Cornell model

Bacterial yield in the Cornell model (Search: Agriculture, 1990) is defined as a static function of feed inputs. Inputs required to predict bacterial yield include: (i) a complete fibre analysis (Van Soest); (ii) total nitrogen and NPN; (iii) acid-detergent-insoluble nitrogen; (iv) neutral-detergent-insoluble nitrogen; (v) protein solubility; (vi) solvent soluble fat; and (vii) ash. These inputs are used to predict the growth rates of three separate pools of microbes as defined by the following equations:

where SCBactj = the yield of structural carbohydrate (SC)-fermenting bacteria from the jth feedstuff (g day :) and NSCBact. = the yield of non-structural carbohydrate (NSC)-ferment-ing bacteria from the jth feedstuff (g day-1). The NSC bacterial pool is further divided into starch-fermenting and sugar-fermenting pools. The efficiency of bacterial yield is calculated according to the following equation:

where Kma = the maintenance requirement of the ath pool of bacteria (a = SC-fermenting, sugar-fermenting, or starch- and pectin-fermenting; g carbohydrate g-1 bacteria h Kdaj = the rate of fermentation or growth of the ath pool of bacteria on the jth feed (h and YGa is the theoretical maximal yield of the ath pool of bacteria (g bacteria g-1 carbohydrate). This type of equation captures the principle that slowly fermented feeds will have a greater proportion of fermentable substrate utilized to maintain bacterial populations than feeds that are fermentable at higher rates. Similar equations are used for sugar- and starch-fermenting bacteria. Nitrogen availability affects the bacterial yield of NSCBact according to the following equations:

RDPep.

KDh.

  1. = DietPB., x --— [16.15]
  2. + Kpj where RDPBb} = rumen degradable protein (b = slow, intermediate, or fast rates of degradation) of protein in the jth feed, Kdj = factorial degradation of the bth protein compartment of the jth, feed, and Kp} = the rate of passage from the rumen of protein in the jth feed. The quantity of protein escaping rumen degradation or bypassing the rumen is a function of rates of degradation in and passage from the rumen, which are specified by input constants. NSCBact utilize peptides generated by protein degradation in support of their growth whereas growth is dependent on carbohydrate availability as described above. Equations describing peptide uptake are of the same form as those used for protein degradation where uptake is a constant function of rumen available peptides. Peptides generated in excess of requirements for NSCBact growth are subject to deamination on passage from the rumen. Therefore, when NSCBact growth rates are slow, rates of peptide use for microbial growth are low and rates of deamination are high. Nitrogen retained by microbes is calculated using the assumptions that microbes are 10% nitrogen and a maximum of 66% of the nitrogen requirement of NSCBact can be provided by peptides. Peptides taken up in excess of this requirement are deami-nated and can be used to meet ammonia requirements of growing microbes. Effects of ionophores are accommodated by reducing the rate of peptide uptake by the NSCBact pool in a stepwise manner. SCBact utilize all of their nitrogen as ammonia.

Rumen ammonia is calculated using the following static, factorial, empirical equation:

+ NSCAMMNR + SCAMMNR)

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