Gut health of poultry in the post-antibiotic era: elucidating the mechanisms underlying successful feed supplements

The recognised risk of antibiotic-resistant bacteria is a global concern that has put increased pressure on non-EU countries to decrease their use of antibiotic growth promoters and EU countries to reduce their therapeutic antibiotic usage in poultry. Therefore, viable non-antibiotic feed supplements are needed to improve bird gut health, immunity and performance, whilst reducing the proposed risk to human health. Many supplements are being produced with this aim, but limited understanding of how they work hampers improvements to efficacy. A natural carbohydrate fraction (NCF) refined from the cell wall of Saccharomyces cerevisiae yeast has been recently developed and proposed to improve gut health, immunity and bird performance. The aim of this research project was to examine possible modes of action behind this feed supplement on gut health, immunity and performance in broiler chickens. A total of 3 bird trials were performed to achieve this aim. Chapter 3 reports on a 6 week trial (Trial 1), designed to examine the performance and gut health of birds fed NCF with a commercial stocking density. The effects of NCF on gut development were examined only at 6 weeks in order to not alter stocking density. The results of this trial demonstrated a positive effect of NCF on performance and limited effects on gut health at 6 weeks. Performance improvements were also age dependent with younger birds showing greatest response to supplementation. Following the conclusion drawn from Trial 1, Trial 2 (Chapter 4) was conducted using weekly sampling from each bird pen to examine the effects of NCF on development of the gut and intestinal morphology in broilers prior to 6 weeks. Trial 2 investigated the effects of NCF on microscopic changes in the gastrointestinal tract (GI tract) from hatch to slaughter and relate effects to performance. In contrast to Trial 1, data from Trial 2 did not show any pronounced effect of NCF on gut health or bird performance. This may be due to the lack of pathogenic challenge or physiological stress on the birds resulting from the decrease in stocking density over the trial. In addition, activity of NCF in the manufactured diets could not be dependably verified to show the intended incremental increases between treatments, which may be due to an issue with the assay or supplement batch itself. Trial 3 (Chapter 5) was performed to investigate the effects in young (0-3 week) broilers of two commercial forms of butyrate salts on gut health, performance, immunity and digestive enzyme activity, both singly and in combination with NCF. Butyrate salts are reported to have similar effects to NCF on gut health and performance but thought to engage different response mechanisms. Again, it was seen that NCF with or without the inclusion of butyrate did not show a pronounced effect on the gut health parameters measured or bird performance in this trial. This project aimed to clarify mechanisms behind the observed effects of gut health supplements in broilers by investigating parameters spanning from the overall performance down to individual gene expression. Chapter 6 reports on upstream factors with the potential to subsequently influence gut health and performance. While NCF had no effect on the digestive enzyme activity or serum immunoglobulin measured, a substantial effect on the gene expression of pathway involving immune response and metabolic processes was observed. Furthermore the project provides evidence that some of the immune adaptations are borne out through alteration to mucin production by goblet cells. It was also indicated in this chapter that the mechanism underlying the effects of NCF depend on the inclusion level: 200g/t NCF altered immune and metabolism genes, 400g/t NCF altered mostly immune genes, whereas 800g/t altered mostly metabolism genes. From the alterations seen in the gene expression and mucin capacity when feeding NCF, it appears that NCF is increasing immune defences to prevent growth impedance, rather than any metabolic, growth promoting effect. From this it may be concluded that performance improvements are more likely to be observed when some level of challenge is placed on the bird