Opportunities to improve nitrogen utilisation in the rumen by reduction in plant-mediated proteolysis

The reaction of ingested forage plant fragments to the rumen environment may contribute to the rapid degradation of plant cell contents in the rumen. A series of experiments were used to explore the hypothesis that plant-mediated proteolysis (PMP) contributes to N loss from fresh forage entering the rumen, and to investigate the potential for identifying cultivars with reduced PMP. During an in sacco time-course experiment, chopped fresh field grown perennial ryegrass (Lolium perenne L.) leaf samples lost 15% of their total N within the first 8 hours in the absence of rumen microbes; whereas 40% was lost from samples incubated with exposure to rumen microbes. Accumulation of free amino acids in the buffer accounted for 75% of the N loss after 24 hours. In a subsequent experiment, in vitro incubation of samples from five perennial ryegrass cultivars detected significant (P<0.05) differences in the extent of PMP for one cultivar. A further in vitro experiment detected significant (P<0.05) variation in PMP among ryegrass genotypes (n=24) within two cultivars. Percentage N loss was not correlated with N concentration in the fresh forage sample. These data indicate PMP contributes to N loss from intact fresh plant cells in the rumen, that it is possible to identify cultivars with reduced PMP, and that there may be potential to further reduce PMP by selection within ryegrass populations.