Influence of major minerals on rumen microbiota.

The supply of minerals needed to meet rumen microbe requirements should match the amount of energy available for fermentation. Therefore, we attempt to assess microbial requirements for phosphorus (P), sulfur (S) and magnesium (Mg) in terms of fermentable organic matter at the rumen level (OMF) or in terms of digestible matter in the total tract (OMD). In vivo, about 5 g of P and 1.8 g of S/kg OMD should be available (a) in the rumen. Pa is provided mostly by salivary secretion, which depends on several dietary factors as well as on the physiological state of the animal; dietary S supply with natural diets depends on dietary S availability, which can be particularly low for some roughages. Dietary Mg concentration should be in the range 1.5-2.5 g/kg OMD. The circumstances in which major minerals may be used to manipulate rumen fermentation are discussed. With a high concentrate diet the addition of a mineral buffer may help to maintain an adequate pH for cellulolysis and enhance the efficiency and quantity of the microbes produced. Some inconsistent results are obtained, possibly because of the sensitivity of rumen microbes to high osmolality. It is concluded that, as the margin between optimal stimulatory concentration and toxic excess is narrow, rumen fermentation should be manipulated cautiously when using minerals.

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

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)/l were infused into the reaction vessels previously inoculated with rumen contents, resulting in Pi concentrations in the vessel contents of 48, 28, 4 and less than 1 mg/l respectively. Various fermentative and synthetic characteristics were examined. In the vessel contents, concentrations of protozoa (about 0.9 X 10(5)/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 less than 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 mumol/l at the highest Pi concentration and fell progressively with each reduction in Pi concentration to a final value of 2.5 mumol/l with the Pi level less than 1 mg/l. 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 less than 1 mg/l were 0.73, 0.41 and 0.31 respectively. Starch digestion was unaffected by Pi concentration and remained at about 0.90 g/g input. 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 less than 1 mg/l. The efficiency of microbial protein synthesis was variable but averaged approximately 25 g N/kg total carbohydrate fermented. 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 over-all P requirement of the microbes was of the order of 5.1 g/kg apparently digested organic matter intake.

Effects of phosphorus deficiency on rumen microbial activity associated with the solid and liquid phases of a fermentor (Rusitec).

The rumen simulation technique (Rusitec) has been used to study the effects of phosphorus (P) deficiency on bacterial protein synthesis and chemical composition and on adenosine triphosphate (ATP) concentrations in the solid and liquid phases of fermentors. 16 g DM of a P-deficient mixed diet was put into each vessel daily and the vessels were infused with about 1 l of a P-deficient (0 mg of P) or P-supplemented (120 mg of P/l) buffer. Two vessels per treatment were used, and during a second experimental period the deficient and supplemented buffers were interchanged to determine the effects of P repletion and depletion. The proportion of bacterial N directly incorporated from dietary amino acids or peptides was greater in solid-associated bacteria (SAB) than in liquid-associated bacteria (LAB) (46 vs 19%). P deficiency increased this proportion in both bacterial populations. Protein synthesis associated with the solid phase represented about 30% of the total protein synthesis in the system. P deficiency induced a marked decrease in microbial protein synthesis in both phases. Microbial yield declined by about 5 points (g of N/kg OMF) in P-deficient conditions. ATP concentrations were greatly reduced in both phases but P deficiency had no effect on protozoal numbers. The effects of P depletion during period II were similar to those of P deficiency in period I, and P repletion showed that the effects of P deficiency were almost entirely reversible.