Metabolic relationships in the supply of nutrients for milk protein synthesis: integrative modeling

The objective of research under the NC-185 regional project is to identify the critical chemical transformations in the rumen, digestive tract, gastrointestinal and splanchnic tissues, and adipose and mammary tissues that define patterns of nutrient utilization in lactating dairy cows. This objective includes research on differences in fermentation, digestion, absorption, and tissue utilization of nutrients in sufficiently different situations to permit estimation of parameters defining various nutrient interconversions. The regional project is utilizing dynamic, mechanistic models of metabolism as tools for integrative analyses of experimental data generated by the group. During the early phases of the project emphasized herein, primary emphasis was on development of models of adipose tissue, mammary gland, liver, rumen, and whole animal metabolism. Serious inadequacies exist in the detail and scope of knowledge of rates of chemical transformations across the wide range of milk yields and nutrient intakes found in production situations. Current knowledge, as described in the various equations and parameters in the models, is presented and discussed. Some characteristics of the current models are illustrated, and methods to utilize the models to identify important experiments are discussed. More cooperative efforts are necessary, including experimental designs that focus on quantification of relationships between input and output, physiological mechanisms that alter patterns of nutrient utilization in lactating dairy cows, and yield estimates of the parameters describing the pre- and postabsorptive uses of feed nutrients.