Comparison of relative calcium bioavailability based on bone ash and apparent ileal Ca digestibility in broiler chickens

Two experiments were conducted to determine relative Ca bioavailability based on bone ash and apparent ileal digestibility (AID) of Ca in reagent-grade calcium carbonate (RCaCO₃) and RCaCO₃ + phytic acid in commercial broiler chickens. In Experiment 1, relative Ca bioavailability of RCaCO₃ and RCaCO₃ + phytic acid was determined in ad libitum-fed broiler chickens using the slope-ratio method. Dietary treatments were corn-soybean meal-based diets. Diet 1 was Ca-deficient (0.30 % Ca; 0.45 % non-phytate P), while diets 2 through 5 were the same as diet 1, except that 2 increasing levels of RCaCO₃ or RCaCO₃ + phytic acid were added to increase the dietary Ca level to 0.45 or 0.60 %. In Experiment 2, AID of Ca in RCaCO₃ with and without phytic acid was determined in ad libitum-fed broiler chickens using corn-based diets (0.83 % Ca and 0.45 % non-phytate P). There were 2 dietary treatments that were identical, except diet 1 contained RCaCO₃ while diet 2 contained RCaCO₃ + phytic acid. In Experiments 1 and 2, there were 5 and 8 replicate pens of 5 chickens per treatment, respectively. In Experiment 1, tibia ash (% and mg/tibia) increased linearly (P < 0.05) as dietary Ca content increased. Relative Ca bioavailability obtained from multiple linear regression of tibia ash content (mg/tibia) on supplemental Ca intake was reduced from 100 to 80 % (P < 0.05) when phytic acid was added. In Experiment 2, phytic acid reduced AID of Ca in RCaCO₃ from 38 to 30 % and relative AID of Ca was reduced to 80 %. In conclusion, phytic acid reduced both relative Ca bioavailability based on bone ash and AID of Ca by 20 %. Furthermore, the relative comparison between the 2 test ingredients in both bioassays was similar, indicating that both assays can potentially be used to assess Ca availability and that Ca digestibility values may be able to be predicted from relative bioavailability values as long as the AID of Ca in the standard reference Ca source is known.