Yield of microbial protein

The yield of microbial protein which becomes available for digestion and absorption post-ruminally by the host has been related to the energy of the diet stated in terms of digestible organic matter (DOM), digestible organic matter digested in the rumen (DOMADR), total digestible nutri¬ents (TDN), net energy for lactation (NEL), metabolisable energy (ME), fermentable organic matter (FOM), fermentable metabolisable energy (FME) and rumen-degradable carbohydrate. The last three eliminate fat and products of fermentation, neither of which are considered to provide energy which can be utilised by the rumen microorganisms. The energy of fermentation products is significant in the case of silages and some distillery and brewery by-products. Hays are not considered to have und gone fermentation though they often contain measurable amount fermentation acids such as acetic and propionic. The routine measure of the contribution of fermentation products to metabolisable energy individual foods has not been a feasible proposition and assumed values are commonly used.
Sophisticated models attempt to relate microbial yield to the rate carbohydrate fermentation and rate of passage, theoretical growth the energy cost of bacterial maintenance and the form of nitrogen available to the rumen microorganisms. Many of the relationships involved such calculations are based on laboratory characterisation of the food, and the value of the model will depend on the validity of the relationships between the laboratory determinations and the values used in models.