A mechanistic model of nutrient digestion in the rumen of dairy cows has been developed to predict rates of nutrient digestion and passage as well as rumen NDF pool sizes at fixed rates of dry matter intake. Different feed fractions according to chemical and physical characteristics are considered. There are 18 rumen state variables representing carbohydrates, nitrogen and microbes in the model. The model is mainly sensitive to the parameters defining rate constants of passage of small particles and digestion rate constant of NDF. For comparative analysis the model has been tested with diets differing widely in intake level and composition. NDF passage and digestibility, NDF pool, total and non-ammonia nitrogen outflow from rumen are well predicted for the whole range of diets used. For some diets, especially those containing maize as a source of starch the predictions of OM and starch digestibility and partition of NAN outflow between dietary and microbial N outflow are not satisfactory.

A dynamic, mechanistic and deterministic model of feed intake regulation in dairy cows is described. The model is based on a conceptual model of feed intake regulation involving an interplay between rumen function and energy transactions. Two interconnected submodels are included, the rumen submodel (RS), adapted from a previously developed rumen model and the feed intake regulation submodel (FIRS). From RS rumen digesta load in terms of kg of NDF is calculated. FIRS not only describes the feed intake regulation but also the digestion, absorption and flow of nutrients beyond the rumen. Energy transactions in the model are related to the simulated difference between the capacity to use energy and the intake of metabolized energy. In the model, energy outputs of the animal include not only the energy needed for maintenance and milk production but also the energy needed to support a genetically determined time course of body reserves. Body reserves, both the absolute level and the rate of mobilization are considered in the regulation of feed intake. A preliminary evaluation of the model behaviour for a typical diet and animal inputs showed predicted values within expected values for feed intake, body condition score and body weight.

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