| Summary: | Synthetic sources of methionine are commonly used to supplement plant protein-based aquaculture feeds. A study was conducted to identify requirement of methionine for juvenile (0.41 g ± 0.02) Pacific white shrimp (Litopenaeus vannamei) fed low-protein (20% CP) diets formulated to be nutritionally-adequate in essential amino acids with the exception of methionine. Graded levels of methionine were prepared by supplementing a semi-purified basal diet with either soybean meal covalently enriched with methionine or with crystalline L-methionine. Covalent diets contained 0.298, 0.362, 0.473, 0.617, 0.766 and 0.958% methionine, whereas crystalline treatments contained 0.298, 0.384, 0.481, 0.576 and 0.748% methionine. Leaching trials with experimental feeds indicated increased, but minimal, leaching of methionine with increased dietary inclusion level and by time for both diet types. Leaching trials with requirement feeds indicated increased, but minimal, leaching of methionine over 60 min. Growth trials showed no significant difference (P>0.05) in survival of shrimp fed either type of diet during the 28-day feeding trial. Percentage weight gain of shrimp fed the covalently-attached methionine ranged from 541 - 643%. Growth of shrimp decreased when fed diets containing covalent methionine levels above 0.617%, suggesting possible inhibition (Im = 0.55% dietary methionine). Percentage weight gain of shrimp fed the crystalline diets ranged from 541 - 683%. Broken-line analysis estimated a methionine requirement level of 0.74% using crystalline methionine supplementation. Leaching trials with requirement feeds indicated increased, but minimal, leaching of methionine over 60 min.
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