<p>The major objective of this doctoral research project was to investigate the influence of varying levels of dietary minerals (Ca, NPP, Na, Fe and Zn) on phytase activity and its subsequent impact on broiler chickens. Along with an extensive review of literature (Chapter 2) related to this subject, the key findings of one in vitro study followed by four feeding trials are summarised in this thesis. Diets of all feeding trials were formulated by considering mineral matrix (Ca, P and Na) value of tested phytase and this matrix value appeared to be correct on the basis of the overall results of all trials. </p> <p>In the in vitro experiment, the effect of different dietary minerals (Ca, Na, Fe and Zn) on phytase activity at different pH was examined (Chapter 3). Calcium (0, 0.6, 0.8 and 1.0 %), Fe (0, 70, 80, 90 mg /diet), Zn (0, 30, 40, 50 mg kg/diet) or Na (0, 0.15, 0.25 and 0.35 %) were incubated (30, 60 and 90 mins) with a Na-phytate (0.27 %) solution, with phytase enzyme (500 FTU kg/ diet) at pH 2.5 or 6.5. There was a reduction (p < 0.05) in phytate hydrolysis by phytase at high concentrations of Ca, Fe and Zn (10 g, 90 and 50 mg respectively), particularly at pH 6.5. Although, increasing Na concentration reduced (p < 0.05) phytate hydrolysis, mostly at pH 2.5, the pattern was indefinite. In the presence of high concentrations of Ca, Zn and Fe, residual phytate content after phytate digestion was higher (p < 0.05) at pH 6.5 than 2.5, while the reverse was the case in the presence of Na. The findings of this in vitro study was further evaluated in four subsequent feeding trial.</p> <p>The influence of different levels of Ca (0.6, 0.8 and 1.0 %) and NPP (0.3 and 0.4 %) with phytase (500 FTU/kg) or without phytase supplementation was evaluated in first feeding trial (Chapter 4). In general, phytase supplementation improved (p < 0.05) the body weight gain (BWG), feed intake (FI) tibia bone breaking strength (BBS), tibia ash content, ileal digestibility of Ca, P and protein. However, the positive effect of phytase on these variables was reversed (p < 0.05) in diets containing high Ca (1.0 %) and low NPP (0.3 %). This combination of minerals and phytase also reduced (p < 0.001) the activities of alkaline phosphatase (AP), Ca-ATPase and Mg-ATPase activity of jejunum. High Ca diet reduced the carcass yield of bird even with phytase supplementation (Ca × phytase, p < 0.041). </p> <p>In Experiment 3 (Chapter 5), the effect of dietary Na (0.15, 0.25 and 0.35 %) on phytase (500 FTU/kg) activity and broiler performance was evaluated and presented. Varying levels of dietary Na, phytase and their interaction did not statistically affect the performance and tibia bone development. High dietary Na (0.35 %) reduced (p < 0.001) excreta dry matter (DM). The ammonia excretion was higher (p < 0.007) in phytase supplemented diets than unsupplemented diets. The negative effect of high Na diet on AME (apparent metabolisable energy), ileal digestibility (Ca and P) and the total tract retention (Ca, P, Na and Mg) of nutrients was countered by phytase supplementation. Supplementation with phytase increased (p < 0.05) the activities of Na-K-ATPase in the jejunum. </p> <p>The activity of phytase in the presence of varying dietary levels of iron (60, 80 and 100 mg/kg) in broiler chickens was investigated in Experiment 4 (Chapter 6). The phytaseinduced improvement in BWG (p < 0.001) and FCR (p < 0.045) at d 35 was significantly reduced by high dietary Fe content (100 mg/kg), indicating significant interaction between Fe and phytase. The combination of high dietary Fe and phytase also reduced (p < 0.001) the ileal digestibility of N, P, Mg and Fe. The high Fe diet reduced the tibia BBS which was counteracted (p = 0.059) by phytase inclusion. High dietary Fe increased (p < 0.001) the deposition of Fe in tibia bone and liver. Phytase improved (Fe × phytase, p < 0.001) the activity of Ca-Mg ATPase, Ca-ATPase and Mg-ATPase in the jejunum when supplemented to diet containing 80 mg Fe/kg. </p> <p>In the final experiment, the response of birds to different levels of dietary Zn (30, 40 and 50 mg/kg) supplemented with phytase were assessed and presented in Chapter 7. The low Zn (30 mg/kg) diet reduced (p < 0.041) FI but only during 1-10d. Irrespective of Zn level, phytase supplementation improved (p < 0.012) the BWG at 1-24d. Bone development of birds was not affected by Zn, phytase or their interaction. Phytase supplemented to low Zn diet improved (p < 0.001) the ileal digestibility of P but reduced (p < 0.05) the Fe and Zn digestibility. The accumulation of Fe (p < 0.001) and Zn (p < 0.002) in liver was increased in birds on high Zn (50 mg/kg) diet. Phytase supplemented to diet containing 40-50 mg Zn/kg improved (p < 0.008) the net energy for production (NEp) and the fat and protein deposition rate in the tissues of broiler chickens. The activities of AP, Ca-ATPase and Mg-ATPase in jejunal mucosa was high (p < 0.001) in birds on the phytase-supplemented mid-Zn diet.</p> <p>In general, it can be concluded that high Ca and Fe had significant negative effect on phytase activity and subsequently on broiler performance. The inhibitory effect of high Ca on phytase activity was more pronounced in low NPP diet. Phytase supplemented to high Zn showed better effect on birds’ performance. The negative effect of high Na only observed on utilization of some minerals and N which was countered by phytase supplementation. Finally, a careful consideration of dietary mineral levels in phytase supplemented diets can be a useful way to sustain the activity of phytase, improve productivity and reduce mineral excretion into the environment.</p>