True Ileal Calcium Digestibility in Soybean Meal and Canola Meal, without and with Microbial Phytase, for Broiler Starter and Finisher

Soybean meal (SBM) and canola meal (CM) are two plant-based protein sources which contain reasonable amounts (2.9 and 6.8 g/kg, respectively) of calcium (Ca). Currently, no published values are available for the Ca digestibility in SBM and limited studies (Anwar et al., 2018; Moss et al., 2018) are available for the Ca digestibility in CM. Microbial phytases are now routinely added in poultry diets to improve the bioavailability of phosphorus (P) bound to phytate (myo-inositol hexaphosphate) and P digestibility. The effect of phytase addition on Ca digestibility, however, is contradictory (Ravindran et al., 2008; Walk et al., 2012). Higher phytase doses, typically over four times the recommended dose and referred to as superdoses, are currently being used by the industry and reported to improve the growth performance and nutrient digestibility (Cowieson et al., 2011). Therefore, two experiments were conducted, with the primary objective of determining the true ileal digestibility of Ca in SBM and CM without and with microbial phytase, during broiler starter (Experiment 1) and finisher (Experiment 2) periods. A secondary objective was to investigate the influence of microbial phytase dose on the true ileal digestibility of Ca in SBM and CM.

Dietary treatments consisted of a 2 × 3 factorial arrangement with ingredient (SBM and CM) and phytase dose (0, 500 and 2000 FTU/kg; Quantum Blue, AB Vista, Marlborough, U.K.) as the main factors. Six experimental diets based on maize-SBM and maize-CM, with three phytase doses, were fed to 6 replicate cages (8 birds per cage) of male broilers (Ross 308) from day 18 to 21 (Experiment 1) or 39 to 42 (Experiment 2) post-hatch. The contribution of Ca from maize was negligible, enabling the use of maize-based diets. A Ca- and P- free diet, with no added phytase, was also developed to determine the endogenous Ca losses. Apparent ileal digestibility was calculated using titanium ratios in the diet and digesta and the true ileal digestibility was calculated by correcting for endogenous Ca losses. Total tract Ca retention was also measured.

Ileal endogenous Ca losses were determined to be 236 and 29 mg/kg of DM intake, respectively, in broiler starter and finisher. True ileal Ca digestibility coefficients of SBM and CM, without added phytase, were determined to be 0.51 and 0.53, respectively, in starter and 0.33 and 0.22, respectively, in finisher. Increasing phytase dose increased (P < 0.05) the true ileal Ca digestibility of CM in both starter and finisher, but true ileal Ca digestibility of SBM increased (P < 0.05) only at the superdose (2000 FTU/kg) in finisher. Total tract Ca retention (P < 0.001) in starter was higher in CM than in SBM and was increased with phytase dose in both ingredients. In broiler finisher, the Ca retention was increased (P < 0.001) by both phytase doses in CM, but only by the superdose (2000 FTU/kg) in SBM, resulting in an ingredient × phytase interaction (P < 0.001).