By Christine Tilley, BVSc, MRCVS
There has been some concern among members about the nutritional value of the food supplements they are using. It is particularly important that the balance of calcium to phosphorous is right, because the uptake of calcium depends on this. Nutrobal (Vetark) was formulated with shelled reptiles particularly in mind and contains ample vitamins and minerals for all tortoises if fed at a daily rate of 0.1g per kg bodyweight (the tip of a teaspoon). The all-important calcium:phosphorus ratio is 46:1.
Many people are accustomed to using Vionate (Sherley’s) which is considerably cheaper, although it does have a higher dosage rate (1/4 teaspoon per kg). The ratio of calcium:phosphorus in Vionate is only 1.4:1, which is sufficient for adult males and non-breeding females, but inadequate for hatchlings, juveniles and egg-laying females, which have an enormous requirement for calcium in the growing shell, bones or eggshells. Therefore if you use Vionate, you must add extra calcium – for example, an equal amount of limestone flour can be mixed in. Other methods of adding calcium are by grating cuttlefish bone onto food; sprinkling on oyster shell grit, available from bird seed suppliers; crushing eggshells onto the food (less effective); or by adding crushed calcium lactate or calcium gluconate tablets. It is even better to use calcium tablets with vitamin D3, which is essential for calcium metabolism. You are then approaching the D3 content of Nutrobal, which is seven times that of Vionate.
Basically, you cannot just sprinkle huge amounts of Vionate on the food to try and match the vitamin and mineral content of Nutrobal; you must adjust the proportion of calcium to keep your captive chelonians healthy.
Further reading: Nutrition and Nutritional Disease in Chelonians by Michelle Barrows, BSc, BVMS, MRCVS. Testudo Vol.5 No.3 (2001).