Effect of calcium source and particle size on the true ileal digestibility and total tract retention of calcium in broiler chickens

The influence of calcium (Ca) source and particle size on true ileal digestibility coefficient (TIDC) of Ca and apparent total tract retention coefficient (ATTRC) of Ca in broilers was assessed. Four experimental diets containing limestone and oyster shell, each as fine (<0.5 mm) and coarse (1-2 mm) particles, were fed to broiler chickens from 21 to 24 days of age. A Ca- and phosphorus-free diet was used to determine the ileal endogenous Ca losses. Limestone and oyster shell were obtained from commercial sources and ground to pass through a set of sieves to obtain the fine and coarse particles. Titanium dioxide was incorporated in all diets as an indigestible marker. Each experimental diet was randomly allotted to six replicate cages (eight birds per cage). Apparent ileal digestibility coefficient and ATTRC of Ca were calculated using the indicator method and the TIDC values were calculated by correcting for endogenous Ca losses. Ileal endogenous Ca losses were determined to be 115 mg/kg of dry matter intake. The main effect of Ca source and the interaction between Ca source and particle size were not significant (P > 0.05) for the TIDC and ATTRC of Ca. Both TIDC and ATTRC of Ca were influenced (P < 0.05) by particle size, with coarser particles increasing the digestibility and retention values. Increased particle size increased the TIDC of Ca in limestone from 0.38 to 0.62 and that of oyster shell from 0.33 to 0.56. The corresponding increases in ATTRC of Ca were from 0.44 to 0.66 in limestone and from 0.40 to 0.60 in oyster shell. Calcium concentration in the gizzard contents of birds was higher (P < 0.05) for limestone as compared to oyster shell and increased with increase in particle size. Calcium concentration in the gizzard contents was strongly correlated with the TIDC (r = 0.81; P < 0.01) and ATTRC (r = 0.82; P < 0.01) of Ca. A strong correlation (r = 0.80, P < 0.01) was also observed between TIDC and ATTRC of Ca.