Efficient N retention by herbivores has consequences for the ecology of their survival, animal health, the economics of animal production, and the environment. Gut microorganisms play a major role in N metabolism and retention in these animals. Ruminants benefit from the growth of microorganisms in the foregut because the microbial amino acids thus formed become available for absorption during passage through the gut. The use of non-protein-N to support microbial growth enables urea recycling within the body and the use of low-quality feeds, both of which help survival under a low plane of nutrition. Microbial synthesis also occurs in the hindgut of non-ruminant herbivores. Coprophagy in some species enables the amino acids thus formed to become available to the animal. In other species, the fate of the amino acids is predominantly to be excreted, although some evidence has been presented that amino acids from caecal digesta can be absorbed. Under more intensive production conditions, protein breakdown by gut microorganisms causes inefficiencies in N metabolism. The breakdown of ingested food protein by ruminal bacteria and of bacterial cells by ciliate protozoa both cause problems to N retention in ruminants. The microorganisms and reactions involved are now quite well understood, although measures to decrease N losses via controlling rumen microbial proteolysis, using antimicrobials, plant secondary compounds, direct-fed microbials and dietary interventions, have met with limited success. Proteolytic activity in the caecum and colon of non-ruminant herbivores is much less than in the small intestine, thus most food proteins will have been digested before reaching the intestine. Nevertheless, microbial proteolysis in the intestine may lead to metabolites that affect or are implicated in animal health, such as vasoactive amines and acute laminitis in horses. Proteolytic microorganisms in the non-ruminant gut have received less attention than in ruminants. Indications are that the processes are similar and that the microbial species are related to, but not the same as, ruminal species carrying out similar reactions.
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