The aim of the present study was to investigate the bioavailability of selenium (Se) from milled soya beans, a plant protein, to lambs after various degrees of heat treatment. Thirty Dohne Merino lambs (ca. 25 kg live weight, 4 months old) were allocated randomly within sex and weight groups into three treatment groups. During the experimental period, they were fed a diet consisting of a large proportion of milled soya beans as their main source of Se and other feeds low in Se to balance the diet. Blood was drawn during this period and analysed for Se and glutathione peroxidase (GSH-Px) activity. Chemical and Se analyses were performed on the feed and soya bean samples as well as on the organs, wool and rumen fluid of the lambs. Although the ADIN concentration of the soya beans increased with increasing heat treatments, more heat damage could have been inflicted on it to differentiate the different soya bean fractions in comparison to each other. On average, the in sacco disappearance of the dry matter (DM), crude protein (CP) and Se was higher for the unprocessed soya beans compared to the two processed soya beans fractions. As a result, the lambs receiving the unprocessed soya beans (control group- C-group) had a significant higher rumen bacterial Se concentration than the lambs receiving the heat¬treated soya beans. Despite this higher concentration, the liver Se concentration of this group was statistically significantly lower compared to the two heat-treated groups. This can be interpreted that the heat processing of the soya beans enhanced its Se fraction to be taken up by the liver. However, those differences were biologically not significant. The average liver Se concentration between the lambs receiving the over- ¬processed- (a-group) and those receiving the ideal treated soya beans (I-group), did not differ, indicating that the excessive heat treatment did not influence the amount of Se reaching the lower digestive tract of the a-group. Although the Se concentration of the heart muscle of the a-group was statistically significantly lower than for the I-and C-groups, those differences biologically were not significant. Despite an increase in the erythrocyte GSH-Px activity over the duration of the trial in all three the treatment groups, no significant differences between the three treatment groups were observed at any stage. It is concluded from this study that the heat treatment of soya beans has not influenced the incorporation and the bioavailability of its Se fraction to any significant degree. / Dissertation (MSc (Agriculturae))--University of Pretoria, 2006. / Animal and Wildlife Sciences / unrestricted

Ce travail vise à développer un modèle générique capable de mieux rendre compte et de prédire les transferts de radionucléides dans le système sol/solution du sol/plante. La première partie de ce travail a été consacrée à l’analyse critique des modèles disponibles dans la littérature pour décrire l’adsorption du césium par les minéraux argileux (principal processus contrôlant sa disponibilité dans les sols). Cette analyse a débouché sur la formulation d’un nouveau modèle mécaniste combinant deux approches: la complexation de surface et l’échange d’ions. Cette approche, a été testée afin de modéliser l’adsorption du Cs sur plusieurs substrats argileux naturels. Ce travail a permis de valider le modèle proposé et de démontrer qu’il constitue un avantage majeur par rapport aux différents modèles existants. La deuxième partie a été consacrée, à la réalisation d’une série d’expérimentations, conduite en milieux contrôlés sur des systèmes dynamiques, et la modélisation de la (bio)disponibilité du Cs dans ces systèmes. A la suite de ces essais, les interactions entre solide et solution observées ont pu être correctement reproduites à partir du modèle proposé en prenant en compte la fraction argileuse du sol uniquement. Ces simulations ont également été comparées aux simulations obtenues à partir des modèles empirique (Kd) et cinétique (EK). Enfin, le développement d’un outil numérique permettant de coupler la description des interactions géochimiques au transfert vers la plante (approche cinétique) a permis de reproduire correctement les essais réalisés en Rhizotests couplant sol, solution et plante et de mieux caractériser la fraction du Cs disponible pour les plantes. / The overall objective of this work is to develop a generic model able to better account and predict the transfer of radionuclides in the soil / soil solution / plant. The first part of the work was devoted to the critical analysis of the models available in the literature to describe the cesium adsorption on clay minerals (the process that mainly controls its availability in the soils). This analysis enabled us to propose a new mechanistic model combining two approaches: surface complexation and cation exchange. Our approach has been tested in order to model the adsorption of Cs on several natural clay substrates, in a wide range of Cs concentrations and physicochemical conditions. This work allowed to validate the proposed mechanistic model and to demonstrate that it constitutes a major advantage over the various existing models. The second part was devoted, to the performing of a series of experiments, carried out in controlled environments on dynamic systems and modeling the (bio)availability of Cs in these systems. Following these tests, the observed interactions between solid and solution could be correctly reproduced with the proposed model taking into account only the clay fraction of the soil. These simulations were also compared with simulations obtained using a empiric (Kd) kinetic models (E-K approach). Finally, the development of a numerical tool for coupling the description of geochemical interactions with transfer to the plant (Michaelis-Menten approach) allowed to reproduce adequately the trials carried out in Rhizotests coupling soil, solution and plant, and to better characterize of the Cs fraction available for plants.