Effects of limestone particle size, phytate, calcium source, and phytase on standardized ileal calcium and phosphorus digestibility in broilers

Work was carried out to determine the effects of limestone (LM) geometric mean diameter (GMD), phytate, Ca source, and phytase on standardized ileal digestibility (SID) of Ca and P in broilers. Twelve treatments (TRT) were tested. One basal corn and corn germ-based diet was prepared without adding inorganic Ca or P (TRT1, 0.02% Ca). Limestone from the same source (800 or 151 mm GMD) and bone (151 mm GMD) were added to TRT 2-4, (0.7% Ca). TRT5 was a corn/ soybean meal (SBM) diet with 800 mm GMD LM (0.77% Ca). Buttiauxella sp. phytase was added (1,000 U/kg) to TRT 1-5 to prepare TRT 6-10, respectively. In addition, 800 or 150 mm GMD LM and monosodium P were added to a nitrogen-Ca-phytate-free diet (TRT 11 and 12, respectively). Treatments were analyzed as a complete block design using SAS mixed procedures and with factorial analysis on subsets of data: 1) LM GMD (151 vs. 800 mm GMD) with or without phytase: TRT 2, 3, 7, and 8; 2) phytate (corn or corn-free) with the same LM at 2 different sizes: TRT 2, 3, 11, and 12; 3) Ca source (LM vs. bone) with or without phytase: TRT 2, 4, 6, 8, and 9; 4) phytate source (corn vs. corn/SBM) with or without phytase: TRT 2, 5, 7, and 10. Broilers (4/pen) were fed mash diets ad lib for 36 h (20-22 d of age). At the end of the trial, distal ileal digesta were collected. Fine LM had lower SID Ca (38.09%) vs. coarse LM (49.18%), irrespective of phytase (P < 0.05). Standardized ileal digestibility of P was lower when the smaller LM was used vs. coarse LM with either 0 or 1,000 phytase U/kg (P < 0.05). Both SID Ca and P were higher without phytate vs. when phytate from corn was present (P < 0.05). Ca from bone was more digestible and its impact on SID P smaller vs. LM regardless of phytase (P < 0.05). In addition, phytate from SBM was more digestible and responsive to phytase as compared to phytate from corn (P < 0.05). In conclusion, dietary factors affected Ca and P digestibility and their response to phytase inclusion.