Investigations into the impact of feed form and age on the standardised ileal amino acid digestibility of feed ingredients for broiler chickens : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science (Poultry Nutrition) at Massey University, Palmerston North, Manawatu, New Zealand

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The importance of measurement of standardised ileal digestibility coefficients (SIDC) of amino acids (AA) in feed ingredients and their value in improving inter alia the precision of broiler feed formulations needs no elaboration. Databases on the SIDC AA of ingredients are accumulating during the past decade and various factors that influence the digestibility have been identified. Two aspects that remain unexplored hitherto in detail are the effects of feed form (FF) and broiler age. Previous AA digestibility assays were accomplished using mash diets because of the simplicity and the fact that research facilities often do not have access to pelleting equipment. As commercial broilers are fed pelleted diets, the data derived from mash diet may not be directly applicable. Another limitation is that, despite the potential age effects on AA digestibility, no study to date has investigated the influence of age on the SIDC AA in ingredients for the whole growth period of broilers. Lack of age-appropriate basal endogenous AA (EAA) flow data for the correction of apparent ileal digestibility coefficients (AIDC) is another constraint. Most of the existing EAA flow data were generated using older birds (21 to 42 d of age). The focus of this thesis research was to address these critical aspects. The first two experimental studies in Chapters 3 and 4 were conducted to determine the effect of FF (mash vs. pellet) on the SIDC AA in four grains (maize, wheat, sorghum and barley) and three protein sources (PS; soybean meal; SBM, meat and bone meal; MBM, and CM; canola meal) in broilers, respectively. Regardless of the grain type, FF had no significant (P > 0.05) influence on the SIDC of any AA, except for Cys and Pro, which were higher (P < 0.05) in mash diets. The effect of FF was more pronounced in the SIDC AA of PS. No influence of FF (P > 0.05) was observed for the SIDC of individual indispensable AA (IAA), except for His, the digestibility of which was lower (P < 0.05) in the pelleted diets. The SIDC of all individual dispensable AA (DAA), average digestibility of DAA and total AA (TAA) were reduced (P < 0.05) by pelleting. Cysteine was the most affected DAA, with a 15.4% reduction in SIDC in pelleted diets in comparison with mash. These results showed that the use of SIDC AA data generated from feeding mash diets may overestimate the AA digestibility in feed ingredients and affect the precision of broiler feed formulations. For this reason, pelleted assay diets were used in succeeding trials. The third study, reported in Chapter 5, was carried out to investigate basal EAA flow at different ages (7, 14, 21, 28, 35 and 42 d post-hatch) of broilers, following feeding of a nitrogen-free diet (NFD). The basal EAA flow of all individual and total AA were reduced quadratically (P < 0.05 to 0.001) as the birds grew older. The EAA flows were higher on d 7, then declined on d 14 and plateaued until d 35. A further decrease in the EAA flow was observed on d 42. These age-specific values were used in the correction of AIDC to SIDC in all ensuing thesis studies. The study presented in Chapter 6 was conducted to determine the influence of age on the SIDC AA in two grain sources (wheat and sorghum) in broilers. Six broiler ages (d 7, 14, 21, 28, 35, and 42 post-hatch) were investigated. Two assay diets were formulated to contain 938 g/kg of each grain as the only source of AA. Each assay diet, in pelleted form, was offered to birds for 4 d during the six periods namely d 3-7, 10-14, 17-21, 24-28, 31-35 and 38-42 prior to ileal digesta collection. In the case of wheat, no age influence (P > 0.05) was observed on the SIDC of average of IAA and DAA, though the average of TAA tended (linear, P = 0.09) to increase with advancing broiler age. In sorghum, the average SIDC of IAA, DAA and TAA were higher (linear or quadratic; P < 0.05 to 0.01) at d 7, declined at d 14 and then evened out. Among the IAA, the SIDC of Arg, His, Ile, Leu, Lys, Thr, Val, and the SIDC of all individual DAA (with the exception of Cys) reduced with advancing age (linear or quadratic, P < 0.05 to 0.001). The study reported in Chapter 7 was conducted to measure the SIDC of AA in maize and barley at six ages (d 7, 14, 21, 28, 35, and 42 post-hatch). Two assay diets were formulated to contain 938 g/kg of either maize or barley as the sole source of dietary AA. In maize, the average SIDC of IAA and TAA was affected in a quadratic manner (P < 0.05) with the values being higher at d 7 that dropped at d 14, increased and evened out between d 21 and 35, and decreased again at d 42. A linear influence (P < 0.05) was observed in the average SIDC of DAA. The SIDC of average IAA, DAA and TAA in barley was influenced (quadratic; P < 0.001) by age. The digestibility values increased from d 7 to 21 and then plateaued until d 42. The SIDC of all AA in barley increased (quadratic; P < 0.05 to 0.001) as the birds grew older. The final study, presented in Chapter 8, was conducted to determine the influence of broiler age on the SIDC AA in two PS (SBM and CM). Six age groups (d 7, 14, 21, 28, 35, and 42) were utilised. Two assay diets were formulated to contain either SBM (413 g/kg) or CM (553 g/kg) as the only source of dietary AA. In SBM, the average SIDC of IAA and TAA were not affected (linear or quadratic; P > 0.05) by age. The average SIDC of DAA in SBM was affected in a quadratic manner (P < 0.05) and the highest value was observed at d 7, that followed by a decline from d 14 to 28, and increased beyond d 35. The average SIDC of DAA and TAA in CM were higher (quadratic; P < 0.05 to 0.001) from d 7 to 14, reduced at d 21 and then increased beyond d 28. The average SIDC of IAA dropped between d 7 and 28, and followed by an increase thereafter. The novelties of this thesis research are that (i) no previous study has investigated the SIDC AA of feed ingredients from hatch to the end of broiler growth cycle, (ii) this is the first study reporting age-appropriate basal EAA flows and (iii) the use of pelleted assay diets that resembled the FF commonly used in the broiler industry. There were several original findings in this thesis. First, pelleting had substantial impact on the SIDC AA of feed ingredients and FF influence was more evident in case of ingredients with higher protein contents. Second, the EAA flow was higher in younger birds and reduced with advancing broiler age. Third, the age effect on the SIDC AA was variable depending on the ingredient and specific AA, and that the age effect on AA digestibility need to be considered in ingredient matrices for precise feed formulation.  
Listed in 2022 Dean's List of Exceptional Theses
Broilers (Chickens), Feeding and feeds, Age, Amino acids in animal nutrition, Dean's List of Exceptional Theses