measured in the three solvents. The amount of N extracted by the three solvents for all feedstuffs was highly correlated between the solvents ie McDougall's buffer vs 0.02 M NaOH (r = 0.91), 0.02 M NaOH vs 0.15 M NaCl (r = 0.79), and McDougall's buffer vs 0.15 M NaCl (r = 0.85). There was a significant correlation between the albumin fraction and N solubility in 0.02 M NaOH and a negative and significant correlation between the glutelin fraction and N solubility in McDougall's buffer. Moreover, there was a significant correlation between the combined albumin and globulin fractions and N solubility in McDougall's buffer and 0.02 M NaOH. The albumin, globulin, prolamin and glutelin fractions were all highly correlated with the CP content of feedstuffs and total true protein.