Evaluation of various sulphur amino acid compounds in the diet of red drum, Sciaenops ocellatus

Abstract

Refinement of diet formulations to enhance the efficiency of red drum production continues to be pursued. Based on previous studies, the sulfur amino acid (SAA) requirement of red drum for methionine plus cystine appears to be most limiting, which may necessitate supplementation if plant proteins are used to replace animal feedstuffs in diet formulations. Therefore, this study was conducted to (1) determine the nutritional value of various sulfur compounds in diets for red drum, (2) determine the effects of the SAA supplements on ammonia excretion, and (3) re-evaluate the cystine sparing value for methionine. Two feeding trials were conducted with juvenile red drum with a semi-purified basal diet (0.54% SAA) supplemented on an equal-sulfur basis with either L-methionine (control), DL-methionine, or methionine hydroxy analog (MHA), N-acetyl-DL-methionine, or liquid MHA (Alimet[TM]) to meet the minimum SAA requirement of red drum (1.06% of diet). Diets containing various ratios of cystine to methionine (40:60, 45:55, and 50:50) also were fed in the second trial to re-evaluate the potential sparing effect of cystine on methionine. All diets were fed to apparent satiation twice daily to triplicate groups of fingerling red drum in 110-L aquaria connected as a brackish (7 ppt) water recirculating system for 8 and 7 weeks in trials 1 and 2. At the end of the second trial, total ammonia nitrogen (TAN) excretion was determined at 4 h postprandial. Fish fed the basal diet had significantly (p [<] 0.05) lower weight gain, feed efficiency and survival than fish fed SAA-supplemented diets in both trials. There were no differences in weight gain, feed efficiency, and protein efficiency ratio or protein conversion efficiency for fish fed diets containing the various SAA-supplemented diets. Results from this experiment indicate red drum were able to use DL-methionine, N-acetyl-DL-methionine, Alimet[TM] and MHA as effectively as L-methionine to meet their SAA requirement. MHA or Alimet[TM] did not influence TAN excretion of red drum. Cystine also was able to spare approximately 50% of the dietary methionine requirement of red drum. This study has provided information to more precisely meet the SAA requirement of red drum.