Assessment of nitrogen maintenance requirement and potential for protein deposition in juvenile Tilapia genotypes by application of an exponential nitrogen utilization model

Assessment of quantitative amino acid requirement data in fish nutrition is predominantly based on dose response studies with supplementation of the dietary limiting amino acid. Studies with modeling amino acid requirements are scarce. Three experiments were conducted with different genotypes of juvenile Tilapia [A: all male O. niloticus; B: red Tilpia (O. mossambicus × O. hornorum); C: hybrid Tilapia (A × B)] using fishmeal-wheat gluten diets with five graded levels of dietary protein supply (17.2-48.5% CP). The dietary amino acid ratio was similar, dietary energy contents were adapted to avoid limiting energy supply. Growth response was studied in a semi-closed in-door water recirculation system with 38 tanks (320 l/tank; water temperature 28 ± 0.5 °C; regulated photoperiod 12 h light/12 h dark). Four replicate groups (25 fish per tank) were utilized in a 70 d growth experiment with three times a day feeding until apparent satiation. Ten fish at the beginning and three fish per tank at the end of the growth study were analyzed for body composition to calculate N-deposition and N-excretion. The nitrogen maintenance requirement (NMR) was established based on nonlinear fitting of N-excretion depending on N-intake. The intersection of the applied exponential function with the y-axis was used as NMR, simulating protein free feeding. The theoretical potential for daily N-deposition of the genotypes under study (ND_maxT) was estimated as threshold value due to exponential fitting of N-deposition depending on N-intake. The observed average NMR (70 mg N/BW_kg^0.67/d) and threshold values for genotype A (ND_maxT = 388 mg/BW_kg^0.67/d), genotype B (ND_maxT = 389 mg/BW_kg^0.67/d) and genotype C (ND_maxT = 319 mg/BW_kg^0.67/d), respectively, are the starting point for the studied exponential N-utilization model and its subsequent application for assessing amino acid requirements depending on growth performance and dietary amino acid efficiency in fish feed.