ABSTRACT Diets with high protein levels but unbalanced amino acid (AA) profiles can lead to poor AA utilization, increasing nitrogenous compound excretion and feed costs. Our study aimed to develop a formulation strategy to maintain a balanced dietary AA profile with a low protein level for Nile tilapia juveniles without compromising growth. Fish (6.75 ± 0.07 g) were fed on one of four isoenergetic diets with varied protein concentrations and AA profiles twice daily to apparent satiation for 41 days. The trial included four dietary treatments containing either 321 (32HighLys and 32LowLys) or 292 and 222 g protein kg –1 (29BAL and 22BAL, respectively) with five replicates per diet, making a total of 20 experimental units in a completed randomized design. The growth of fish fed the 29BAL diet was not compromised; weight gain (WG) and thermal growth coefficients were similar to those fed the 32HighLys and 32LowLys diets. However, the protein gain of those fish fed the 32HighLys diet was significantly higher than that of those fed the other diets. The 22BAL diet promoted the lowest growth, and its higher protein-energy ratio led to increased body lipid content. Therefore, the formulation strategy to supply balanced dietary AA to Nile tilapia reduced digestible protein from 32 to 29 %, without compromising fish growth and allowing a 12 % decrease in the excretion of nitrogenous waste. (AA utilization costs 6.75 675 6 75 (6.7 007 0 07 0.0 4 days HighLys LowLys 22 1 – BAL respectively 2 design compromised WG (WG However proteinenergy energy content Therefore 3 waste 6.7 67 7 (6. 00 0. 6. (6 (

ABSTRACT Protein quality is related to amino acid composition and digestibility. Accurate evaluation of apparent digestibility coefficients (ADCs) of nutrients in commonly used feedstuffs is paramount for formulating efficient aquafeed. ADCs of soybean meal (SBM) and poultry by-product meal (PBM) were evaluated using reference diets formulated with two types of ingredients (semi-purified [SP] and practical [P]) for juvenile Nile tilapia ( Oreochromis niloticus, Linnaeus) of the GIFT strain. Groups of 20 juveniles (65.05 ± 12.37 g) were fed twice a day to apparent satiety with one of the four experimental diets (SBM-SP, SBM-P, PBM-SP, and PBM-P) in quadruplicate for 30 days. After the last feeding, feces were collected by siphoning hourly and the ADCs of dry matter, protein, and amino acids (AAs) were calculated. Nile tilapia exhibited a high capacity to digest SBM and PBM, with most ADCs exceeding 90 %. The type of reference diet affected the ADCs of protein and AAs on the test ingredients, with the SP reference diet providing the highest ADC, mainly in SBM. Digestibility data generated with a P-type reference diet demonstrated more practical relevance than those generated with an SP-type reference diet. They can be applied in digestibility studies for Nile tilapia. (ADCs aquafeed (SBM byproduct product PBM (PBM semipurified semi purified [SP P [P] niloticus Linnaeus strain 2 65.05 6505 65 05 (65.0 1237 12 37 12.3 g SBMSP, SBMSP SP, (SBM-SP SBMP, SBMP P, SBM-P PBMSP, PBMSP PBM-SP PBMP PBM-P 3 days feeding matter (AAs calculated 9 % ADC Ptype SPtype [P 65.0 650 6 0 (65. 123 1 12. 65. (65 (6