The use of intact proteins and purified amino acids in determining the methionine requirement in practical diets of pacific white shrimp, Litopenaeus vannamei

Abstract

As there is no consensus on the efficacy of purified amino acids in shrimp feed, two approaches were designed to evaluate the methionine requirement in practical diets for Litopenaeus vannamei. The first approach used intact proteins to produce both a deficient and a replete diet. These diets were then co-mixed to create varying levels of methionine using intact proteins. The second approach supplemented a basal diet with pure methionine to create a replete diet and again co-mix the two diets to produce different methionine levels. Hence, three main diets were made, which included a deficient basal diet (B, 0.48 % methionine), a replete diet (M, 0.85 % methionine) that used DL-Met for the first trial, and a dipeptide Met (Met-Met)-supplemented diet for the second trial, and a replete diet (C, 0.84 % methionine), which contained corn protein concentrate to increase the level of methionine. Ten experimental diets were produced by blending the deficient diet with the replete diet which resulted in graded levels of methionine - B100, B70:M30, M100, B90:C10, B80:C20, B70:C30, B60:C40, B40:C60. B20:C80, and C100. Experimental diets were then fed to shrimp (15/aquaria) in 60 randomly assigned aquaria (55.8 L) with a mean initial weight of 0.45 ± 0.002 g over a 54-day growth trial (Trial 1) and 0.23 ± 0.0001 g over a 42-day growth trial (Trial 2). All diets were formulated to be isonitrogenous and isolipidic (36 % protein and 8 % lipid, as is), with the basal diet formulated with fishmeal and lentil meal as the primary protein sources and whole wheat as a carbohydrate source. Significant differences were observed in weight gain as well as shrimp whole body amino acids namely alanine, arginine, glycine, histidine, phenylalanine, proline, and taurine for Trial 1 and cysteine, glycine, threonine, and taurine for Trial 2. The optimal dietary methionine requirement of L. vannamei, estimated by a one-slope broken-line regression analysis model based on weight gain% was 0.67 % of the dry diet (equivalent to 1.85 % of dietary protein on a dry-weight basis) for Trial 1 and at 0.56 % of the dry diet (equivalent to 1.55 % of dietary protein on a dry-weight basis) for Trial 2. For thermal growth unit coefficient the optimal dietary methionine requirement of L. vannamei, estimated by a one-slope broken-line regression analysis model was at 0.66 % of the dry diet (equivalent to 1.82 % of dietary protein on a dry-weight basis) for Trial 1 and at 0.61 % of the dry diet (equivalent to 1.68 % of dietary protein on a dry-weight basis) for Trial 2. Findings from these trials confirm that, in the presence of replete cystine, that a conservative methionine requirement of 0.61 % diet (1.68 % protein) is recommended.