Independent and interactive effects of limestone solubility, dietary calcium content and phytase dose on growth...

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.