The effect of pre-weaning rearing environment and post-weaning provision of zinc oxide on the gastrointestinal tract health and microbiome in pigs

The use of pharmacological levels of zinc oxide in pig diets for 14 days post-weaning has been common practice for many farmers across the EU. High dietary zinc oxide frequently improves growth performance and reduces the incidence of diarrhoea immediately after weaning, thereby reducing mortality and in turn improving profitability for farmers. However, environmental concerns and reports of contribution to the spread of antibiotic resistance genes in pathogenic bacteria, have resulted in the upcoming ban of pharmacological levels of zinc oxide in weaner pig diets across the EU in 2022. Alternative strategies to improve pig performance and health after weaning are continuously being sought, with the potential for rearing piglets outdoors before weaning, showing such benefits. Therefore, the research presented in this thesis aimed to investigate whether pre-weaning rearing environment and post-weaning supplementation of zinc oxide improved pig performance from farrow to finish and whether there are components of the gastrointestinal tract microbiome that showed similarities between rearing environment and dietary treatment. It was found that pigs reared outdoors showed lifetime performance benefits; enabling the majority of pigs reared outdoors to be sent to slaughter sooner than those reared indoors, whilst zinc oxide supplementation improved performance of pigs during the first two weeks after weaning but did not provide lifetime advantages, in support of previous findings. Remarkably, similar shifts in the composition of some bacteria of the small intestine were detected when comparing indoor pigs without zinc oxide supplementation to all outdoor pigs as well as indoor pigs receiving zinc oxide supplementation. Both outdoor rearing and the supplementation of zinc oxide in the diets of indoor-reared pigs, improved performance during the period of treatment after weaning, with added lifetime performance improvements of outdoor-reared pigs. This suggests that outdoor rearing and zinc oxide supplementation might cause similar changes in the microbiota that could be associated with improved pig performance immediately after weaning. The significance of these findings are discussed with a focus on continuing the work to investigate associations between the microbiota and pig performance.