The biological value of microbial protein is 66 to 87 percent. Dietary protein may be improved or reduced in biological value by rumen microbes, depending on the quality of protein fed.
Most ruminal bacteria can use ammonia nitrogen as a source of nitrogen.
Some species of bacteria require additional nitrogen compounds such as intact protein or carbon chains of certain amino acids for most efficient or rapid growth. Ammonia is derived in the rumen through microbial degradation of dietary protein and dietary nonprotein nitrogen, from hydrolysis of recycled urea to the rumen, and from degradation of microbial crude protein. Rumen ammonia disappears from the rumen in different ways, such as incorporation of the nitrogen by the microbes, absorption through the rumen wall, and flushing to the omasum. Ammonia not taken up by the microbes is absorbed directly through the rumen wall. The rate of absorption is dependent on the pH of the rumen environment and the concentration of ammonia.