Protein Level Reduction in Corn and Soybean Meal-Based Diets Supplemented with Essential Amino Acids During the Fattening Phase of Nile Tilapia

This study evaluated the effect of reducing dietary protein levels with essential amino acid (EAA) supplementation in soybean meal and corn-based diets on the performance of Nile tilapia (Oreochromis niloticus) during the fattening phase. A total of 195 GIFT-lineage tilapia (initial weight 297±5.06 g) were randomly assigned to five treatments, with three replicates per treatment. The fish in each treatment were fed for 70 days with a diet containing one of five crude protein (CP) levels (300, 275, 249, 222, or 197 g kg⁻¹). All diets were isoenergetic and isoaminoacidic for EAA. Fish fed higher CP levels (249, 275, and 300 g kg⁻¹) had significantly better performance in final weight (553.66–560.37 g; P = 1.44e−05), weight gain (256.35–260.35 g; P = 6.06e−06), specific growth rate (0.88–0.90; P = 2.68e−05), and feed conversion ratio (1.56–1.58; P = 9.69e−06) compared to fish fed 197 and 222 g kg⁻¹ CP. Protein efficiency ratio was highest (2.78; P = 0.0132) with CP level 249 g kg⁻¹, while carcass protein content was significantly higher with CP levels 275 and 300 g kg⁻¹ CP (17.35 and 17.08 g 100 g⁻¹, respectively; P = 0.0118). Regression analysis using the broken-line model estimated dietary requirements of 229 g kg⁻¹ digestible protein (DP) and 248 g kg⁻¹ CP. This study demonstrates that reducing CP from 300 to 248 g kg⁻¹ (269 to 228 g kg⁻¹ DP), with adequate EAA supplementation in soybean meal and corn-based diets, maintains growth performance in Nile tilapia weighing 300–530 g, whereas further reductions impair performance.