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.

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.

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 in vitro 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.

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.

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 in vitro and in vivo. This project elucidated the complex interactions between dietary NSP and NSP-degrading enzymes in birds challenged with or without NE. The in situ enzymatic release of bioactive compounds was likely promising with supplementation of xylanase from GH family 11 to wheat-based diets.

The objective of this study was to evaluate the effect of 2 different doses of a partially buffered formic acid product (Amasil NA; 61% formic acid, 20.5% sodium formate), and a monoglyceride blend of short- and medium-chain fatty acids (BalanGut LS P) on necrotic enteritis (NE) infected broilers in terms of performance, intestinal microbial population and shortchain fatty acids concentrations in the gastrointestinal tract. A total of 528-day-old as hatched Ross 308 broilers were allocated to 48 pens with 11 birds in each pen. Six dietary treatments applied in the study were: T1) nonsupplemented diet (Control); T2) antibiotic supplemented diets; T3) and T4) high (Starter: 0.5%; Grower and Finisher: 0.5%) and low (Starter: 0.3%; Grower and Finisher: 0.2%) dose of Amasil NA; and groups T5) and T6) high (Starter: 0.3%; Grower and Finisher: 0.2%) and low dose (Starter: 0.3%; Grower: 0.15%; Finisher: 0.075%) of (BalanGut LS P). All birds in this study were fed starter (d 0-10), grower (d 11−24) and finisher (d 25−35) diets and challenged with NE. To induce subclinical NE, oral administrations of Eimeria oocysts (d 9) followed by inoculation of Clostridium perfringens strains (d 14 and 15) were applied. Results showed that birds fed the high dose of Amasil NA, had a higher feed conversion ratio (FCR, P < 0.05) compared to the nonsupplemented group during the starter period. Antibiotic supplementation reduced FCR during the grower (P < 0.001), finisher (P < 0.05) and overall (P < 0.001) periods of the experiment. Both levels of BalanGut LS P and low levels of Amasil NA enhanced overall FCR (P < 0.05) compared to the birds in the nonsupplemented group. Compared to the nonsupplemented group, high levels of Amasil NA and low levels of BalanGut LS P improved FCR in the finisher stage (P < 0.05). On d 16, cecum digesta of birds fed with antibiotic supplemented diets showed a significantly lower number of C. perfringens (P < 0.001) compared to the nonsupplemented and high level of BalanGut LS P group. Bacillus (P < 0.01) and Ruminococcus numbers were significantly lower in the birds fed with high level of Amasil NA (P < 0.05) compared to the antibiotic supplemented diets. High doses of Amasil NA, showed the highest propionate concentration in the cecum (P < 0.001). The study suggests that supplementation of BalanGut LS P and Amasil NA at different feeding phases may achieve optimal performance improvement in broilers under NE challenge.