Dietary leucine requirement for growth and maintenance, and its utilization efficiency for fingerling of Nile tilapia, Oreochromis niloticus (Linnaeus)

Two separate 12-week feeding trials were conducted to investigate the leucine requirement for maintenance and its utilization efficiency for growth of fingerlings of Oreochromis niloticus. In the first experiment, six diets (35% CP; 13.71 kJ g-1 DE) containing graded levels of leucine (0.71, 0.95, 1.26, 1.49, 1.72 and 1.95% dry diet) were fed to the fish (3.74±0.11 cm, 0.65±0.05 g) near to satiation to determine its leucine requirement. Highest performance in terms of absolute weight gain (AWG, 5.83 g fish-1), protein gain (PG, 1.02 g fish-1), leucine gain (LG, 82.19 mg fish-1), RNA/DNA ratio (4.74) and best feed conversion ratio (FCR, 1.51) were recorded at 1.49% dietary leucine. Quadratic analysis of AWG, PG, LG against varying levels of leucine indicated the requirement at 1.56, 1.61 and 1.57% dry diet, corresponding to 4.45, 4.60 and 4.48% dietary protein, respectively. In the second experiment, six experimental diets (35.0% CP; 13.8 kJg-1 DE) at 5, 25, 45, 65, 85 and 95% of optimum level of leucine determined in experiment I (1.58% dry diet) corresponding to 0.08, 0.39, 0.71, 1.03, 1.34 and 1.50 g of leucine 100g-1 of dry diet were given to the same sized fish under almost identical laboratory conditions to estimate the leucine maintenance requirement and its utilization efficiency which were calculated by solving the linear equations of protein gain and leucine gain data to leucine intake levels. The maintenance requirement based on protein gain and leucine gain calculated at Y = 0 amounted to 8.23 and 9.62 mg kg-1 BW0.7day-1. Extrapolating the linear equations of protein gain and leucine gain to the Y intercept showed that zero leucine intake resulted in a net daily loss of 91.9 mg whole body protein and 5.65 mg whole body leucine kg-1 BW0.7. The maintenance need for leucine represented 6·07% of the total need for leucine. Slope of the leucine gain line showed that the efficiency of leucine utilization above the maintenance level is 58.7%