Dietary magnesium requirement for fingerlings of Rohu (Labeo rohita)

The present study was performed to quantify the dietary magnesium requirement of fingerling Labeo rohita (5.9 ± 0.14 cm; 1.73 ± 0.06 g) by feeding six casein-gelatin based (350 g kg−1 crude protein; 16.72 kJ g−1 gross energy) purified experimental diets with incremental levels of magnesium (0.06, 0.27, 0.46, 0.66, 0.87 and 1.07 g kg−1 diet) to triplicate groups of fish at 08:00, 12:00 and 16:00 h. Live weight gain (LWG), specific growth rate (SGR), feed conversion ratio, protein efficiency ratio, protein gain (PG) and whole body crude protein improved with the increase in dietary magnesium levels up to 0.46 g kg−1 and then stabilized (P > .05). Hepatosomatic index, viscerosomatic index and condition factor improved with increasing levels of dietary magnesium from 0.06 g to 0.46 g kg−1 of diet. However, magnesium concentration in whole body, muscle, vertebrae and scales increased significantly with increase in dietary magnesium levels up to 0.66 g kg−1 of diet, whereupon the response plateaued. Variation in dietary magnesium levels significantly affected liver malondialdehyde content and superoxide dismutase, catalase, glutathione peroxidase and serum alkaline phosphatase (ALP) activities. Broken-line regression analysis of LWG, SGR and PG against dietary levels of magnesium indicated the optimal dietary magnesium requirement of fingerling L. rohita at 0.44, 0.44 and 0.45 g kg−1 of the diet, respectively, whereas that based on magnesium concentrations in whole body and vertebrae and serum ALP activity, it was found to be at 0.66, 0.68 and 0.74 g kg−1 of the diet, respectively.