Efficiency of nitrogen capture

The efficiency with which nitrogen is captured by the microorganisms of the rumen depends not only upon the speed and extent of breakdown but also upon the synchronous provision of a readily available, utilisable source of energy to fuel the synthesis of microbial protein. Failure to achieve this balance can result in too rapid and extensive a breakdown, and the syn¬thetic powers of the rumen microorganisms may be overwhelmed. Wastage may then occur since the excess ammonia is absorbed and largely excreted as urea; some, though, is recycled via the rumen wall and contributes fur¬ther to the nitrogen economy of the rumen. The extent of recycling has been estimated as about 70 per cent of nitrogen intake for diets of low protein content (about 5 g/kg) and as little as 11 per cent for foods with about 20 g/kg.
That part of the food crude protein which is immediately degradable is unlikely to be as effective a source of nitrogen for the microorganisms as that which is more slowly degraded. It is generally considered that the slowly degraded nitrogen fraction is incorporated into microbial protein with an efficiency of 1.0, whereas that immediately degraded is less efficiently used. Estimates of the efficiency with which immediately degraded protein is incorporated vary, but 0.8 is a commonly used figure.