Nutrient Digestibility and Retention of Potential Feed Ingredients for Rainbow Trout (Oncorhynchus mykiss W.) Aquaculture in Iran

To overcome the aquafeed challenges for aquaculture in Iran, the world’s largest rainbow trout producer, the nutrient digestibility and retention of eight rendered poultry by-products, and two plant protein sources were investigated. The test ingredients were added to a casein-based semipurified reference diet as control diet and fed to juvenile rainbow trout in three runs. During the experimental runs, test ingredients were added to the control diet in a ratio of 30 : 70. The diets were then fed to the animals in four replicates. TiO2 was used as marker and feces were collected by settling method. The apparent protein digestibility coefficient for poultry by-product meals (PBM), feather meals (FeM), poultry protein concentrate (PPCon), blood meal (BM), canola meal (CM), and Iranian soybean meal (IRSBM) were 73%–93%, 73%–96%, 100%, 87%, 94%, and 97%, respectively. The nitrogen productive value for PBM ranged 27%–46%, both PPCon and IRSBM 37%, CM 24%, BM 23%, and FeM from −6% to 18%. Citric acid supplementation of the alkaline thermochemically hydrolyzed FeM improved the digestibility of crude lipid and organic matter from 6% and 88% to 55% and 92%, respectively. The assessment of digestibility as well as nutrient retention analysis for tested feed components indicated that refined PBM and PPCon, both CM and IRSBM could be used in rainbow trout diets as value-added and eco-friendly protein sources. The findings of this study can assist fish nutritionists and key players in the aquafeed industry in adopting a sustainable approach to aquaculture by selecting locally available raw materials that demonstrate high nutrient retention in rainbow trout.