Exploring the undigestibles in broilers: enhancing gut health and performance through enzymes

Abstract

<p>A series of experiments were conducted to develop nutritional strategies that minimise the undigested components in the diet and improve growth performance in broiler chickens offered a wheat- or maize-based diet. The general hypotheses tested in this thesis were that indigestible carbohydrate would: (i) bulk up the undigested dietary components; (ii) be reduced by enzymatic intervention, and (iii) release bioactive compounds in response to tailored exogenous enzymes, which lead to improving bird gut health and growth performance.</p> <p>Experiment 1 (Chapters 3 and 4) characterised the dietary components remaining undigested along the gastrointestinal tract in broilers. Common wheat- or maize-based diets were offered to birds without supplementation of exogenous enzymes. The results from this experiment indicate that a wheat- or maize-based diet could support high level growth performance of birds; however, more than 30% of nutrients provided remained unutilised in the excreta, regardless of bird age and diet type. On average on the two diets, 102 g/kg NSP and 92 g/kg crude protein were not utilised by birds based on results of 12 days of age; whereas, on average, 98 g/kg NSP and 84 g/kg crude protein were excreted at 35 days of age for both diets. Detailed analysis on the undigested NSP fraction along the gastrointestinal tract illustrated marked differences between the two diets in the amounts and types of NSP delivered to the different gut sections. Soluble NSP level was higher in birds fed the wheat-based diet compared to those fed the maize-based diet in all gut sections. Accumulation of insoluble NSP in the gizzard was noted in birds fed both diets but was more pronounced in birds fed the maizebased diet than those fed the wheat-based diet.</p> <p>Experiment 2 (Chapter 5) investigated the efficacy of various feed enzymes on degrading NSP residues present in the gizzard, jejunal and ileal digesta collected from Experiment 1. The <i>in vitro</i> digestion was conducted at 42°C for 2 hours (pH 6.0), partly mimicking the chicken small intestine environment. The results demonstrated that the extent of enzyme-induced NSP degradation was variable, depending on the enzyme preparations, digesta type and diets, indicating the distinct modes of action and target substrates of each enzyme preparation. A combination of xylanase and beta-glucanase resulted in the greatest NSP degradation for both diets when using the ileal digesta as substrate. In general, meaningful hydrolysis of soluble NSP following the incubation with enzymes was observed only with the digesta obtained from birds fed the wheat-based diet.</p> <p>Experiment 3 (Chapter 6 and 7) examined NSP-degrading enzymes in birds fed the wheat- or maize-based diets with or without subclinical NE challenge. This study was arranged in a 2 × 2 × 4 factorial with factors as follow: NE challenge (no or yes), diet type (wheat or maize) and enzyme treatments (control, GH10 xylanase, GH11 xylanase and mannanase). All the supplemental enzymes improved the growth performance in birds fed the wheat-based diet, regardless of the NE challenge. This improvement resulted from the marked reduction of jejunal and ileal digesta viscosity, and higher caecal concentrations of volatile fatty acids (VFA) as a result of enzyme addition. Moreover, supplementation GH11 xylanase to the wheat-based diet released pentose-based oligosaccharides in the ileum, with degradation of soluble and insoluble NSP. In birds offered the maize-based diet, both GH10 xylanase and GH11 xylanase tended to increase body weight compared to the non-supplemented birds when NE was absent. However, challenged birds offered the maize-based diet supplemented with the two xylanases presented poor growth performance, with increased necrosis lesions, due to elevated digesta viscosity. Supplementation of mannanase resulted in improved growth performance when supplemented to the wheat- but not maize-based diet, regardless of NE. Collectively, wheatbased diets hold an advantage over maize-based diets in bird performance in response to enzyme supplementation.</p> <p>Overall, the outcomes from this thesis show that more than 30% of feed provided ended up in the excreta in nutritionally adequate wheat- or maize-based diets, with NSP accounting for 28% of the indigestible fraction, irrespective of base cereal. This highlights that nutritional strategies to improve overall nutrient digestibility should focus on NSP utilisation in birds. The application of novel NSP-degrading enzymes shows promise in the degradation of dietary NSP <i>in vitro</i> and <i>in vivo</i>. This project elucidated the complex interactions between dietary NSP and NSP-degrading enzymes in birds challenged with or without NE. The <i>in situ</i> enzymatic release of bioactive compounds was likely promising with supplementation of xylanase from GH family 11 to wheat-based diets.</p>