Towards a digestible calcium system for broiler chicken nutrition: A review and recommendations for the future

There is growing awareness that variability in the nutritional quality of Ca sources has significant effects on gastrointestinal pH, P and amino acid digestibility, exogenous enzyme efficacy and the microbial population in the gastrointestinal tract of broilers. In this regard, the interest in moving toward a digestible Ca system is gaining traction and the apparent ileal digestibility (AID) or standardized ileal digestibility (SID) coefficients for Ca in ingredients, such as limestone (n = 55), dicalcium phosphate (DCP; n = 21), meat and bone meal (MBM; n = 20), monocalcium phosphate (MCP; n = 12), and plant-based ingredients are available. The aim of this review was to compile the literature evaluating Ca digestibility of feed ingredients for broilers, highlight factors that may influence the digestibility coefficients, and suggest recommendations to move toward a digestible Ca system. There were 11 papers published between 1974 and 2019 focused on the evaluation of Ca digestibility of feed ingredients for broilers. The average ileal Ca digestibility coefficient for limestone was 0.53, DCP was 0.42, MCP was 0.35 and MBM was 0.46. There were no differences in the AID or SID Ca digestibility coefficients within ingredient. The ileal Ca digestibility coefficient for limestone increased (quadratic, P < 0.05) as particle size increased. The ileal Ca digestibility coefficients for DCP were lower (P < 0.05) in corn- compared with semi-purified-based basal diets; whereas this was the opposite for MCP. Increasing the adaptation time ≥72 h increased (quadratic, P < 0.05) the ileal digestibility coefficients determined for DCP, but there was no effect for MCP. Widening the analyzed Ca:non-phytate phosphorus (NPP) ratio increased the ileal Ca digestibility coefficients for MCP (quadratic, P < 0.05) and MBM (linear, P < 0.05) and decreased the ileal digestibility coefficients for DCP (quadratic, P < 0.05). There has been considerable progress in the reported Ca digestibility coefficients for ingredients. However, limitations for reproducibility of Ca digestibility coefficients still exist. Future evaluations of ileal Ca digestibility coefficients using corn-based diets, standardizing the adaptation period to 72 h for diets with a Ca:NPP ratio> 1:1 or to 16 h for diets with a Ca:NPP ratio < 1:1 may minimize the effect of the animal adaptation response on digestibility coefficients. Other considerations include using phytase at saturation levels, measuring and reporting Ca and mineral concentrations, origin, and particle size of ingredients to allow characterization and future predictions of Ca digestibility and repeatability of results.