Effect of various limestone sources, solubility, and inclusion of phytase on apparent ileal digestibility of calcium and phosphorus in broiler chickens

A study was conducted to determine the effect of 18 limestone sources from 10 countries and inclusion of phytase on apparent ileal digestibility (AID) of Ca and P. The 1,728 day-old male broiler chickens were raised in battery and fed a starter diet from days 1 to 19 post-hatch. Each limestone source was added at 1.5% in 36 experimental diets without and with phytase at 1,000 FYT/kg, which were provided ad libitum to 2 replicate cages of 6 birds from day 19 to 21 in each of 4 repeated experiments to yield 8 replications per treatment. Titanium dioxide (0.5%) was added as a marker for AID calculations. Digesta were collected from the distal one-half of the ileum. Samples of digesta and diets were analyzed for titanium, P, and Ca. Eighteen limestone sources and phytase at 0 and 1,000 FYT/kg in the diet were used in an 18×2 factorial arrangement in 4 randomized complete blocks; in addition, 3 categories of limestone solubility of slow, medium, and fast, and two levels of phytase at 0 and 1,000 FYT/kg in the diet were used in a 3 × 2 factorial arrangement in 4 complete randomized blocks. Experiment repetition was set as a block for each analysis and significance was determined at P < 0.05. There was an interaction between limestone source and phytase for AID of Ca (P < 0.006) and P (P < 0.001), with phytase increasing AID of Ca and P more in some limestones than others. Likewise, there was a significant interaction between limestone solubility and phytase for AID of Ca (P < 0.008) and P (P < 0.009). The medium and fast solubility limestones showed higher AID of Ca, but lower AID of P compared with slow, whereas the improvements in AID of Ca and P by phytase were greater for medium than for slow and fast solubility limestones. These results indicate that AID of Ca, P, and the magnitude of the increase associated with phytase varied among limestone sources and solubility. This emphasizes that the per se Ca and P value of diets and, the effect of phytase, may need to be adjusted according to specific limestone characteristics.