Effect of different levels of phosphorus on rumen microbial fermentation and synthesis determined using a continuous culture technique

Abstract

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 Piconcentrations 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 piconcentration. Total volatile fatty acids (VFA) produced averaged about 6.83 mmol/h with Pilevels of 48 and 28 mg/l. Reduction in Piconcentrations 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 Piconcentrations, also increased with the lowest level of Pito about 240 mg/l. ATP concentrations averaged about 14 μmol/l at the highest Piconcentration and fell progressively with each reduction in Piconcentration to a final value of 2.5μmol/1 with the Pilevel < 1 mg/1.3. At Piconcentrations 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 Piconcentrations 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 Piconcentrations down to 4 mg/l. There was, however, a significant reduction to 0.26 g/d with Piconcentrations 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 Piconcentrations 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.