Synthesis of Microbial Nitrogen Compounds in the Rumen and Their Subsequent Digestion

Ammonia (NH3 and/or NH4+) appears to be incorporated rapidly into rumen bacteria in the form of amide groups and used for synthesis-first of glutamate, aspartate and alanine and then other amino acids. For greatest economy, ammonia in rumen fluid should be present in the minimum concentration needed to maintain an adequate supply within bacterial cells. This will be partly determined by rate of transport of ammonia across cell walls. Loss of ammonia by other routes will occur but substantial direct absorption from the rumen is unlikely under ‘normal’ conditions so that reducing its release rate from an NPN source is unlikely to improve efficiency of N utilization. Observed yields of microbial protein in terms of energy fermented in the rumen are generally less than the theoretical maximum. The maximum is likely to be approached, with ensuing practical benefits, if the bacterial population is growing vigously. Increasing dilution rate may help to achieve this, but it is important to prevent deficiencies of nutrients such as S and P. Certain preformed compounds (possibly methionine, phenylalanine and branched chain carbon skeletons) may also stimulate growth. A controlled supply of energy is of paramount importance for encouraging efficient synthesis of microbial N compounds. Cooked starch appears to be more satisfactory for this purpose than soluble sugars, but chemical modification of the latter may improve their value. Manipulation of energy supply is likely to be more successful in improving NPN use than manipulation of ammonia release. Protein and nucleic acids form the bulk of bacterial N compounds entering the small intestine. The latter are rapidly digested and absorbed either as nucleosides or bases. There is no indication that protein digestion in the ruminant differs fundamentally from that in the nonruminant although little comparative work has been done.