Abstract
It has been shown with terrestrial animals that the genetic improvement increases the nutrient requirements of the animal, which becomes more efficient in using these nutrients to achieve their higher growth potential. This study was conducted to estimate the dietary methionine requirements of different generations of the Genetically Improved Farmed Tilapia (GIFT) strain of Nile tilapia at the juvenile stage (17.51 g-19.55 g initial body weight). To achieve this objective, a completely randomized 2 × 6 factorial design was applied, with the genetic background of fish (generation) as the first independent variable with two levels, the 16th and 17th generations, and the methionine content of the diet as the second independent variable, with 6 graded levels, 0.52, 0.62, 0.84, 0.94, 1.04 and 1.27% of methionine in the diets, respectively. At the end of the 42-day experiment, the interaction effect of the generation × diet was not significant (P > 0.05) for any of the response parameters studied. The genetic improvement led to 15% more growth in the 17th generation than the 16th generation of the GIFT strain of Nile tilapia, which was accompanied with better feed conversion ratio, protein productive value, and energy productive value in the former generation (P < 0.05). Whatever the generation considered, the optimum dietary methionine requirement for the growth of the improved GIFT strain of Nile tilapia was estimated in this study, using the broken-line model, between 0.75% and 0.80% of the diet (with a cysteine level of 0.50% of the diet), which is higher than 0.49% of the diet previously estimated for conventional Nile tilapia at the juvenile stage. Therefore, the genetic improvement applied to the GIFT strain of Nile tilapia likely led to a higher methionine and total sulfur amino acid requirement than the conventional strains. The results of this study support the need to update the existing nutrient requirement databases of fish, in order to ensure that diets are formulated to effectively satisfy the requirements of not only conventional, but also improved strains of fish, for the full expression of their accrued growth potential of the latter strains.