Phosphorus in phytate is largely unavailable to chickens unless they are provided with dietary phytase. Phytase was shown to increase phytate degradation in the crop and proventriculus-gizzard and very little phytate degradation occurred in the duodenum-jejunum or ileum. These previous investigations were conducted on chickens fed corn based diets but not with wheat based diet. Increase in digesta passage or mean retention time (MRT) along the gastrointestinal tract could enhance phytase efficacy as the prolonged reaction time between substrates and phytase may further facilitate phytate dephosphorylation. Dietary fat and fibre supplementation have been shown to influence intestinal MRT in chickens therefore it is expected that inclusion of both dietary fat and fibre could be manipulated to further improve phytase efficacy in broiler chickens. Three experiments and preliminary studies were conducted: 1) to assess the effect of methodology on estimation of phytate hydrolysis and P digestibility in young broilers and 2) to investigate whether or not dietary fat and fibre could further improve phytase efficacy in broilers fed diets containing wheat by delaying digesta MRT. In the preliminary study, in order to facilitate analysis of limited digesta samples, a sample preparation protocol involving H2SO4 digestion for sequential analysis of titanium and other minerals in feed and digesta was established. It was also found that blood protein interfered with the colorimetric analysis of P and myo-inositol, thus deproteinization is required. Continuous feeding prior to sampling was recommended to obtain adequate amount of digesta for digestibility related analyses. In the first study, feeding duration (1 h or 5 h) and 1 h feeding followed by feed withdrawal and refeeding prior to sampling significantly affected the concentration of phytate, inositol phosphates and measured degradation and digestibility of phytate-P in different segments of gastrointestinal tract, which could lead to overestimation or underestimation of degradation and digestibility values. Therefore, it is also recommended to collect digesta samples at least 3 h after the start of photoperiod and avoid sampling 4 h prior to dark period when lighting program is applied. In the second study, 5% fat inclusion had no significant effect on growth performance, while phytase supplementation at 1500 FTU/kg improved feed intake (FI) and body weight gain (BWG). However, interactions between both factors additively increased FI from 804g to 1,221g, BWG from 630 g to 904 g, ileal phytate degradation by 44% and ileal P digestibility by 17%. Adding cellulose as filler in pellet-crumbled diet has diluted the nutrient concentration in 5% fat diet that eventually led to poorer FCR. However, phytase supplementation eliminated the negative effect of nutrient dilution by improving performance of chickens as good as those fed those fed 5% fat diet without cellulose addition. In the third study, a combination of 1500 FTU/kg phytase supplementation with 5% fat increased crop digesta MRT and improved phytate-P degradation compared to with 1% fat, providing evidence for the role of fat in improving phytase efficacy in broilers. This thesis has shown the importance of methodology standardization in estimating phytate hydrolysis and P digestibility and the role of dietary fat and fibre in improving phytase efficacy in broiler
