Growth and metabolic responses of juvenile Nile tilapia demonstrate DL-Methionine is superior to MHA-Ca by Karthik Masagounder, Evonik Operations GmbH, Hanau-Wolfgang, Germany Sofia Engrola & Rita Teodósio, Centre of Marine Sciences (CCMAR), Faro, Portugal & Cláudia Aragão, Universidade do Algarve, Faro, Portugal
S
oybean meal is a commonly used plant protein ingredient in aqua feeds, including tilapia feed, due to its high protein content and relatively well-balanced amino acid profile. However, plant ingredient-based diets are commonly limited in methionine and require its supplementation in order to fulfil the dietary methionine requirements for
fish. DL-methionine (DL-Met) and methionine hydroxy analogue (MHA; DL-2-hydroxy-4-methylthiobutyrate and its calcium salt) are commonly used supplemental methionine sources in animal feeds. DL-Met is a racemic mixture of D- and L-isomers of methionine, whereas MHA is a racemic mixture of D- and L-isomers of MHA. The chemical structure of MHA is similar to that of methionine however, as it contains a hydroxyl group instead of the amino group, it is not an amino acid. These chemical differences are likely to be reflected in absorption kinetics and metabolism and may result in different biological efficiencies as a methionine source. Based on the published data, NRC (2011) concluded that it is reasonable to assume that the biological efficacy of MHA in fish is 75-80 percent that of DL-Met on an equimolar basis (63-67 percent on a weight basis for MHA-Ca).
The objectives of this study were to evaluate how the dietary methionine sources affect growth performance and feed utilisation of juvenile Nile tilapia (Oreochromis niloticus) as well as to understand methionine metabolism and utilisation; all factors which are important for the aquaculture industry. To understand how methionine sources are absorbed and metabolised in tilapia juveniles, a time-course metabolic trial was performed using radiolabeled DL-Met and MHA-Ca.
Meeting the minimum requirements for Nile tilapia
Three experimental diets were formulated to be isonitrogenous and isoenergetic. The reference diet (REF Diet) was a negative control, without fishmeal inclusion, formulated to be 40 percent below the methionine requirement for Nile tilapia. No methionine was supplemented to this diet. Experimental diets were the REF diet supplemented with 0.15 percent DL-methionine (DLM diet) and 0.18 percent calcium bis-methionine hydroxyl analogue (MHA diet; equal on molar basis to 0.15 percent DL-methionine). Diets were formulated to meet the minimum requirements of amino acids for Nile tilapia juveniles, except for methionine. Juvenile Nile tilapia (2.3g, initial body weight) were allocated into nine tanks at an initial density of 50 fish per tank (1.1kg/m3). Triplicate tanks were randomly assigned to one of the three dietary treatments (REF, DLM and MHA). Fish were fed to visual satiety by hand, three times a day and
22 | October 2021 - International Aquafeed