Challenge and improvement of a model of post-absorptive metabolism in dairy cattle.

Abstract

AbstractWe have been conducting research to improve quantitative descriptions of metabolism in lactating dairy cows depicted in an existing mechanistic, computer-assisted model. The objective was to challenge this model with data collected in in vivo and in vitro experiments on high-producing dairy cattle fed a range of energy. Cows that varied in genetic propensity for milk production and dietary fat intake were used. Dietary inputs, milk component outputs, body fat, nutrient concentrations in blood and maximal velocity and substrate sensitivity of adipose-tissue metabolic reactions were observed. The model simulated yields of milk components within 5% of observed means. Simulated lipid metabolism and accumulation of body fat were adequate in many situations; however, the model response to changes in energy intake was too sensitive. This inadequacy was especially noticeable in later lactation, when body fat accumulation was simulated to be too high. Lack of precision in longterm dynamic changes indicates inadequacy in parameters describing energy-utilizing reactions. This severe challenge of the model supports its functionality. The model can be used to test hypotheses concerning the relation of body fat and protein use to milk production. Current knowledge as depicted in the model would suggest that a wide range of body fat at calving does not change milk production or body protein loss for cows fed adequately. In simulations, decreasing body protein at calving decreases milk production, while increasing body protein results in a greater loss of body protein without altering milk production. Unknown variation in use of amino acids by muscles hinders our ability to predict milk protein responses to changes in nutrient intakes. Further experiments must be designed to determine utilization of nutrients in viscera, muscle and adipose tissue, and must encompass sufficient range in genetic ability, nutrient input and time to adequately describe the dynamic and integrated nature of metabolic reactions.