Method for measuring microbial growth in rumen content.

Radioactive sodium sulfide was used to label the sulfide pool of rumen contents in vitro. Microbial protein synthesis was calculated from the size and rate of dilution of label in the sulfide pool, and from the radioactivity incorporated into protein together with a conversion factor specifying the nitrogen-sulfur ratio determined for microbial protein. The microbial cell yield, calculated on the basis of the adenosine triphosphate (ATP) available from fermentation, was 13 to 14 g (dry weight) per mole of ATP, which is in good agreement with the values obtained for pure cultures of bacteria. Calculation of microbial protein yield per kilogram of ration agreed fairly well with previous estimates for similar rations.