Modelling of threonine and methionine requirements of Oreochromis niloticus due to principles of the diet dilution technique

Abstract

Two growth studies (56 days) were conducted with juvenile all male Oreochromis niloticus, using six mixed diets with threonine and five mixed diets, with methionine as the first limiting amino acid (LAA), respectively. Diets were randomly assigned to four repetition tanks with 25 fish per tank. Growth results and body nutrient analyses yielded protein deposition data depending on the uptake of the LAA. For further modelling, a nonlinear N-utilization model was utilized for assessing the dietary efficiency of threonine and methionine utilization, respectively. By use of the observed dietary AA efficiency, threonine and methionine requirement data were established, depending on the daily protein deposition. The optimal dietary AA concentrations (8.3 g kg(-1) versus 9.9 g kg(-1) threonine; 7.3 g kg(-1) versus 8.8 g kg(-1) methionine) were derived for 70% of the potential for protein deposition (NDmaxT) and predicted daily feed intake (3% versus 2.5% of BW). Observed methionine requirements are imposing for the upper limit for methionine replacement by cystine and minimal catabolism of methionine to yield cystine. Generally, the modelling provided requirement data in line with current recommendations. Variation of the dietary AA efficiency by modelling procedure ranks this factor as very important for the conclusion of amino acid requirement data.