Comparing effects of feeding crystalline or coated methionine to soybean meal based diets on the Nile tilapia (Oreochromis niloticus) growth performance and protein quality

A sixty days feeding trial was conducted to investigate the influence of including different forms of dietary methionine on growth performance and plasma methionine levels in Nile tilapia diets (Oreochromis niloticus). One hundred and thirty two Nile tilapia fingerlings (Oreochromis niloticus) mono sex of mean initial body weight 24.32 ± 0.31g were randomly distributed into 12 closed system 120 liter tanks. Fish in each three tanks were fed one of four diets. 1) Control positive (C+) (fish meal as main protein source covering methionine requirement for Nile tilapia), 2) Control negative (C-) (soybean meal as primary protein source deficient in methionine), 3) soybean meal supplemented with crystalline DL methionine (CRM), or 4) soybean meal supplemented with coated methionine (COM). Before the end of the experiment by 48h., blood samples were obtained at 1.5, 3.5, 5.5, 12.5 and 24h after feed administration to measure methionine uptake in blood plasma.  Results revealed that supplying fish diet free of animal protein source with coated methionine (COM) had comparable final body weight(FBW), weight gain (WG), feed conversion ratio (FCR) and specific growth rate (SGR) with the control group (P>0.05). The poorest growth performance and feed utilization was exhibited by the group fed the negative control (C-) diet. Apparent protein digestibility were significantly higher (P<0.05) in groups fed diets supplemented with either coated or crystalline methionine compared with groups fed un-supplemented diets (C+ and C-). Dorsal muscle protein and total essential amino acids (ΣEAA %) content were not significantly affected by dietary treatments. Plasma methionine concentration was significantly influenced by both protein source and methionine form, where the highest plasma methionine concentration throughout the measuring period was revealed by fish fed diet supplemented with crystalline methionine (CRM), while the lowest plasma methionine concentration resulted from group fed (C-). Both group of fish fed (C+) or (COM) had moderate plasma methionine concentration.  Based on these results, it appeared that coated methionine effectively slow the release of free methionine in a way to be near the animal bounded protein. Accordingly, it could be recommended to supplement soybean- based diets with coated methionine which had shown no adverse effect on Nile tilapia growth performance and protein utilization efficiency.