Protein is broken down by rumen microorganisms via peptides and amino acids to produce ammonia at rates which frequently exceed microbial requirements for N. Much of the ammonia-N formed in this way is eventually excreted as urea. If any of the steps in the degradation sequence could be inhibited, excessive ammonia production would be reduced. More protein, peptides or amino acids would escape fermentation in the rumen, thereby improving the protein nutrition of the host animal.

The breakdown of peptides to amino acids is a central part of the degradation sequence. The main enzymic mechanism by which peptides are hydrolysed in the rumen is a bacterial dipeptidyl aminopeptidase, which cleaves dipeptides from the N-terminus of the peptide chain (Wallace & McKain, 1990). Little carboxypeptidase activity appears to be present. The present experiments were undertaken to find out to what extent blocking the N-terminus of peptides enables them to survive degradation in rumen fluid, and to determine which peptides can be protected in this way.