Rumen digestion of crude protein (CP), and protein available at the intestinal absorptive site as metabolizable protein (MP), is predicted using static equations in the NRC model (NRC, 2001). There are a number of required feedstuff chemical analyses and feedstuff biological determinations, as well as some characteristics of the animals to which the feedstuffs and ration are to be fed, that are required as model input. The process proceeds stepwise to create numerous empirical equations that are finally combined to constitute an evaluative/predictive model.
The first step is to predict microbial crude protein (MCP) from 'discounted' total digestible nutrients (TDN) intake (g day-1) as:
Calculation of discounted TDN is itself a summative equation utilizing several feedstuff chemical assays, as well as an estimate of ruminai digestion of neutral detergent fibre. The 'discount' refers to the reduction in the energy (TDN) content of the diet as its relative intake increases. Microbial crude protein production is subsequently used to calculate the rumen degraded protein (RDP) requirement using the assumption that 18% more RDP is required than appears as MCP. It is assumed that the oversupply of RDP is lost from the rumen as ammonia. However, if the supply of RDP is less than 118% of TDN predicted MCP, then MCP is calculated as:
These relationships are based on the assumption that RDP composition and availability in the rumen does not affect rates of micro bial growth. This simplification fails to capture interactions between protein and carbohydrate availability, although NRC (2001) addresses this issue by the statement that'... when rumen fermentation is normal, there is little additional benefit of altering carbohydrate or protein degradation rates, or their level of synchrony, on microbial protein synthesis'. The assumption that the required oversupply of RDP is a constant proportion of RDP is probably inadequate as well, since this flux is largely dependent upon rumen ammonia concentrations.
Difficulties with these calculations are that fat in the diet contributes to discounted TDN, whereas there is no experimental evidence that this occurs, and that there is no limit to the proportion of RDP that can be provided from non-protein nitrogen (NPN) sources, such as urea, to support microbial growth. Thus, high fat diets will elicit a calculated RDP deficiency for microbial growth that can be corrected by dietary addition of urea.
Once the RDP requirement of the luminal microorganisms is calculated, the RDP and rumen undegraded protein (RUP) proportions of feed CP are calculated based on the assumption that feed CP can be divided into three fractions, referred to as A, B and C, with a first order rate of degradation constant for fraction B. The in situ approach is recommended to determine these fractions, with the assumption that fraction A crude protein is rapidly and completely degraded in the rumen, fraction B crude protein is potentially degraded at a first order rate (fcd) and fraction C crude protein is indigestible. That the in situ procedure is highly variable among and within labs, which has led to its widespread abandonment in the scientific literature, and that all accepted assumptions relative to fractions A, B and C have been found to be false, is little addressed. Nevertheless the feed proportions of RDP are undegraded dietary protein (UDP) are calculated as:
Where the rates of passage (/cp) of various feedstuffs are calculated from three equations for wet forages (eqn 16.5), dry forages (eqn 16.6) and concentrate feeds (eqn 16.7) as:
where X: = DM intake, % of body weight; X2 = concentrate, % of diet DM; X3 = NDF %, % of DM.
Questions related to how to define feed-stuffs into the three categories, the origin of the data used to generate these calculations (except that they are based on unspecified experiments using rare earth elements), and any validation of the equations, are not addressed.
Endogenous CP (ECP) is calculated directly from DM intake as:
After all sources of CP flowing to the intestine (i.e. MCP, UDP, ECP) have been estimated, their MP fractions are estimated as fixed constants of 0.64 for MCP and 0.40 for ECP. Intestinal digestibility of RUP is considered to be predictable based on the mobile bag technique, or a two-stage in vitro digestion procedure, but not as acid detergent insoluble CP which was judged unacceptable. The conversion of digestible UDP into MP is considered to be 100%. That the mobile bag procedure is known to overestimate the MP content of RUP, as it measures CP disappearing from the bag in the large intestine to be MP, is not addressed.
In the NRC (2001) »/stem, microbial growth is a function of discounted TDN and (or) RDP intake. Feed protein escape from the rumen depends on its division into soluble, degradable and undegradable fractions, with a first order rate of degradation of the degradable fraction, as well as calculated rates of rumen passage which are based on total DM intake of the target animal, its concentrate proportion and the neutral detergent fibre (NDF) content of the feedstuff. Endogenous protein is calculated as a function of DM intake. Conversion of these three crude protein flows into the intestine into MP assumes separate constants. Numerous assumptions are inherent to all of these calculations and virtually all of them have been shown to be false.
The calculations depend exclusively on empirical equations derived from data sets, of variable sizes and degrees of definition, derived from published studies. Validation of these empirical equations by use of independent data sets is not attempted.
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