Metabolic Control: Improvement of a Dynamic Model of Lactational Metabolism in Early Lactation

摘要

A mechanistic, dynamic model of metabolism exists to allow testing of complex hypotheses on nutrient metabolism of the dairy cow. The model describes protein synthesis and degradation in the viscera and muscle, lipogenesis and lipolysis in the adiposetissue, and milk synthesis. It is dynamic, integrating rates over the course of a few hours or an entire lactation. The model performs very well at describing milk production given nutrient intakes. Errors still exist in descriptions of visceral, muscleand adipose metabolism, especially in early lactation. Over the last 15 years, we have had as an objective the task of improving the performance of this model for research into metabolic control in dairy cattle. Areas of emphasis have been lipogenesis and lipolysis in the adipose tissue, body protein use and basal energy use. A series of model challenges has determined that descriptions of basic processes (such as ion pumping, protein and fat turnover rates, increased metabolic costs associated with increased intake) in this model are inadequate. Errors in the descriptions of viscera and muscle tissue result in excess energy accumulation in the adipose tissue. A hypothesis based on experimental evidence is that simulated rates of energy use in the viscera, due to the hormonal and nutrient intake changes that take place in early lactation, are too low. A corollary hypothesis is that rates of energy use by the body, especially in protein turnover and associated metabolic costs is also too low in the model. Simulations increasing these energetic costs in the model resulted in realistic reductions in body fat compared to observed experimental data. These improvements become hypotheses for a framework of focused experimentation to improve our quantitative knowledge of metabolism in lactating cows.