Effects of dietary magnesium on the growth, carapace strength and tissue magnesium concentrations of soft‐shelled turtle, P elodiscus sinensis ( W iegmann)

A feeding trial was conducted to evaluate the effects of dietary magnesium on the growth, carapace strength, tissue and serum Mg concentration of soft‐shelled turtles, P elodiscus sinensis ( W iegmann). Juvenile soft‐shelled turtles of approximate 5.4 g body weight were fed diets with seven levels of Mg (48, 206, 369, 670, 955, 1195 and 1500 mg Mg kg −1 ) for eight weeks. No significant difference ( P  ≥ 0.05) was found in weight gain ( WG ), feed conversion ratio or protein efficiency ratio among treatments. However, the WG of turtles continued to increase with increasing dietary Mg levels up to 670 mg kg −1 , beyond which the WG levelled off. The plasma alkaline phosphatase activity and the muscle, bone Mg concentrations of the turtles increased with the increasing dietary Mg levels between 48 and 955 mg kg −1 , beyond which the tissue Mg concentrations remained relatively constant. Furthermore, the carapace strengths of turtles fed with the control diet of 48 mg Mg kg −1 were significantly weaker ( P  < 0.05) than that of turtles fed with diets containing higher Mg levels. Based on a broken‐line modelling analysis, the required dietary Mg level for the optimal WG of juvenile soft‐shelled turtles was estimated to be approximately 650 mg kg −1 . By contrast, the required dietary Mg levels for turtles to reach the optimal muscle and bone Mg concentrations were 1050 and 1000 mg kg −1 respectively. The required dietary Mg level for maximal alkaline phosphatase activity was approximately 980 mg kg −1 .