1. A continuous culture technique was used to study the phosphorus requirements of rumen micro-organisms. Solutions of artificial saliva containing 120, 80, 40 and 0 mg inorganic phosphorus (Pi)/1 were infused into the reaction vessels previously inoculated with rumen contents, resulting in Pi concentrations in the vessel contents of 48, 28, 4 and < 1 mg/l respectively. Various fermentative and synthetic characteristics were examined.
2. In the vessel contents, concentrations of protozoa (about 0.9 x 105/ml) were not significantly affected by pi concentration. Total volatile fatty acids (VFA) produced averaged about 6.83 mmol/h with Pi levels of 48 and 28 mg/l. Reduction in Pi concentrations to 4 and < 1 mg/l resulted in significant reductions in total VFA to approximately 6.25 and 3.75 mmol/h respectively, accompanied by a rise in pH from 6.5 to 7.3. Ammonia-nitrogen values, which averaged about 131 mg/l at the higher Pi concentrations, also increased with the lowest level of Pi to about 240 mg/l. ATP concentrations averaged about 14 μmol/l at the highest Pi concentration and fell progressively with each reduction in Pi concentration to a final value of 2.5μmol/1 with the Pi level < 1 mg/1.
3. At Pi concentrations of 48 and 28 mg/l, the digestibilities of xylose, arabinose and cellulose-glucose were maintained at about 0.90, 0.62 and 0.70 g/g input respectively. At lower Pi, concentrations these digestibilities fell significantly and corresponding values at Pi < 1 mg/l were 0.73, 0.41 and 0.31 respectively. Starch digestion was unaffected by Pi concentrations and remained at about 0.90 g/g input.
4. The amount of microbial-N synthesized averaged 0.48 g/d and was maintained with Pi concentrations down to 4 mg/l. There was, however, a significant reduction to 0.26 g/d with Pi concentrations of < l mg/l. The effiency of microbial protein synthesis was variable but averaged approximately 25 g N/kg total carbothdrate fermented.
5. It was estimated that the minimum Pi concentrations required in rumen fluid in vivo to maintain maximum degradative and synthetic microbial activities was in the range 75–100 mg/l and that the over-all P requirement of the microbes was of the order of 5.1 g/kg apparently digested organic matter intake.