A net carbohydrate and protein system for evaluating cattle diets: I. Ruminal fermentation

Abstract

The Cornell Net Carbohydrate and Protein System (CNCPS) has a kinetic submodel that predicts ruminal fermentation. The ruminal microbial population is divided into bacteria that ferment structural carbohydrate (SC) and those that ferment nonstructural carbohydrate (NSC). Protozoa are accommodated by a decrease in the theoretical maximum growth yield (.50 vs .40 g of cells per gram of carbohydrate fermented), and the yields are adjusted for maintenance requirements (.05 vs .150 g of cell dry weight per gram of carbohydrate fermented per hour for SC and NSC bacteria, respectively). Bacterial yield is decreased when forage NDF is < 20% (2.5% for every 1% decrease in NDF). The SC bacteria utilize only ammonia as a N source, but the NSC bacteria can utilize either ammonia or peptides. The yield of NSC bacteria is enhanced by as much as 18.7% when proteins or peptides are available. The NSC bacteria produce less ammonia when the carbohydrate fermentation (growth) rate is rapid, but 34% of the ammonia production is insensitive to the rate of carbohydrate fermentation. Ammonia production rates are moderated by the rate of peptide and amino acid uptake (.07 g of peptide per gram of cells per hour), and peptides and amino acids can pass out of the rumen if the rate of proteolysis is faster than the rate of peptide utilization. The protein-sparing effect of ionophores is accommodated by decreasing the rate of peptide uptake by 34%. Validation with published data of microbial flow from the rumen gave a regression with a slope of .94 and an r2 of .88.

Relationship between efficiency of nitrogen utilization and isotopic nitrogen fractionation in dairy cows: contribution of digestion v. metabolism?

“…SAB mostly comprise bacteria attached to particles, especially fiber, and this fraction is richer in cellulolytic species than LAB (Michalet-Doreau et al, 2001). Microbes degrading structural carbohydrate (cellulolytic) use ammonia N as their main N source and so may fractionate N isotopes to a greater extent than microorganisms degrading non-structural carbohydrate (amylolytic), which use ammonia, peptides and AA as N sources (Russell et al, 1992). In this respect, despite a close correlation between the two rumen bacteria fractions (r = 0.82), the expected negative relationship between rumen ammonia concentration and δ 15 N values of rumen bacteria (Wattiaux and Reed, 1995) was only significant for SAB.…”

Effects of alfalfa silage storage structure and roasting corn on ruminal digestion and microbial CP synthesis in lactating dairy cows

“…Microorganisms utilizing cellulose and hemicelluloses as energy substrates grow slowly and use NH 3 as their primary N source (Russell et al, 1992). Atasoglu et al (2001) estimated that 80% or more of the N incorporated by ruminal cellulolytic bacteria in vivo was derived from ruminal NH 3, but overall bacterial growth was stimulated when peptides or amino acids were included in the growth medium.…”

Predicting the profile of nutrients available for absorption: from nutrient requirement to animal response and environmental impact

“…In general, empirical protein evaluation systems predict MPS using a constant EMPS value. In some systems, EMPS is variable and depends on, e.g., level of feed intake (AFRC, 1993), fractional passage rate (Thomas, 2004) or fractional degradation rate (Russell et al, 1992). Yet there are many mechanistic factors that affect EMPS.…”

“…However, substrate degradation and bacterial growth have been shown to increase with addition of AA or peptides in fibrolytic and amylolytic bacteria (Bach et al, 2005). In various models, the theoretical improvement in EMPS when all N required is provided by AA rather than ammonia varied widely: 19% (Russell et al, 1992), 59% (Dijkstra et al, 1992) and 77% (Baldwin, 1995). A major part of the variation in these values can be explained by differences in the units in which EMPS is expressed (i.e.…”