Modelling Mammary Metabolism in the Dairy Cow to Predict Milk Constituent Yield, with Emphasis on Amino Acid Metabolism and Milk Protein Production: Model Construction

Abstract

Previous efforts to simulate mammary metabolism have focused on energy, mostly considering amino acids (AA) in aggregate. The main objective of this work was to build a model of mammary metabolism, based on data from arterio-venous difference studies, which considered AA in sufficient detail to predict yields of milk solids. The model contains 19 state variables and considers the removal of 37 metabolites from blood, including 22 AA. It is driven by blood flow and arterial concentrations, and outputs include milk protein, milk lactose, and three classes of milk fat (by chain length). The model was parameterized using a balance version of it and the mean observations from four arterio-venous difference experiments, with a limited number of assumptions, and evaluated against these experiments. In assembling the balance model, milk protein output was not predicted satisfactorily, as some essential AA were not present in quantities great enough to support the rates of milk protein synthesis observed experimentally. Tryptophan showed the greatest deficit, followed by tyrosine plus phenylalanine, methionine, and histidine. In addition, significant quantities of pyruvate were needed to synthesize serine, glycine, and alanine. The supply of alpha-ketoglutarate plus glutamate to synthesize proline and glutamine was provided in part by catabolism of arginine; the remainder was derived from catabolism of other AA and energetic substrates.

Modelling Mammary Metabolism in the Dairy Cow to Predict Milk Constituent Yield, with Emphasis on Amino Acid Metabolism and Milk Protein Production: Model Evaluation

“…Apparent responses to other EAA have been observed (Schwab et al, 1976;Clark et al, 1978;Vanhatalo et al, 1999). The model described by Hanigan et al (2001) contains a representation of the single limiting AA theory and provides for possible limitations by methionine, lysine, histidine, phenylalanine, and threonine. Application of the model to experiments designed to manipulate AA supply to the udder should provide a quantitative assessment of the validity of the theory and provide an evaluation of the model.…”

“…A number of models have been developed to examine AA metabolism in the mammary gland (France et al, 1995;Hanigan & Baldwin, 1994;Maas et al, 1997;Waghorn & Baldwin, 1984). Until recently, there has not been a comprehensive representation of AA metabolism in the udder of the lactating dairy cow (Hanigan et al, 2001). The model of Hanigan et al (2001) has been constructed and parameterized, but not tested for sensitivity against parameter estimates and inputs, nor evaluated against an independent data set.…”

“…Until recently, there has not been a comprehensive representation of AA metabolism in the udder of the lactating dairy cow (Hanigan et al, 2001). The model of Hanigan et al (2001) has been constructed and parameterized, but not tested for sensitivity against parameter estimates and inputs, nor evaluated against an independent data set.…”

“…Application of the model to experiments designed to manipulate AA supply to the udder should provide a quantitative assessment of the validity of the theory and provide an evaluation of the model. The objectives of the current study were (1) to evaluate the model of Hanigan et al (2001) against experimental data and, thereby, the concepts represented in the model and (2) to test the model for sensitivity to inputs and parameter estimates.…”

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A model of mammary metabolism has been constructed and parameterized, with milk protein synthesis represented as a function of five essential amino acids (EAA) (Hanigan et al, 2001). Herein the model is evaluated using both the data used to construct the model (reference data) and an independent data set (literature data), and sensitivity to inputs and parameter estimates is assessed.

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