The Cornell Net Carbohydrate Protein System(CNCPS) partitions crude protein into fractions A, B, and C, depending on their rate and extent of degradability in the rumen (Sniffen et al., 1992; Licitra et al., 1996; NRC, 2001). Fraction A represents the non-protein N (NPN) (ammonia, peptides, amino acids) and is considered to be completely soluble; fraction B, subdivided into B 1, B 2, and B 3, consists of true protein with progressively declining ruminal degradability. Fraction C is unavailable true protein.
Broadly, these crude protein fractions are categorized into rumen degradable protein (RDP) and rumen undegradable protein (RUP). The rumen degradable protein meets protein requirements for ruminal microbial growth and protein synthesis.
Once reaching the rumen, feed and protein degradation is a function of microbial activity (McDonald et al., 1988: Van Soest, 1994). Rumen microbial activity, growth and protein synthesis is primarily limited by the rate and extent of carbohydrate fermentation in the rumen (Bondi, 1987; Van Soest, 1995). Consequently, dietary fiber fractions in the forage determine the animal response to feed.
Microbial protein and rumen undegradable protein reaching the small intestine are absorbed to meet the ruminant’s protein requirement (McDonald et al., 1988; Klopfenstein, 1996). When rumen degradable protein exceeds the capacity of the rumen microbes to assimilate it, ammonia builds up in the rumen. This is followed by absorption of ammonia into the blood, conversion into urea by the liver, and excretion in the urine ( Butler , 1998). The conversion of ammonia to urea costs the dairy cow energy that could otherwise be used for milk production (Van Soest, 1994; Butler, 1998). This loss of dietary crude prrotein and energy reduces the utilization efficiency of rumen degradable protein and therefore, reduced ruminant production (NRC, 2001). It also causes a negative energy balance that leads to a reduced fertility ( Butler , 1998).