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.