True ileal digestibility of calcium in limestone and dicalcium phosphate are additive in diets of broiler chickens.

The experiment was conducted to test if the true ileal digestibility (TID) of Ca in limestone and dicalcium phosphate (DCP) are additive in mixed diets for Ross 708 broiler chickens from day 22 to 27 post-hatching. The birds were fed a standard broiler starter diet from day 1 to 22 post-hatching. A total of 504 birds were grouped by BW into 8 blocks of 9 cages with 7 birds per cage and assigned to 9 dietary treatments in a 3 × 3 factorial arrangement of 3 Ca sources including limestone, DCP, and their mixture at a ratio of 1:1, and 3 dietary Ca concentrations at 3.3, 4.3, and 5.3 g/kg. Potassium phosphate was added to adjust the dietary P concentration and maintain a 1.1:1 total Ca:non-phytate P ratio. The results showed that the Ca intake, ileal Ca digested, and Ca retained linearly increased as the Ca concentration increased (P < 0.001). No Ca concentration effect was observed on apparent ileal digestibility of Ca (P = 0.13). By regressing digested Ca per bird against Ca intake per bird, the TID of Ca was determined at 63.7, 67.1, and 67.8% for limestone, DCP, and their mixture, respectively. The expected TID for Ca in limestone and DCP mixture was predicted to be 65.1% based upon the Ca contribution coefficient calculated to be 0.59 for limestone and 0.41 for DCP. The predicted TID of Ca (65.1%) in the mixture was not different (P = 0.63) from the determined TID of Ca (67.8%). In conclusion, the TID of Ca in limestone and DCP are additive in semi-purified diets for chickens.