The dynamic model of Baldwin et al. (1987a,b,c) is more complex than the NRC and Cornell models, and, therefore, requires more inputs. These include soluble sugars, organic acids, pectins, soluble and insoluble starch, plant lipids, feed fats, soluble and insoluble protein, non-protein nitrogen, hemi-cellulose, cellulose, lignin, and soluble and insoluble ash. The various feed components enter large particle, small particle or soluble pools of the rumen depending on their chemical and physical characteristics. Rates of rumination influence rates of conversion of large into small particles. Components of small particles are hydrolysed to chemical constituents by associated microbes or passed from the rumen. The inclusion of three pools for feed inputs and association and growth of microbes in association with small particles is required to explicitly accommodate digestion lag times as observed in vivo.
Cellulose and hemicellulose digestion are dependent on microbial attachment and are influenced by rumen pH and peptide concentrations. As concentrations of cellulolytic microbes increase, rates of attachment to small particle-structural carbohydrate increase and rates of hydrolysis of feed particles increase. This results in concomitant increases in rates of entry of peptide and amino acid nitrogen into the soluble pool. Utilization of peptide N is handled somewhat differently compared with the Cornell model. Peptides and amino acids act to enhance theoretical maximal yields of microbes and are used for SC-bacterial growth with a maximal yield of SC-bacteria from peptide and amino acid nitrogen of 50%. The following equations describe the microbial growth components of the model.
where dMi/dt represents the change in the microbial pool (kg) with respect to time. The size of the Mi pool is determined at any time by integrating the equation. Mi partitioning between large particle small particle, and soluble pools is based on the proportion of DM present in each pool. UMiG represents the input of microbes to the pool due to microbial growth where:
UmG = YATP X ATPg X NH4Adj X FatAdj
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