Dietary magnesium requirement of soft-shelled turtles, Pelodiscus sinensis, fed diets containing exogenous phytate

•Mg requirement is 715 mg/kg for soft-shelled turtle with 1.1% added phytic acid.•Mg requirement increases about 10% from turtles fed a phytate-free diet.•Dietary Mg for optimal ALP activity is similar to turtles fed a phytate-free diet.•Dietary intake of 970 mg Mg/kg enhances carapace strength.

A feeding study was conducted to evaluate the effects of an exogenous phytic acid diet on the dietary magnesium (Mg) requirement for soft-shelled turtles. Juvenile soft-shelled turtles, Pelodiscus sinensis, were fed diets that contained 36, 160, 357, 630, 947, 1249, or 1560 mg/kg Mg and 11 g/kg of phytic acid for 8 weeks. Weight gain of turtles generally increased as dietary Mg increased up to approximately 630 mg/kg followed by a plateau stage till 1560 mg/kg. The plasma alkaline phosphatase (ALP) activity, carapace strength and concentration of Mg in the tissue generally increased as dietary Mg concentrations increased from 36 to 947 mg/kg. However, increases in Mg beyond 947 mg/kg caused the Mg concentrations in the tissue and carapace strength to remain constant. When 11 g/kg of phytic acid was added to the diet and broken-line analysis used weight gain as the indicator, the dietary Mg requirements for soft-shelled turtles was estimated at 715 mg/kg. This is a 10% increase from the requirement reported for turtles fed a phytate-free diet. A dietary intake of 970 mg Mg/kg is recommended to further enhance carapace strength and plasma ALP activity in soft-shelled turtles.