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1 Department of Animal Sciences, Washington State University, Pullman 99164-6351
2 Department of Animal Science, University of California, Davis 95616
We have been conducting research to improve quantitative descriptions of metabolism in lactating dairy cows depicted in an existing mechanistic, computer-assisted model. The model is dynamic and deterministic and is based on biochemical equations describing ruminal fermentation and chemical interactions in body tissues. 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 (7045 to 12,909 kg of milk/305 d), lactation number (1 to 4), stage of lactation (–30 to 345 d in milk), and rate of intake (14 to 29 kg/d of dry matter) and that were fed various energy-yielding feedstuffs 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. Model simulations were conducted; simulated yields of milk components for a 305-d lactation were 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 because of inadequate representation of dynamic responses over periods more than a few weeks long. The model behaves consistently with biochemical principles, behavior was in the correct direction, and precision was adequate for many variables. Lack of precision in long-term dynamic changes indicates that the parameters describing energy-utilizing reactions are inadequate. This severe challenge of the model supports its functionality. Further experiments must be designed to determine how nutrients in viscera, muscle, and adipose tissue are used; these experiments must encompass sufficient range in genetic ability, nutrient input, and time to adequately describe the dynamic and integrated nature of metabolic reactions.
Key Words: lactation • adipose • model • fat
Submitted on November 16, 1998
Accepted on June 21, 1999
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