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Arabinoxylan digestion and endoxylanase functionality in ageing broilers fed wheat-based diets

Boek - Dissertatie

Wheat (Triticum aestivum L.) is a major energy component in European broiler feeds. However, the dietary fibre fraction of wheat largely consists of non-starch carbohydrates (NSC), mainly arabinoxylans (AX) which are known to evoke antinutritional effects in the gastrointestinal tract (GIT) of broilers. To overcome these antinutritive effects, endo-β-1,4-xylanases are frequently added to broiler diets. Since 1990 three mechanisms of action of feed endoxylanases are put forward. They include the removal of the physical barrier, and hence the nutrient encapsulating effect, composed of wheat cell walls and the reduction in intestinal viscosity. The third mechanism of action is of more recent date and attributes the beneficial effects upon endoxylanase addition to the formation of prebiotic arabinoxylan-oligosaccharides (AXOS). These would reduce the risk of pathogen infection and positively affect the microbial composition and gastrointestinal health. In spite of the knowledge gained on these mechanisms, part of the large inconsistency in response to endoxylanases as a function of farm, flock and time remains unexplained. This implies that there is still a lack of understanding of how these enzymes exactly work in vivo in the GIT of broilers. Moreover, insight into the predominant mode of action of feed endoxylanases is often obscured by the wide variety in broiler-, diet- and enzyme-related factors that highly influence the efficacy of these enzymes. As a consequence, only little progress in the optimisation of these feed enzymes and their application has been made during the last decade. Against this background, this doctoral dissertation aimed to contribute to a better understanding of AX digestion in broilers and the functionality of dietary endoxylanases and AX included in wheat-based broiler feeds. To this end, an in-depth evaluation of AX hydrolysis and fermentation in the GIT of broilers as a function of age was performed. The impact of dietary AX type and endoxylanase type and dose on this AX digestion and endoxylanase efficacy was also investigated more in detail. Broiler age and the age-related development of the microbial community revealed to be very important factors in determining AX digestion, and hence endoxylanase functionality in the hindgut of broilers. The incapability of the first colonising microbiota to digest the dietary AX was noticed at young broiler ages by the very low AX digestibility values, but an evolution towards a microbial community capable to digest wheat AX well at slaughter age was observed by the significant increase in AX digestibility values at this age. As the prebiotic mechanism is nowadays sometimes pointed out as a main working mechanism of endoxylanases, the contribution of AXOS as AX hydrolysis products to the age-related AX digestion in the hindgut of broilers was examined in more detail. Therefore, beyond the well-documented health and prebiotic effects, the additive effect of AXOS addition at a level of 0.50% to wheat-based diets containing no endoxylanases was investigated further. Our results indicate that the provision of AXOS in broiler diets has the potential to kick-start the AX digestion capacity of the young broiler microbiota, thereby resulting in steady digestion of the dietary fibre fraction of the feed from young broiler ages onwards. Plausible mechanisms explaining this kick-starter effect are not fully elucidated but are most likely the result of enhanced colonisation by microbial species harbouring a greater armoury of AX-degrading enzymes and/or a stimulus of gastrointestinal hormone responses which could beneficially affect the physiology of the GIT. As AXOS is a rather expensive dietary fibre source to add on top of a wheat-based diet, the potential of a readily available by-product of the milling industry, i.e. wheat bran with reduced particle size (RPS-WB) on AX digestion was investigated further in a second broiler trial. In this trial, we set out to examine if this RPS-WB source was able to induce a similar kick-starter effect at young broiler ages. In addition, the potentiating of this kick-starter effect upon addition of endoxylanases in combination with 1.0% RPS-WB was investigated more in detail. Results indicated that RPS-WB could alter the microbial community by introducing a greater abundancy of butyrate-producing microbiota, as observed by increasing levels of butyric acid in the caeca of broilers towards slaughter age. Despite this beneficial effect, no improved digestion of dietary AX was observed with 1.0% RPS-WB addition compared to the control. On the contrary, endoxylanase addition resulted in a marked increase in AX digestion, especially at young broiler ages. No synergistic effects on AX digestion were observed when both an endoxylanase and RPS-WB were added to the broiler's feed. It was however assumed that in larger-scale trials a combination of these supplements in wheat-based broiler diets can result in both an improved health and nutritional response. The chemical and physical appearance of AX in the diet hence largely dictate microbial responses and endoxylanase efficacy. To back these results, further research should aim to characterise microbial populations conferring to the kick-starter effect. In a final broiler trial, the influence of two types and three dosages of commercial endoxylanase preparations on AX digestion profiles as a function of broiler age were examined. Introducing an endoxylanase in the broiler's feed generally improved the feed conversion ratio (FCR) compared to the control as long as the dose applied was sufficiently high. Adding two different types of endoxylanase preparations resulted in the formation along the hindgut of a heterogeneous pool of AX products with distinct structural and physicochemical properties. An endoxylanase produced from B. subtilis was able to stimulate precaecal digestion of dietary AX by creating a large pool of solubles, without negatively affecting intestinal viscosity. An endoxylanase produced from N. flexuosa mainly triggered caecal AX fermentation by delivering easily fermentable AX substrates having a low degree of polymerisation and a low arabinose to xylose ratio. Endoxylanases should modify the dietary AX in such a way that both antinutritional effects displayed by these AX are reduced and the AX degrading capacity of the young microbiome is stimulated. This strategy will most likely generate the largest benefits regarding broiler health and performance. This doctoral dissertation led to new insights in and strategies based on feed endoxylanase functionality, enabling them to stimulate the AX degrading capacity of the young microbiome and increase the overall dietary fibre digestibility. Broiler age and the form under which AX is present in the dietary matrix were shown to be two very important factors determining AX digestion and endoxylanase efficacy. These factors should hence be taken into account when formulating broiler feeds to allow a further optimisation of the functional and nutritional value of dietary fibre in animal feed. Advancements in analytical tools that would allow continuous tracking of how the dietary fibre fraction is digested and which microbial metabolites are produced upon this fibre digestion in vivo, can aid to further unravel the full complexity of the mechanisms of action of feed enzymes in general and endoxylanases in particular in the near future.
Jaar van publicatie:2020
Toegankelijkheid:Open