Bone quality, selected blood variables and mineral retention in laying hens fed with different dietary concentrations and sources of calcium

The objective of this study was to determine the influence of limestone particle size in the diets with different contents of Ca on the biomechanical and geometrical measurements of tibia and femur bones, digestibility of nutrients, and selected biochemical blood variables. The experiment was conducted with 108 laying hens, allocated to 9 treatments of 6 replicates (cages), with 2 layers in each cage. A 3×3 factorial arrangement, with 3 dietary concentrations of Ca (3.20%, 3.70%, and 4.20%) and 3 levels of dietary substitutions (0, 25, and 50%) of fine particles of limestone (FPL; diameter, 0.2–0.6mm) with large particles of limestone (LPL; diameter, 1.0 to 1.4mm), was used. The hens were fed with experimental diets from 25 to 70wk of age. At wk 45 a balance experiment was conducted, and after termination of the experiment, i.e., at wk 70, tibia and femur bones, and blood samples, were collected for analysis. Neither dietary Ca concentration nor limestone particle size had an effect on dry matter, organic matter, ether extract, N-free extracts, crude fiber, and crude ash digestibility, and P retention and excretion; however, Ca excretion increased linearly and Ca relative retention decreased linearly with increasing Ca dietary concentration (P<0.05). No effect of limestone particle size on tibia and femur biomechanical and geometrical measurements, tibia and femur mineralization, serum alkaline phosphatase activity, and serum Ca and P concentration, was observed. Increased dietary Ca concentration enhanced linearly tibia and femur bone breaking strength, yielding load, stiffness, and Ca concentration (P<0.05). Serum alkaline phosphatase activity decreased linearly with increasing Ca dietary concentration (P<0.05). In conclusion, the results of this study demonstrated that a content of 3.20–3.70% Ca in a layer's diet is not sufficient through the entire laying cycle to maintain optimal bone quality; however, partial substitution of fine- with large-particle limestone does not improve Ca and P retention and bones quality variables.