A critique of the Cornell Net Carbohydrate and Protein System with emphasis on dairy cattle. 2. The post-rumen digestion model

The Cornell Net Carbohydrate and Protein System (CNCPS) post-rumen digestio n model calculates nutrient absorption in the small intestine from microbia l cell, feed residue and endogenous secretion composition, but thereafter i t only makes use of one class of nutrient, the absorbed amino acids (AA). T his is because apparently digested protein, carbohydrate and fat are amalga mated in the calculation of total digested nutrients (TDN) as a measure of energy supply, which is then converted to digested energy (DE), metabolizab le energy (ME) and net energy for lactation (TEL). At the maintenance level of feeding, this results in TDN values for feeds 0.5 to 2.5% below those l isted in NRC (1988). ME values predicted at a typical production feeding le vel (L = 4) are about 6% below the listed maintenance level values, when a reduction of only about 5% would be predicted. The model makes no direct us e of NRC (1988) tabulated values for the TDN, DE, ME or NEL of feeds. A sen sitivity test of the Supply model revealed that variations of +/- 10% in ma ny of the input parameters were without significant effect upon the nutrien t supply measured as TDN or metabolizable protein (MP) supply. Variations i n dietary crude protein (CP) concentration affected rumen N and peptide sup ply, and MP supply from undegraded intake protein (UIP). Plasma urea N and milk urea N varied +/- 20% because of a multiplier effect within the model, twice the imposed CP% variation. Variations in neutral detergent fibre con centration (NDF%) affected effective NDF (eNDF) and microbial protein synth esis. Variations in the rate of degradation of cell walls (NDF) caused a me an change of +/-7.7% in microbial efficiency, whereas variations in the deg radation rates for the A and BI carbohydrate (CHO) fractions were without e ffect on microbial efficiency. Variations in starch content (BI fraction of CHO) quantitatively affected microbial production, but this was offset by a consequential reduction in the size of the A fraction (sugars and soluble CHO) of CHO, if all other parameters were held constant. Variations in the fat content of the diet produced significant effects upon predicted TDN an d ME values, because of the high gross energy of fat and the multiplier (2. 25) used on fat percentage in the calculation of TDN values. Published test s of the CNCPS claim that it can predict non-ammonia nitrogen (NAN) outflow s from the rumen of dairy cattle adequately. There are weaknesses in the st atistical analysis of the NAN data, particularly the pooling of data from g rowing cattle with that from dairy cows. Later work has shown that NAN supp ly in the CNCPS is over predicted with high undegraded protein diets, since any shortage of rumen degraded protein does not reduce microbial protein s ynthesis.