Rumen effective degradability of amino acids from soybean meal corrected for microbial contamination

Rumen degradation kinetics and effective degradability of individual amino acids, total analysed amino acids (TAA) and crude protein (CP) of soybean m eal were measured on four rumen-cannulated wethers using the nylon bag tech nique. Microbial contamination of the incubated residues was corrected usin g a continuous N-15 intraruminal infusion and isolated solid associated bac teria as a reference sample. TAA showed a lower soluble fraction (14.9 vs. 20.8%; P < 0.01), a similar insoluble-degradable fraction (79.0 vs. 79.2%) and a higher degradation rate (11.5 vs. 8.4%.h(-1); P < 0.05) than CP. As a consequence, effective degradability was similar for TAA and CP (74.7 vs. 75.7%). Degradability values of individual amino acids varied moderately (r ange: +/-6% of TAA degradability). Valine, isoleucine, leucine, alanine, as partic acid and tyrosine showed significantly lower degradability than TAA, while the opposite effect was observed for histidine, threonine and glutam ic acid. Degradability of individual amino acids was related to their solub le fraction (r = 0.877; P < 0.001).