Microbial digestion and assimilation of forage proteins in the rumen: some new insights

When ruminants ingest fresh forages, plant proteins are degraded to peptides, amino acids and ammonia. Plant cells remain alive in the rumen after forages are ingested but stressful conditions may promote an accelerated 'senescence' during which plant proteases begin the protein degradation process. Because most rumen bacteria can grow with ammonia as their only N source, the Australian Feeding Standards as yet do not consider bacterial requirements for peptides/amino acids when assessing microbial requirements. We present results describing 15N release from labelled fresh ryegrass ingested by sheep showing that peptides are a major N source for rumen bacteria and are apparently assimilated by bacteria without ever entering the mixed rumen fluid. Feeding standards in Australia may need to be modified to account for use of non-ammonia N sources in the future.