Chromium-methionine chelate can reduce protein content in Nile tilapia diets: an analysis of growth, carcass, biochemical and hematological parameters, and economic return

This study aimed to evaluate the effects of adding chromium-methionine (Cr-meth) chelate in extruded diets containing reduced protein content on growth performance, carcass composition, nutrient retention, and hematobiochemical responses in Nile tilapia juveniles. The fish were fed with two control diets (no Cr-meth): positive control (311 g kg⁻¹ of crude protein, estimated 258 g kg⁻¹ of digestible protein) and negative control (255 g kg⁻¹ of crude protein, estimated 207 g kg⁻¹ of digestible protein). Another five experimental diets (mean 251 g kg⁻¹ of crude protein, mean estimated 204 g kg⁻¹ of digestible protein) were supplemented with 0.2, 0.4, 0.6, 0.8, and 1.0 mg Cr-meth kg diet⁻¹. Juveniles (34.31 g) were organized in a randomized design (seven treatments in triplicate) in 21 tanks (250 L) and fed until apparent satiety. The experiment lasted 9 weeks. Fish fed 0.8 and 1.0 mg Cr-meth kg diet⁻¹ showed zootechnical variables similar to the positive control. The reduced protein diets showed lower costs, and supplementation with 0.91 mg kg⁻¹ of chromium matched the profitability observed in the positive control diet. Carcass gross energy and energy retention, plasma cholesterol (total and LDL), and triglycerides values were higher in fish fed 0.6 mg Cr-meth kg diet⁻¹ than in the positive control. Treatments with dietary Cr-meth (mainly between 0.4 and 0.8 mg kg⁻¹) decreased carcass Cr, selenium, and copper retention and increased carcass nitrogen retention and muscle glycogen levels compared to the positive control. This treatment (positive control) also had lower values for total leukocytes and monocytes, respectively, than fish fed 0.2 and 0.4 mg Cr-meth kg diet⁻¹ and for eosinophils and lymphocytes than fish fed 0.8 mg Cr-meth kg diet⁻¹. In conclusion, we recommend supplementation with 0.8 mg Cr-meth kg diet⁻¹ for diets with low protein content, as it produces a dietary protein-sparing effect, reducing costs, improving immune defense, muscle glycogen, and nitrogen retention without compromising growth performance, physiology, and fish metabolism.