Metal-amino acid complexes (Zn, Se, Cu, Fe and Mn) as a replacement of inorganic trace minerals in commercial diets for Nile tilapia (Oreochromis niloticus) reared under field conditions: Effects on growth, feed efficiency, gut microbiota, intestinal histology, and economic return

Abstract

An 80-day trial was carried out to evaluate the effects of replacing dietary inorganic trace minerals (ITM), with metal-amino acid complexes (MAAC) on the growth rates, feed utilization, economic return, body composition and mineralization, gut microbiota, and intestinal histology of Nile tilapia (Oreochromis niloticus) reared in in-pond hapas, under commercial conditions. The fish (93.7 g) were stocked in 2-m3 hapas, in quadruplicates, at a density of 30 fish m−3 (60 fish hapa−1). Five isonitrogenous (30% CP), isoenergetic (17 MJ kg−1) extruded (floating) commercial tilapia diets were produced, containing 0, 25%, 50%, 75% and 100% MAAC as a replacement of the commercial ITM premix. In the beginning, the diets were fed to the fish at a daily rate of 3% of their body weight (BW), but the feeding rate was reduced to 2.7% BW at the beginning of the second month. The best growth performance and feed efficiency were achieved at 50% MAAC level, with no further improvement registered above this level. Except for lipid content, tissue proximate composition was significantly affected by MAAC supplementation. Economic analysis indicated that MAAC-based diets were more profitable than the control, ITM-supplemented diet, with 50% replacement showing the highest return (21% increase) over the control diet. Tissue Fe and Mn levels did not differ, while tissue Cu was higher in fish fed ITM than in those fed MAAC diets. Tissue Zn and Se increased with increasing MAAC up to 50% and 75% replacement and decreased with further MAAC supplementation. Bone Cu and Mn were not affected by dietary mineral premix; whereas Fe, Zn and Se were significantly altered. Bone Fe concentration significantly decreased at 75% and 100% MAAC levels, whereas bone Zn increased with increasing MAAC to 50%. Bone Se was lower in fish fed ITM than in those fed MAAC at 25% replacement, however, Se was not significantly affected by further increases in MAAC concentrations. Digestive enzyme activity was improved with increasing supplemental MAAC up to 50% substitution level. Fish fed MAAC showed significantly higher abundances of intestinal beneficial bacteria (Bacillus and Lactobacillus.) and lower abundances of putative pathogenic bacteria (Staphylococcus and Streptococcus) than the control group. The length of the intestinal folds and the number of goblet cell counts also significantly increased in fish fed MAAC up to 50% inclusion. Quadratic regression analyses revealed that about 50–55% MAAC is required for optimum performance, body mineralization, intestinal absorptive capacity, and beneficial intestinal microbiota.