LAUSR

This study evaluated the effect of limestone solubility on the capacity of a novel consensus bacterial 6-phytase variant (PhyG) to improve phosphorus (P) and calcium (Ca) digestibility, retention and utilization in low-Ca broiler diets containing no added inorganic phosphate (Pi). Male Ross 308 broilers (n = 1,152) were fed one of 16 experimental diets from 11 to 21 d of age in a randomized complete design (12 birds/cage, 6 cages/treatment). Diets comprised three positive controls (PC3, PC2 and PC1) containing 1.8, 1.2 or 0.6 g/kg MCP-P and 7.7, 7.0 or 6.2 g/kg Ca, respectively, and a negative control (NC) containing no added Pi (4.4 g/kg P; 2.8 g/kg phytate-P) and 5.5 g/kg Ca from either low or high solubility limestone (LSL or HSL respectively [with 42 and 97% solubility after 5 min at pH3.0]), supplemented with 0, 250, 500, 1,000 or 2,000 FTU/kg of PhyG. Fecal samples collected on d 18 to 20 and ileal digesta collected on d 21 were analyzed for titanium dioxide, Ca, P, and phytate (IP6, inositol hexakisphosphate). Tibias (d 21) were analyzed for ash content. Data were analyzed by factorial analysis (2 limestone solubilities × 4 MCP-P levels and 2 limestone solubilities × 5 phytase dose levels) and exponential regression. Increasing dose levels of PhyG resulted in an exponential increase (P < 0.01) in the apparent ileal digestibility (AID) of P, ileal digestible P content of the diet, ileal IP6 content and IP6 disappearance in birds fed either HSL or LSL diets, but AID Ca and ileal digestible Ca were exponentially increased by the phytase only in HSL diets (P < 0.01). Relative to HSL, the LSL increased AID P, ileal digestible P and IP6 disappearance (P < 0.05) but reduced AID Ca, ileal digestible Ca and retainable Ca (P < 0.05), resulting in reduced retainable P and tibia ash. Phytase exponentially increased the apparent total tract digestibility of P, retainable P and tibia ash in HSL and LSL diets, but at or above 500 FTU/kg values were higher in HSL than LSL (interaction P < 0.05). The findings highlight that phytase dose-response effects on mineral digestibility and utilization are different for high- and low-solubility limestones and it is therefore recommended to use digestible rather than total Ca content during diet formulation to ensure an optimal balance of Ca and P, especially in low-Ca diets. In diets containing HSL, higher phytase dose levels may be needed to compensate for the low digestible P content of the basal diet.