Dietary calcium requirement and effects on growth and tissue calcium content of juvenile grass carp (Ctenopharyngodon idella)

A growth trial was conducted to estimate the optimum concentration of dietary magnesium (Mg) for grass carp (Ctenopharyngodon idella). Triplicate groups of grass carp (5.56 ± 0.02 g) were fed diets containing graded levels (187, 331, 473, 637, 779 and 937 mg kg−1) of Mg for 8 weeks. Weight gain, specific growth rate and feed efficiency were linearly increased up to 637 mg kg−1 dietary Mg and then levelled off beyond this level. For body composition, dietary Mg levels higher than 473 mg kg−1 significantly decreased the moisture content but increased the lipid content of whole body, muscle and liver. Dietary Mg levels higher than 473 mg kg−1 significantly decreased the ash contents of vertebrae, scales and muscle. Mg contents in whole body, vertebrae, scales and plasma were increased up to 637 mg kg−1 dietary Mg and then levelled off beyond this level. However, Ca and P contents seem to be inversely related to dietary Mg. Dietary Mg levels higher than 473 mg kg−1 significantly decreased Zn and Fe contents in whole body and vertebrae. Broken-line analysis indicated that 687 mg kg−1 dietary Mg was required for maximal tissue Mg storage, as well as satisfied for the optimal growth.