Research, improvement and application of mechanistic, biochemical, dynamic models of metabolism in lactating dairy cattle

Models are representations of reality. The fields of nutrition, metabolism and biomedicine have used models to aid in research and its applications since before World War II. A model, or a modeling approach, to research may also be defined as an ordered way of describing knowledge of some ‘real’ system. Such models have been useful in ordering our knowledge into practical systems to describe nutrient requirements of agricultural animals. The ability to describe metabolic transactions, and their resultant affects on nutrient requirements, is critical to the continued ability to raise food-producing animals in efficient ways around the world. Models of increasing complexity, ever grounded in validated research data, will continue to improve. The only way to eventually define the true complexity of the organisms that we are dealing with is to have an ordered model approach which, in a planned iterative fashion, asks complex questions and increases our knowledge with the clear answers we receive. A mechanistic, dynamic model of metabolism exists in the dairy cow (Molly, Modelling Ruminant Digestion and Metabolism. Chapman & Hall, New York, USA, p. 469.) and allows testing of complex hypotheses on the nutrition of the dairy cow. A major area for which we still lack full understanding is metabolism in early lactation. A series of model challenges has determined that descriptions of basic processes (such as ion pumping, protein and fat turnover rates, and increased metabolic costs associated with increased intake) in this model are inadequate. Milk production can be described very well from feed inputs. However, errors in the biochemical transactions of viscera and muscle tissue result in excess energy accumulation in adipose tissue. A hypothesis, based on validated experimental evidence explicit in the Molly model, and on observations made since construction of the model, is that simulated rates of energy use in the viscera, probably due to the hormonal and nutrient intake changes that take place in early lactation, are too low in the model. A corollary hypothesis is that rates of energy use by the body, especially in protein turnover and its associated metabolic costs are also too low. Simulations increasing these energetic costs resulted in realistic reductions in body fat relative to observed experimental data. These hypotheses become the framework for continued experimentation and incorporation of new information into the model.