Evaluation of different sulfur amino acid compounds in the diet of red drum, Sciaenops ocellatus, and sparing value of cystine for methionine

Relatively large substitution of plant protein feedstuffs for animal feedstuffs in the diet of red drum will likely require supplementation of methionine to meet this species' metabolic requirement. Therefore, two feeding trials were conducted to determine the value of different sulfur compounds in diets for red drum, and to re-evaluate the cystine replacement value for methionine. The basal diet in both trials was formulated to contain 35% crude protein from red drum muscle and crystalline amino acids. The feeding trials compared growth characteristics of red drum fed different sulfur amino acid (SAA) compounds including L-methionine (control), DL-methionine, crystalline methionine hydroxy analog (MHA), N-acetyl-DL-methionine, or Alimet(TM), a liquid MHA, supplemented to the basal diet on an equal-sulfur basis to meet the minimum SAA requirement of red drum. Various ratios of cystine to methionine (40/60, 45/55, and 50150) also were added to the basal diet to re-evaluate the potential sparing effect of cystine for methionine. All diets were fed to triplicate groups of fingerling red drum in 110-l aquaria connected as a brackish (7parts per thousand) water recirculating system for 8 and 7 weeks in trials 1 and 2, respectively. Fish fed the basal diet had significantly (P<0.05) lower survival and reduced weight gain, feed efficiency (FE), protein conversion efficiency (PCE), and protein efficiency ratio (PER) values compared to fish fed SAA-supplemented diets. There were no differences in these responses of fish fed diets containing the various SAA compounds. Proximate composition of whole-body and muscle also were significantly altered in fish fed the basal diet compared to fish fed L-methionine and the other SAA compounds. Results from this study indicate red drum were able to use the various SAA compounds equally as effective as L-methionine to meet their SAA requirement. Cystine also was able to spare approximately 50% of the dietary methionine requirement of red drum. (C) 2004 Elsevier B.V. All rights reserved.