Rumen microbes use fermentable organic matter from the ration to form microbial protein. The microbial protein usually constitutes the largest portion of the intestinal digestible protein and has a favorable amino acid pattern that correlates well with the synthesis of milk protein.
In the old intestinal digestible protein system (1991), microbial protein production was assumed to be constant: 150 g microbial protein/kg fermentable organic matter. In the current intestinal digestible protein system (2007), the amount of organic matter actually fermented in the rumen (Rumen Fermentable Organic Matter) is considered when calculating microbial protein production. In addition, the amount of energy released during rumen breakdown and which is therefore available for microbial protein production differs per carbohydrate fraction. The efficiency with which the energy and protein released are used by the micro-organisms for the production of microbial protein is also taken into account. This depends on how it is present in the rumen, i.e. dissolved in the rumen fluid or in somewhat coarser particles.