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dc.contributor.authorMoate, Peter
dc.date.accessioned2012-09-14
dc.date.available2012-09-14
dc.date.issued2007-08-28
dc.identifier.urihttp://hdl.handle.net/2123/8662
dc.descriptionDoctor of Philosophy(PhD)en_AU
dc.description.abstractDuring the past 30 years, there have been considerable advances in models dedicated to describing nutrient digestion and milk production in dairy cows. The most sophisticated of these is Molly, a dynamic mechanistic scientific model developed by R. Baldwin, J. France, D. Beever, M. Gill and J. Thornley. The most successful from the point of view of adoption, has been the Cornell Net Carbohydrate System (CNCPS), a static production model developed by researchers at Cornell University, D. Fox, R. Russel, C. Sniffen, J.O’Connor and P. Van Soest. The system of equations that make up the CNCPS has been incorporated into the CNCPS Program and also into the CPM-Dairy program. At the start of the new Millennium, despite the fact that the above software systems had sophisticated models to describe the digestion and metabolism of arbohydrates and protein, their equations to describe the digestion and metabolism of fat were quite primitive. During the last twenty years there has been a growing recognition of the importance of fat in dairy cow diets and that the optimization of the inclusion of fat in diets might have beneficial and profitable consequences for milk production, total milk fat concentration,the concentration of individual fatty acids in milk and for cow fertility. Clearly, at the time of the commencement of the work of this thesis, there was a need for a more sophisticated approach to the modeling of fat nutrition of dairy cows. Therefore, the main purpose of this thesis was to integrate into a general model of dairy cow nutrition (CPM-Dairy), a large amount of information on a system which iv encompasses intake of fat, ruminal transformations of individual fatty acids, intestinal absorption of individual fatty acids and incorporation into milk of the major individual fatty acids. Additional purposes of this work were the derivation of mechanistic principles underlying the behaviour of this system, the estimation of important parameters that describe the system and the identification of important gaps in our knowledge about the system. The general techniques of model development employed in this thesis involved: • collation of data from the scientific literature, • recognition of patterns within the data that could be accounted for by mechanistic principles, • choice of appropriate mathematical equations to describe the data, • estimation of parameters • assembling equations into sub-models to describe aggregated phenomena e.g. flow of specific long chain fatty acids (LCFA) to the doudenum • statistical validation of sub-models by employing the developed sub-models to make predictions about independent data-sets, and comparing these predictions with data. The work of this thesis resulted in the development of four models: 􀂙 A static sub-model to describe ruminal transformations and intestinal absorption of LCFA (fat sub-model)v 􀂙 A static sub-model to describe the output in milk of the major milk fatty acids (milk fatty acid sub-model) 􀂙 A dynamic model to describe in vitro ruminal lipolysis and biohydrogenation (BH) of unsaturated LCFA 􀂙 A dynamic model of plasma non-esterified fatty acids (NEFA) kinetics The static fat sub-model describes the intake of dietary fat based on 10 major dietary fatty acids, ruminal lipolysis of fat, ruminal BH of unsaturated LCFA, ruminal de novo synthesis of LCFA and intestinal absorption of 10 main individual LCFA. This model is currently influencing fat nutrition of dairy cows since it has been incorporated into CPMDairy and is used by more than 2000 dairy nutritionists and scientists throughout the world to formulate dairy rations. The fat sub-model has helped to elucidate key aspects of ruminal fat metabolism and of intestinal absorption of fatty acids. In particular, the fat sub model contains the first published estimates of rates of lipolysis of 25 common dairy feeds. The fat sub-model also contains the first published equations to predict how estimated rates of BH of hexadecenoic, octadecatrienoic, octadecadienoic, transoctadecenoic and cis-octadecenoic acid vary in response to different concentrations of total free (non-esterified) LCFA in the rumen. It also includes the first published equations to predict the de novo production of fatty acids in the rumen. The fat sub-model utilizes separate coefficients to describe the intestinal absorption of 10 individual nonesterified LCFA and for 10 individual rumen ‘by-pass’ (esterified) LCFA. These findings together constitute a significant advancement in our knowledge of ruminal metabolism and intestinal absorption of LCFA. vi The static milk fatty acid sub-model utilizes as principle drivers, predicted amounts of 10 major fatty acids absorbed from the intestine (obtained from the fat sub-model). These predictions along with other dietary and cow factors are used in multiple regression equations to predict the output in milk of 26 major milk fatty acids. This constitutes the second major achievement of this thesis since this is the first time that a model has been developed that predicts the production of all of the major milk fatty acids. In developing the milk fatty acid sub-model, significant advancements have been made in our understanding of the important principals underlying the fatty acid composition of milk fat. Chief among these is a finding that production of each individual de novo fatty acid in milk constitutes a relatively fixed proportion of the total production of de novo fatty acids (C4 – C15). The equations developed to predict the production of individual preformed milk fatty acids reveal some hitherto unrecognized and potentially important effects of dietary components on the production of specific fatty acids. In particular, one equation predicts that production of cis-9, trans-11 linoleic acid (CLA) may be enhanced by inclusion of buffer (sodium bicarbonate) in the diet and reduced by the inclusion of magnesium oxide in the diet. These unexpected associations have the potential to influence human health and therefore warrant further research. Although the equations of the milk fatty acid sub-model hold much promise for contributing to the eventual goal of predicting the fatty acid composition of milk, validation of these equations must await publication in the scientific literature of additional appropriate data. Nevertheless, the vii equations in the milk fatty acid model constitute a significant advancement in this field of research.The novel, non-linear dynamic model to describe in vitro ruminal lipolysis and BH of unsaturated LCFA, employs Michaelis-Menten kinetics and inhibition kinetics. This model may prove to be an important advancement in the field of modeling ruminal transformations of LCFA since it demonstrates that this type of modeling can provide new insights regarding BH of unsaturated LCFA. From this modeling, a novel finding is that vaccenic acid, at high concentrations, appears to inhibit its own BH. A second novel and perhaps surprising finding is that the BH of rumenic acid appears to proceed at a faster rate when triglyceride is present in the incubation medium compared to when it is absent from the incubation medium. These two findings warrant further research. The work involved in the development of the milk fatty acid sub-model identified that there is an important gap in the scientific knowledge regarding the quantification of lipolysis of adipose tissue. The novel dynamic model of plasma NEFA kinetics developed in this thesis, makes use of data obtained from a relatively simple procedure, an intravenous glucose tolerance test (IVGTT) to enable estimation of the rate of lipolysis of adipose tissue. Although further work is foreshadowed to validate this new model, it is considered that this model has the potential to make significant contributions to the advancement of nutritional research of ruminants and health related research into diabetes and metabolic syndrome in humans. viii The overall goal of a complete model to predict the fatty acid composition of milk is still tantalizingly just outside of our reach. However, the work of this thesis constitutes a significant advancement in our understanding of the system that controls the production of individual milk fatty acids, and is a substantial foundation upon which to build towards the goal of predicting the fatty acid composition of milk.en_AU
dc.publisherUniversity of Sydney.en_AU
dc.rightsThe author retains copyright of this thesis.
dc.subjectfatty aciden_AU
dc.subjectcomposition of milken_AU
dc.titleTowards a model to predict the fatty acid composition of milken_AU
dc.typePhD Doctorateen_AU
dc.date.valid1008-01-01en_AU


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