Combined effects between dietary selenium forms and dietary protein levels on performance, intestinal structure, hemato-biochemical indices, and immune response of Nile tilapia

Aquatic species require selenium (Se) as a trace element for healthy growth, stress tolerance, and appropriate development. Fish feed has extensively studied both organic and inorganic sources of selenium; their distribution, metabolism, and absorption processes differ. The present study was the effect of different dietary protein levels suboptimum level (25%) and optimum level (35%), with different selenium forms inorganic (sodium selenate) or organic (selenomithionine), and their interaction on growth, intestinal structure, hemato-biochemical, and immune responses of Nile tilapia. Two experimental diets were formulated to contain approximately 25 % and 35% crude protein, each diet supplemented with selenium forms with 0 (normal level in ingredients), 0.5 mg kg⁻¹ sodium selenate and 0.5 mg kg⁻¹ selenomethionine, therefore six diets were formulated. Nile tilapia, Oreochromis niloticus, fingerlings (2.02 ± 0.03 g) were fed formulated diets in triplicates with 5% of total biomass three times a day for 84 days. Results showed that, fish fed diet containing 35% crude protein and supplemented with 0.5 mg selenomethionine recorded the highest final body weight (FBW), weight gain (WG) and specific growth rate (SGR). However, no significant (P > 0.05) differences were recorded in FBW, WG, SGR and feed conversion ratio (FCR) between fish fed diet containing 35 % crude protein without Se supplementation and fish fed diet containing 25 % crude protein supplemented with Se form. The highest values of villi height/width and goblet cell count were seen in fish fed diets enriched with selenomethionine and containing either 25% or 35% crude protein. Red blood cell count (RBC), hematocrit (Hct), and hemoglobin (Hb) values were highest in fish fed a meal with 35% protein level and 0.05 mg kg⁻¹ selenomethionine supplemented. Interaction between different levels of protein (25% and 35 %) and different forms of Se (inorganic and organic) significantly affected C4 (P= 0.006) and C3 (P= 0.004) values, while no significant differences were found in IgM. The highest values of complement 4 (C4) and complement 3 (C3) were obtained for diet of 35 % crude protein and supplemented with 0.05 mg kg⁻¹ diet selenomethionine. The highest immunoglobulin M (IgM) value was obtained for fish fed diet containing 35 % crude protein and supplemented with selenomethionine form. The highest GH was obtained in 35% crude protein with organic Se followed by 25 % protein with inorganic selenium. As such, Nile tilapia benefit from the addition of organic selenium to their diets.