Feed particle size, whole wheat inclusion and xylanase supplementation in broiler diets : influence on the performance, digesta characteristics and digestive tract development : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Poultry Nutrition at Massey University, Palmerston North, New Zealand
The first three experiments of this thesis examined the effects of particle size reduction of grains in relation to feed form (mash vs. pellet), grain type (wheat vs. maize) and xylanase supplementation on broiler performance, energy utilisation, digestive tract development and digesta parameters. The fourth experiment examined the interaction between wheat hardness and xylanase supplementation. The effects of insoluble fibre source and whole wheat inclusion were studied in the fifth experiment. In the first experiment (Chapter 4), pelleting reduced nitrogen-corrected apparent metabolisable energy (AMEn), but broiler performance was superior in birds fed pelleted wheat-based diets compared to those fed mash diets. Feed form had a greater effect on various measured parameters than did particle size. Pelleting evened out differences in particle size distribution between treatments and, as a result, wheat particle size had no effect on the performance of broilers fed pelleted diets. In contrast, the second experiment (Chapter 5) showed that differences in particle size distribution persisted between diets after pelleting and, as a result, coarse grinding of wheat or maize improved broiler performance compared to those fed diets based on fine particles. These results may be related, in part, to changes in size distribution following pelleting. In mash diets, inconsistency in performance reponses were found. In the first experiment (Chapter 4), coarse grinding of wheat improved weight gain and feed per gain compared to medium grinding. In the third experiment (Chapter 6), however, grinding particle size had no influence on broiler performance. The observed discrepancy suggests involvement of other factors such as wheat cultivar and grain hardness. Data reported in Chapter 6 showed that xylanase supplementation improved feed per gain of birds fed the coarse particle size diet, but had no effect on those fed the medium particle size diet. In Chapter 7, there was a significant interaction between wheat hardness and xylanase supplementation due to the improved feed per gain and AMEn of birds maintained on hard wheat-based diet, while there was no effect of xylanase on sort wheat-based diet. These findings suggest that the efficiency of exogenous enzymes is influenced by both particle size and wheat hardness. Data reported in Chapter 7 showed that inclusion of soft or hard whole wheat pre-pelleting produced different particle size distributions in the pelleted diets. This suggested that hardness of the grain must be considered when choosing whole wheat for inclusion in broiler diets. Data on the effect of feed particle size on its subsequent distribution in poultry digesta are scanty. Results reported in Chapters 4 and 5 showed that there was no effect of feed particle size within feed form on duodenal digesta particle size. On the other hand, particle size of duodenal digesta was influenced by feed form (mash vs. pellet). Wheat hardness was also found to influence the particle size of proximal (duodenum and jejunum) intestinal digesta (Chapter 7). These results indicated that the gizzard does not uniformly reduce the size of all particles. However, the gizzard appears highly efficient in grinding large particles, although some large particles escape the grinding. The final experiment demonstrated that the effects of insoluble fibre on digestive tract development and broiler performance differed depending on the fibre source. Wood shavings, a source of coarse insoluble fibre, increased relative gizzard size and improved corrected feed per gain and ileal starch digestibility. In contrast, cellulose, a source of fine insoluble fibre, had no influence on these parameters. In conclusion, dietary manipulations, which stimulated gizzard development, positively influenced broiler performance and starch digestibility. The findings of this thesis suggest that energy savings during feed processing could be achieved by coarse grinding of grains with no adverse effect on broiler performance and that cereals used in broiler diets can be ground more coarsely than the current practice. Wheat hardness appears to be an important criterion to consider when choosing a cultivar for whole wheat inclusion in broiler diets. Another major finding was that the effectiveness of exogenous xylanase in wheat-based diets could be improved by considering factors such as particle size and wheat hardness.