Growth performance, real-time gizzard pH and calcium solubility in the gut of broiler chickens is dependent on the interaction between dietary calcium concentration and limestone particle size.

1.The purpose of this study was to determine the effect of limestone particle size and dietary Ca concentration on performance, real-time gizzard pH, and Ca and P solubility in the gastrointestinal tract of 21d broiler chickens.2. A total of 576, one-day-old Ross 308 male broilers were randomly allocated among 4 treatments, with 8 replicate pens, and 18 birds per pen. Treatments were arranged as a 2 × 2 factorial consisting of two particle sizes of limestone (coarse, CL, 1200 µm; fine, FL, 44 µm) and two Ca concentrations (9.6 or 6.0 g/kg). On d 19 and 20, four birds per treatment were administered Heidelberg pH capsules and readings monitored for 3 h.3. Reducing Ca concentration from 9.6 to 6.0 g/kg had no effect on d 21 weight gain or FCR of birds fed CL; however, feeding FL at 9.6 g/kg Ca increased weight gain by 10% and reduced FCR by 5% compared to FL at 6.0 g/kg Ca (P < 0.001).4. Average gizzard pH readings ranged from pH 0.67 to 3.01 across all treatments. Birds fed CL at 6.0 g/kg Ca had lower average gizzard pH compared to birds fed 9.6 g/kg Ca, while birds fed FL at 6.0 g/kg Ca had higher gizzard pH than birds fed CL at both Ca concentrations, but was comparable to birds fed FL at 9.6 g/kg Ca (P < 0.001).5. For birds fed CL at 6.0 g/kg Ca, soluble Ca in the gizzard was approximately 40% lower (P < 0.05) than all other treatments, and reduced by 44% (P < 0.05) in the small intestine compared with birds fed FL at 6.0 g/kg Ca.6. These findings demonstrated that the effect of Ca concentration on gizzard pH, Ca solubility and broiler performance was dependent on limestone particle size and suggested that solubility per se had little relevance to performance.