Measurement of true ileal phosphorus digestibility in maize and soybean meal for broiler chickens: Comparison of two methodologies

An experiment was conducted to determine the true ileal P digestibility of maize and soybean meal for broiler chickens using two methodologies based on the regression method. The two methods differed in dietary protein content and dietary Ca:P ratios in the assay diets used. In Method 1, the test ingredient was used as the sole dietary source of protein and P and a dietary Ca:non-phytate P ratio of around 2:1 was maintained by the addition of limestone. In Method 2, dried egg albumen was used as the additional protein supplement and a dietary Ca:total P ratio of 1.3:1 was maintained by the addition of limestone. For the maize-based diets, disodium phosphate was used as an additional P supplement. Two sets of sequential semi-purified diets containing 200, 460 and 720g/kg of maize or 400, 510 and 620g/kg of soybean meal were formulated. Each set was evaluated as a 2×3 factorial arrangement of treatments, which included two methods and three inclusion levels. The experiment was conducted as a randomised complete block design and a total of 288 of 21-day old male broilers (Ross 308) were assigned to four blocks based on body weights. Each block had 12 cages (six birds per cage) and the 12 test diets were assigned to a cage within each block. Apparent ileal digestibility coefficient of P was determined by the indicator method and linear regression was used to determine true ileal P digestibility. In both methods, ileal P output and ileal digestible P increased linearly with increasing inclusion levels of maize (P<0.01) and soybean meal (P<0.001). Apparent ileal P digestibility coefficients of P determined for maize were higher (P<0.001) in Method 2 compared with Method 1 whereas the opposite trend was observed for soybean meal. True ileal P digestibility coefficients determined for maize in Methods 1 and 2 were 0.728 and 0.426, respectively, and were different (P<0.01). True ileal P digestibility coefficients determined for soybean meal in Methods 1 and 2 were 0.740 and 0.523, respectively, and were different (P<0.01). The present findings showed that estimation of true ileal P digestibility in maize and soybean meal was influenced by the assay methodology employed.