| A series of experiments (EXP) were conducted to investigate the interactive effects of inorganic P (iP), phytase source and level, citric acid, and wheat middlings (WM) on growth, bone, and carcass response variables. Experiments 1 and 2 were conducted from d 7 to 18 or 0 to 21 (EXP 1 and 2, respectively) in brooders with feeders and nipple drinkers. Chicks obtained from a commercial hatchery were placed on a common chick starter diet for 6 d for EXP 1 or placed immediately on experimental diets for EXP 2. On d 7 or 0, birds were wing-banded, weighed, and placed on experimental diets. Dietary treatments consisted of a corn-soybean meal (C-SBM) basal diet, with two or three levels of iP 9 (0.25 (EXP 2), 0.35, or 0.45% (EXP 1 and 2)), two or three levels of WM (0, 10 (EXP 1 and 2), or 15% (EXP 1)) and three levels of citric acid (0, 3, or 5%). In EXP 1, WM had no effect on weight gain and feed efficiency (P > 0.05), but 15% WM increased feed intake (P < 0.05). The addition of 5% citric acid decreased weight gain and feed intake (P < 0.05). In EXP 2, increasing dietary iP levels increased growth and bone responses (P < 0.05). The addition of 3% citric acid increased bone strength (P < 0.05). Experiments 3, 4, and 5 were conducted to investigate interactions among citric acid, WM, and phytase in P-deficient corn soybean diets and their impact on growth and P utilization. Each EXP consisted of 15 treatments replicated six, five, or four times with five or 20 birds per replicate (in EXP 3, 4, and 5, respectively). The EXP utilized standard-curve methodology, with monobasic potassium phosphate (KH 2PO4) serving as the standard. Diets I to 4 consisted of a P-deficient diet with graded levels of iP from KH2PO4, diets 5 to 15 consist of three levels of phytase (0, 300, or 600 FTU/kg), two levels of citric acid (0 or 3%), or two levels of WM (0 or 10%), alone or in combination. Increasing iP supplementation produced a linear increase in growth and bone response variables (P < 0.05). Phytase increased weight gain, feed efficiency, adjusted bone breaking strength (ABBS), tibia ash, and estimated P release (P < 0.05). The inclusion of WM to broiler diets decreased gain, feed intake, and ABBS (P < 0.05) in EXP 3 but had the tendency to increase weight gain during the finisher and withdrawal phases of EXP 4 and 5. Citric acid increased growth and bone response and estimated P release in EXP 3 (P < 0.05). In EXP 4 and 5 the inclusion of 300 FTU/kg of phytase released at least 0.09% P and 600FTU/kg released at least 0.087% P, whereas citric acid and WM released a maximum of 0.046% P. Experiment 6 was conducted to investigate the impact of phytase source, citric acid, and WM on growth performance, bone strength, carcass yield, and P release in broilers fed marginally-deficient P diets from d 21 to 55. Broilers fed the fungal-derived phytase (FP ) diets had lower carcass yield, whereas broilers fed the E. coli-derived phytase (EP) diets had lower wing yield (P < 0.05). Phytase and citric acid released at least 0.100% P, whereas the inclusion of WM alone released 0.051% P using ABBS data. The combination of phytase and WM increased P release by 67% (P < 0.05), whereas phytase, citric acid, and WM released at least 0.148% P. |
