RELATIONSHIP BETWEEN NITROGEN SOLUBILITY AND IN-SITU PROTEIN DEGRADABILITY IN RUMINANT FEEDSTUFFS

The relationship between in situ protein degradability and N solubilit y in three solvents was investigated in a range of ruminant feedstuffs with different physical characteristics and crude protein (CP) conten t. In addition, relationships between N solubility in the three solven ts 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 mat ter (DM) degradation in sacco and protein solubility in the three solv ents, but there was no dose relationship between protein degradation i n sacco and protein solubility in any of the three solvents used, or b etween CP content of feedstuffs and ruminal protein degradation, or be tween the estimates of ruminal protein degradability and the rate cons tant for protein disappearance from the rumen. However, when the feeds tuffs 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 = -0.6 0 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 rel ationship between 1.5 or 3 h protein disappearance from the rumen and protein solubilities measured in the three solvents. The amount of N e xtracted by the three solvents for all feedstuffs was highly correlate d 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 0.1 5 M NaCl (r = 0.85). There was a significant correlation between the a lbumin fraction and N solubility in 0.02 M NaOH and a negative and sig nificant correlation between the glutelin fraction and N solubility in McDougall's buffer. Moreover, there was a significant correlation bet ween the combined albumin and globulin fractions and N solubility in M cDougall'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.