Dynamics of fermentation of a purified diet and microbial growth in the rumen

Ruminal fermentation and disappearance of glucose, starch, and cellulose, and incorporation of glucose and starch into microbial cells were estimated in a fistulated Jersey cow fed twice daily a purified diet containing urea as the sole nitrogen source. Estimated rumen volume was 59.8 liters. Turnover time and rate of passage of rumen contents were 33.4 h and 1.8 liters per h. Turnover times of glucose, starch, and cellulose were .17, 4.7, and 14.2 h. Fermentation times of glucose, starch, and cellulose were .17, 5.5, and 25.1 h. Percentages of glucose, starch, and cellulose utilized in the rumen were 99.4, 85.4, and 60.6. Thus, 18.5% of the carbohydrate fed bypassed rumen fermentation, and 81.5% was utilized in the rumen. All glucose disappeared from the rumen within an hour. An average of 32.1, 43.0, and 14%, respectively, of glucose utilized was incorporated into microbial cells, volatile fatty acids, and carbon dioxide. Percentage of starch incorporated into cells varied, with time being highest 2 h after feeding at 40% and lowest at 20%, 10 h after feeding. Respective percentages of starch incorporated into microbial cells, volatile fatty acids, and carbon dioxide were 32.4, 45.9; and 13.3. Total microbial protein and cell yields per kilogram carbohydrate utilized in the rumen were 77.1 and 117.5 g. Microbial cell yield per mole (estimated) of adenosine triphosphate was 16.2 g.