Independent and interactive effects of limestone solubility, dietary calcium content and phytase dose on growth performance, mineral digestibility and utilization in broilers

Effects of limestone solubility, dietary Ca concentration and dose of a 6-phytase (PhyG) on broiler growth performance, mineral digestibility and utilization were investigated. A randomized complete block design (2 × 3 × 2 arrangement; 12 treatments) incorporated 2 limestones [high solubility (HSL), and low solubility limestone (LSL), 94 and 58 % soluble, respectively, after 5 min at pH 3; geometric mean diameter 57 and 400 μm, respectively], 3 Ca levels: Low, Medium, High (0.65, 0.80, 0.95 % during 0 to 10 d of age; 0.55, 0.70, 0.85 % during 11 to 20 d of age; 0.45, 0.60, 0.75 % during 21 to 35 d of age), and two phytase doses [500 and 1,500 phytase units (FTU)/kg]. Diets were formulated with retainable P using a phytase matrix and fed as pelleted corn-, wheat- and soybean meal-based diets ad libitum to Ross 308 males (2,160 birds, 6 pens/treatment, 30 birds/pen). Digesta was collected on d 20 d to determine ileal digestibility (AID) of minerals. Tibias were collected on d 10, 20 and 35 for bone ash determination. There were 2-way interactions between limestone solubility and Ca level, limestone solubility and phytase dose and Ca level and phytase dose on final BW, overall BW gain and feed intake; all were reduced (P < 0.05) by increasing dietary Ca, with HSL vs. LSL, and increased with high vs. low phytase. Tibia ash was increased by high (vs. low) phytase dose across diets. The AID of P at d 20 was increased by LSL vs. HSL, reduced by increasing dietary Ca, and increased by high (vs. low) phytase dose; AID of Ca was reduced by LSL vs. HSL. The highest d 35 BW (2,571 g) was achieved with LSL, Medium Ca and 1,500 FTU/kg PhyG. Achieving optimal growth performance depended upon the combined effects of all 3 factors; in diets containing HSL and High Ca, increasing the phytase dose may maintain performance and reduce the negative effects of excess Ca.