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Weaning in the early stage of the piglets’ life can lead to post-weaning diarrhoea. For several decades, it was tried to overcome weaning problems by the application of antibiotics, but this has led to the risk of antibiotic resistance. In the last years, the search for natural alternatives as feed additives was immensely enhanced, as it is summarized in Chapter 1.
Chapter 2 reviews the current literature about the physiological development in the GIT of piglets with a special emphasis on the critical phase of weaning. Diverse studies in rodent and pig models have revealed that specific dietary factors (such as trace metals or probiotics) can have a major impact on gene expression and on other digestive physiological functions. Therefore, an overview is given on two feeding strategies (probiotics and the trace element zinc) used for this PhD thesis. Besides the ’classical field’ of animal nutrition, other aspects and approaches including enzymatic analysis of digestive enzymes and the respective gene expression, or western blot analysis of specific proteins relevant for digestion and so on, have been taken into account when evaluating feed additives and the respective mode of action.
Chapter 3 explains the main aims and hypotheses of this PhD thesis. The main work of the current PhD thesis consists of three published manuscripts summarized in Chapter 4, 5 and 6. The main interest was to increase the knowledge of possible underlying mechanisms of the probiotic Enterococcus faecium NCIMB 10415 and the trace mineral zinc. Therefore, three studies with suckling or weaning piglets were conducted within this thesis.
Furthermore, several protocols for quantitative real-time RT-PCR plus diverse enzyme assays have been established in order to measure activities of disaccharidases (lactase, maltase, sucrase), aminopeptidase-N and intestinal alkaline phosphatase and the respective gene expression in jejunal brush border membranes of the piglets (methods have been published in Martin et al., 2012; Chapter 4).
In the first feeding trial, the impact of E. faecium NCIMB 10415 on performance parameters, digestibility and small intestinal digestive measures such as gene expression and digestive enzyme activities has been evaluated in piglets before and after weaning (Chapter 4). The results of the study suggest E. faecium supplementation of sow and piglet diets did not markedly affect physiological parameters associated with digestive function, likely due to good sanitary conditions in this trial. The age-dependent effects on brush border disaccharidase activities and epithelial morphology reflected the weaning transition, as well as maturation and adaptation to feed. Taking all results from the collaboration into consideration (see: Chapter 7), the study failed to prove a positive impact of the tested Enterococcus strain.
Whether changes in the activity of intestinal alkaline phosphatase can be associated with other parameters, such as the intestinal microbial ecology or influences on the immune system due to probiotic application, will be determined in future trials. Also of great interest are specific effects of certain probiotic strains when animal health is impaired or when other stress factors exert negative effects on digestive physiological parameters. Therefore, further investigation of probiotic effects in pigs should include challenge conditions via conscious provocation tests with viruses or enteropathogenic bacteria, or studies should be conducted under sanitary conditions which are closer to the situations in farms.
The essential trace element zinc as a constituent of over 300 metalloenzymes has been used for decades in high concentrations as a feed additive to prevent or treat diarrhoea in animals and humans. It plays a vital role in the regulation of gene expression, and is seen as a key factor in maintaining different physiological processes. While zinc deficiency impairs growth performance and clinical signs as parakeratosis, high amounts of dietary zinc oxide have been proven in the literature to reduce the incidence of post-weaning diarrhoea, and improve the performance of pigs after weaning (Chapter 2.3). Intestinal zinc uptake is facilitated through members of the zinc transporter families SLC30 (ZnT) and SLC39 (ZIP). Whereas ZIP proteins help to increase intracellular zinc concentration, zinc transport proteins from the ZnT family reduce intracellular concentrations through transport into the extracellular matrix (ZnT1) or into vesicles (ZnT2). The divalent metal-ion transporter 1 (DMT1) appears to play a minor role in zinc homeostasis. Although it has been shown that low levels of dietary zinc can modify the expression of these transporters in the gut and other organs of rodents, the reaction to pharmacological zinc levels is still not fully clarified. Older studies on true zinc digestibility in rats have suggested a homeostatic regulation of zinc absorption. To date, studies about influences of dietary zinc on homeostatic mechanisms in the jejunum of piglets are still lacking.
In the studies in Chapter 5 & 6, it was aimed to feed piglets with marginal, normal and high dietary zinc amounts for four weeks after weaning as compared to current nutritional recommendations (GfE, 2006). The main interest of the two studies was to determine whether these different dietary zinc concentrations would have an influence on the performance of the weaned animals, on jejunal mRNA expression or activities of digestive enzymes. Furthermore, investigations on gene expression of the above mentioned specific key zinc transporters (ZnT1, ZnT2, ZnT5, ZIP4, DMT1, MT) played a main role because of their essentiality in maintaining a homeostatic balance in the GIT of the piglets. Furthermore, it has been aimed looking at short-term (1-2 weeks) versus long-term (3-4 weeks) effects of feeding high dietary zinc levels.
In both of the current zinc studies, piglets in the HZn group showed a higher ADFI and ADG in the first week post-weaning, suggesting a stimulus on appetite in the HZn group. Furthermore, it was possible to show that high levels of dietary zinc can influence the expression of zinc transporters and zinc-binding proteins in the jejunum of weaned piglets. A decreased jejunal expression of zinc transporter ZIP4 and the increase of the jejunal expression of ZnT1 and MT suggest increased intracellular zinc concentrations, an increased zinc export from intestinal tissues into extracellular compartments and a decreased zinc uptake from the gut lumen at high dietary zinc supply. If there are zinc-dependent transcription factors such as the Krüppel-like factor 4 (KLF4) involved in these processes, needs to be further elucidated for clearance. As the in vitro experiments with the intestinal porcine cell line IPEC-J2 have revealed, the adaptive process appears to be established within 24h; however, it does not prevent the pigs from tissue zinc accumulation in their bone, kidney, pancreas, intestine and liver tissue of the piglets suggesting an overstrained homeostatic regulation. Whether this is the cause for impaired performance during longer supply, as observed four weeks after weaning, needs further clarification.
In conclusion, high dietary zinc level improved the performance of piglets in the short-term which means the first two weeks after the weaning. Although we could show that jejunal mRNA levels of key zinc transporters changed with a high dietary zinc supply, this did not prevent the animal from zinc accumulation in various tissues suggesting an outbalanced homeostatic regulation. Besides the evaluation of critical points in the use of high zinc concentrations in pigs’ diets, Chapter 7 tried to contribute to the on-going discussion about the balance between environmental issues and animal health with a model calculation of the use of different zinc dosages in modern pig production.