Effect of two commercial limestone sources with different solubility on the efficacy of two phytases in 0-21 d old broilers

A study evaluated the effects of limestone sources with different solubility on the efficacy of two phytases in broilers. A 2×5 factorial arrangement was employed with two commercial limestone sources and five diet treatments; a positive control (PC) diet with 7.2 g/kg P and 9.6 g/kg Ca, and a negative control diet containing reductions of 1.87 g/kg available P, 1.99 g/kg Ca and 0.4 g/kg Na, supplemented with either Buttiauxella phytase (PhyB) or Escherichia coli phytase (PhyE) at 500 or 1000 FTU/kg diet. The two limestone sources were feed-grade commercial products with different particle sizes and solubility (fast-soluble (FS) 100%; slow-soluble (SS) 26% soluble after 30 minutes at pH 3) containing similar levels of Ca. Diets were fed to one-day-old Ross 308 males (n=2,400) with 30 birds/pen and eight pens/treatment in two phases (starter 0-10 d and grower 10-21 d). On d 21, ileal digesta was collected from 12 birds/pen to determine apparent ileal digestibility (AID) of P and Ca, and myo-inositol hexakisphosphate (IP6) disappearance, and tibias from four birds/pen for ash determination. The SS limestone improved body weight gain (BWG), feed intake (FI) and FCR vs FS limestone in starter/grower phases (P<0.05), and improved AID of P (P<0.05) and IP6 disappearance (P<0.05) at d 21. There was an interaction between limestone and phytase on BWG, FI and FCR in the grower phase (P≤0.05) whereby FS (vs SS) limestone reduced BWG at either dose of PhyE, but only at 500 FTU/kg of PhyB. At an equivalent dose, PhyB had higher BWG and feed intake than PhyE (P<0.05). At 1000 FTU/kg, performance was equivalent (BWG and FI) or superior (FCR) to the PC, PhyB produced greater tibia ash, AID of P and IP6 disappearance (P<0.05). The findings showed that the effects of limestone particle size on phytase efficacy varied with phytase source and dose.