This thesis was written with the aim of modelling the lactation process in dairy cows and
goats by applying a hierarchical Bayesian approach. Information on cofactors that could
possibly affect lactation is included in the model through a novel approach using covariates.
Posterior distributions of quantities of interest are obtained by means of the Markov chain
Monte Carlo methods. Prediction of future lactation cycle(s) is also performed.
In chapter one lactation is defined, its characteristics considered, the factors that could
possibly influence lactation mentioned, and the reasons for modelling lactation explained.
Chapter two provides a historical perspective to lactation models, considers typical lactation
curve shapes and curves fitted to the lactation composition traits fat and protein of milk.
Attention is also paid to persistency of lactation.
Chapter three considers alternative methods of obtaining total yield and producing Standard
Lactation Curves (SLACâs). Attention is paid to methods used in fitting lactation curves and
the assumptions about the errors.
In chapter four the generalised Bayesian model approach used to simultaneous ly model more
than one lactation trait, while also incorporating information on cofactors that could possibly
influence lactation, is developed. Special attention is paid not only to the model for complete
data, but also how modelling is adjusted to make provision for cases where not all lactation
cycles have been observed for all animals, also referred to as incomplete data. The use of the
Gibbs sampler and the Metropolis-Hastings algorithm in determining marginal posterior
distributions of model parameters and quantities that are functions of such parameters are also
discussed. Prediction of future lactation cycles using the model is also considered.
In chapter five the Bayesian approach together with the Wood model, applied to 4564
lactation cycles of 1141 Jersey cows, is used to illustrate the approach to modelling and
prediction of milk yield, percentage of fat and percentage of protein in milk composition in
the case of complete data. The incorporation of cofactor information through the use of the
covariate matrix is also considered in greater detail. The results from the Gibbs sampler are
evaluated and convergence there-of investigated. Attention is also paid to the expected
lactation curve characteristics as defined by Wood, as well as obtaining the expected lactation
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curve of one of the levels of a cofactor when the influence of the other cofactors on the
lactation curve has be eliminated.
Chapter six considers the use of the Bayesian approach together with the general exponential
and 4-parameter Morant model, as well as an adaptation of a model suggested by Wilmink, in
modelling and predicting milk yield, fat content and protein content of milk for the Jersey
data.
In chapter seven a diagnostic comparison by means of Bayes factors of the results from the
four models in the preceding two chapters, when used together with the Bayesian approach, is
performed. As a result the adapted form of the Wilmink model fared best of the models
considered!
Chapter eight illustrates the use of the Bayesian approach, together with the four lactation
models considered in this study, to predict the lactation traits for animals similar to, but not
contained in the data used to develop the respective models.
In chapter nine the Bayesian approach together with the Wood model, applied to 755 lactation
cycles of 493 Saanen does collected during either or both of two consecutive year, is used to
illustrate the approach to modelling and predicting milk yield, percentage of fat and
percentage of protein in milk in the case of incomplete data.
Chapter ten provides a summary of the results and a perspective of the contribution of this
research to lactation modelling.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-11032006-105824 |
| Date | 03 November 2006 |
| Creators | Lombaard, Carolina Susanna |
| Contributors | Prof PCN Groenewald |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-11032006-105824/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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