The first step of protein degradation in the rumen includes attachment of bacteria to feed particles, fol­lowed by activity of cell-bound microbial proteases (Brock et al., 1982). A large number of different microbial species form a consortium that attaches to a feed particle, acting symbiotically to degrade and ferment nutrients, including protein.  Products resulting from this process are peptides and amino acids (AA) (Wallace et al., 1997).

Peptides and AA resulting from the extracellular rumen proteolytic activity are transported inside microbial cells (Figure 1). Peptides can be further degraded by peptidases to AA, and the latter can be incorporated into microbial protein or further deaminated to VFA, CO2, and ammonia (Tamminga, 1979; Castrillo and Balcells, 2002). The fate of absorbed peptides and AA once inside the microbial cell will depend on the availability of energy. If energy is available and protein supply is high, bacteria readily degrade it yielding ammonia and various metabolic acids the ammonia is used to construct microbial protein. If available energy is scarce AA are deaminated to keto acids that are fermented to VFA and small amounts of the branched chain VFA — isobutyrate, iso valerate, and 2 - methylbutyrate — that arise from valine, leucine, and isoleucine, respectively (Wallace et al., 1997). Dietary plant amides, nitrites and nitrates, and endogenous salivary urea (nonprotein nitrogen compounds) when fermented also yield ammonia (Tamminga, 1979; Castrillo and Balcells, 2002).

Because proteolysis occurs rapidly in the rumen, it usually exceeds the rate of utilization by microorganisms with an accumulation of ammonia. This excess ammonia is absorbed across the rumen wall, converted to urea in the liver, released as blood urea N (BUN), and can be excreted into urine or recycled back to the rumen via saliva or diffusion through the rumen wall. This inefficient cycling of N is energetically costly to the animal when it exceeds a threshold below which the BUN could help support more microbial protein synthesis (MPS) (Firkins and Reynolds, 2005).