Different mathematical approaches to estimating microbial protein supply in ruminants

Many of the amino acids that are available for absorption in ruminants are derived from microbial protein that has been synthesized in the reticulorum en. This paper focuses on the prediction of the microbial protein supply an d evaluates different approaches to represent mathematically the process of microbial protein synthesis. In current protein evaluation systems for rum inants, the microbial protein supply is predicted using empirical equations that relate microbial protein production to the amounts of ruminally avail able energy and nitrogen. In contrast, mechanistic models of rumen function endeavor to describe quantitatively the microbial protein production that is based on underlying identifiable processes. A brief description is prese nted of two culture techniques used to examine microbial ecosystems, namely , batch culture and chemostat culture. The mathematical equations describin g these cultures are helpful in understanding key parameters of microbial p roduction for inclusion in models, including specific growth rate, growth y ield, and substrate affinity. The availability of carbohydrates is a primar y determinant of microbial protein production in the rumen, and the adequac y of mathematical representations of this relationship in empirical and mec hanistic models is assessed. The representation of substrate utilization fo r nongrowth processes and the relationship between microbial protein produc tion and the availability of various nitrogen sources are discussed. A vari able part of the synthesized microbial protein does not reach the duodenum but is degraded in the rumen, and its representation is examined. The predi ction of microbial protein supply should be based on a sound representation of the underlying mechanisms, including the interactions among microbes an d between microbial activity and substrate degradation.