<b>Optimizing Calcium and Phosphorus Utilization in Broiler Chickens: </b><b>The Role of Phytate and Supplemental Phytase</b>

<p dir="ltr">The objective of this dissertation was to investigate the role of phytate and supplemental phytase in optimizing calcium <b>(Ca)</b> and phosphorus <b>(P)</b> utilization for broiler chickens. Therefore, 5 studies were carried out to evaluate this objective. All studies employed the randomized complete block design, and all trials were conducted in battery cages.</p><p dir="ltr">Study 1 investigated the digestible Ca (<b>dCa</b>) equivalency value of phytase in a corn-soybean meal basal diet fed to 2 ages of broiler chickens. A total of 1,152 male Cobb 500 broiler chickens were used in an experiment in which 8 diets consisting of graded levels of Ca supplied with limestone or graded levels of phytase were fed. The 8 dietary treatments were based on a corn-soybean meal diet containing 5.1 g/kg of Ca and 5.1 g/kg of P as negative control <b>(NC)</b>; the NC + 1.3, 2.6, or 3.9 g/kg of Ca from limestone; and the NC + 500, 1000, 2000, or 4000 FYT/kg of phytase. The dCa equivalence for dietary phytase at 1,520 to 7,661 FYT/kg ranged from 1.55 to 2.02 g/kg in the 10-d-old birds fed for 3 d and from 0.52 to 0.64 g/kg in 21-d-old birds fed for 14 d. The results showed the reduction in Ca level that could be accommodated by phytase supplementation, which is markedly different between the younger and older birds.</p><p dir="ltr">Study 2 investigated Ca utilization changes in the absence or presence of phytate-P <b>(PP)</b> and without or with added phytase in broiler chicken diets. A total of 320 male broiler chickens were offered experimental diets based on potato protein concentrate from d 18 to 21 post hatching. Diets were nutrient-adequate positive control (4.8 g non-PP/kg; 0 g PP/kg) and 2 nutrient-reduced (2.8 g non-PP/kg) containing either 0 or 4.4 g PP/kg solely from rice bran and added phytase at 0 or 4,000 FYT/kg in a 1 + 2 × 2 factorial. Phytate-P reduced apparent ileal digestibility <b>(AID)</b> and apparent total tract retention <b>(ATTR)</b> of Ca and P. Phytase supplementation improved the AID and ATTR of P in 4.4 g PP/kg but not in 0 g PP/kg diet due to a lack of the substrate PP, which resulted in a PP by phytase interaction. Furthermore, added phytase increased ATTR of Ca by a greater magnitude (25 percentage points) in the 4.4 g PP/kg diet than (8.5 percentage points) in the 0 g PP/kg diet, resulting in a significant interaction.</p><p dir="ltr">Study 3 was designed to determine the additivity of ileal digestibility and total tract retention of Ca for broiler chickens when fed a mixed diet formulated with limestone, meat and bone meal <b>(MBM)</b>, and canola meal <b>(CM)</b> without or with exogenous phytase. A total of 444 male broiler chickens were used, and dietary treatments were arranged as 4 × 2 factorial with 4 diets based on limestone or MBM or CM as the sole contributor of Ca, or a combination of limestone, MBM, and CM <b>(Mixed)</b>, and phytase supplementation at 0 or 1,000 FYT/kg. Phytase only improved standardized ileal digestibility (<b>SID</b>) and standardized total tract retention (<b>STTR</b>) of Ca in the single diet formulated with CM but not in the other diet types, leading to an interaction effect. The predicted SID and STTR of Ca were not different from the determined values in the mixed diet, regardless of phytase supplementation.</p><p dir="ltr">The fourth study examined the interactive effects of graded total Ca-to-total P ratios <b>(Ca:P)</b> and phytase on the SID of P and bone characteristics in diets formulated with soybean meal as the sole source of P. Male broiler chickens were fed 6 semi-purified diets prepared in a 2 × 3 factorial treatment arrangement with 2 levels of phytase (0 or 1,000 FYT/kg of diet) and 3 levels of Ca:P (1.1, 2.0, or 2.8) for 72 hours from d 18 to 21 post hatching. Increasing the Ca:P linearly decreased the SID of P from 54 to 40%, whereas the supplementation of phytase improved the SID of P from 26 to 67%. The supplementation of phytase increased the bone-breaking strength, whereas tibia ash was decreased.</p><p dir="ltr">The last study was designed to determine the additivity of PP<b> </b>contribution to total P<b> </b>utilization in broiler chickens when fed 2 different combinations of mixed diets formulated with corn, soybean meal <b>(SBM)</b>, canola meal <b>(CM)</b>, or rice bran <b>(RB)</b> without or with exogenous phytase. A total of 960 male broiler chickens were used in the trial from d 18 to 21 post hatching. Dietary treatments were arranged as 6 × 2 + 3 factorial with 6 diets based on corn or SBM or CM or RB as the sole source of PP, or a combination of corn, SBM, and RB <b>(CSR)</b>, or corn, CM, and RB <b>(CCR)</b>, respectively, all with exogenous phytase at 0 or 750 FTU/kg. The 3 remaining diets were formulated with graded concentrations of monosodium phosphate as the only P source at 0, 9 or 18 g/kg to standardize digestibility measurement and correct for P contribution from non-phytate-P. The inclusion of phytase improved the AID and SID of P in all diet types except in the corn diet. Phytase supplementation resulted in the variable disappearance of myo-inositol 1,2,3,4,5,6-hexakis dihydrogen phosphate <b>(InsP</b>_{<strong>6</strong>}<b>)</b> across all diet types, leading to an interaction effect. Regardless of supplemental phytase, the CCR diet resulted in lower SID of total P, PP, and InsP₆ disappearance than the CSR diet. The SID of PP was additive in the CSR and CCR diets when exogenous phytase was absent. Despite improving the SID of PP, phytase supplementation did not exhibit an additive effect in the CCR diet and showed a tendency towards non-additivity in the CSR diet.</p>