The impact of limestone particle size on the digestibility of dietary calcium and phosphorus and phytase efficacy in broilers

The major feed ingredients used in broiler feed formulation are deficient in calcium (Ca), therefore Ca is supplemented, predominantly in the form of limestone, to meet the bird’s requirements. Previous research has highlighted several constraints with limestone supplementation relating to intrinsic and extrinsic characteristics that potentially alter the digestibility of Ca from limestone and indirectly the digestibility of phosphorus (P) from the diet. The particle size of limestone is one characteristic that has not been evaluated extensively, and its effects on Ca and P digestibility are inconclusive. The supplementation of phytase enzymes is a common practice in the broiler industry. While several phytase sources are commercially available, their efficacy may differ significantly. Therefore, the objective of the study was to assess the effects of three limestone samples (LS) sourced from the same mine, each with different particle sizes on Ca and P digestibility and their interaction with two phytase sources. The study comprised two experiments. Experiment one consisted of a 3x2 factorial design with three LS (LS1-125 μm, LS2-174 μm, LS3-733 μm) supplemented in treatment (TRT) diets, with or without the inclusion of phytase (0 or 1 000 FTU/kg feed of phytase A (PhyA) – a Buttiauxella spp. phytase). The second experiment consisted of a similar 3x2 factorial design, with each LS supplemented with either PhyA or phytase B (PhyB – an Escherichia coli phytase) at 1 000 FTU/kg feed. On day (d) 18, 528 birds closest to the average flock weight were moved to a metabolism house (six birds/cage) and randomly assigned to dietary TRTs (eight replicates/TRT). On d 21, ileal digesta was collected and the samples were analysed for Ca, P, and chromium (Cr). Data were analysed in RStudio version 13, using a full factorial for each experiment and means were separated using protected least-square means separation (P < 0.05). The inclusion of LS3 in TRT diets, irrespective of phytase, improved (P < 0.05) the apparent ileal digestibility of Ca (AID) from limestone (AIDLimestone Ca) (LS3: 50.17%; LS2: 42.21%; LS1: 38.05%) and P (AID P) (LS3: 58.26%, LS2: 42.69%, LS1: 37.93%) compared to LS2 and LS1. The supplementation of PhyA at 1 000 FTU/kg feed increased (P < 0.05) AID Ca and AID P compared to unsupplemented diets. Differences in phytase source supplementation were evident (P < 0.05) with the supplementation of PhyA resulting in a 14.18% improvement in AIDLimestone Ca and a 15.92% improvement in AID P compared to the inclusion of PhyB. The results from the study emphasise the significant impact of limestone particle size on Ca and P digestibility, favouring larger particle sizes in broiler diets. Furthermore, the study confirmed the importance of phytase supplementation, while underscoring the varying efficacy between phytase sources.