Efficacy of three methionine sources in diets for Pacific white shrimp, Litopenaeus vannamei

To examine the need for supplemental dietary methionine and to determine the ability of different methionine sources to meet this need for Pacific white shrimp, Litopenaeus vannamei, a growth trial was conducted. A control diet (30% crude protein) was formulated to be deficient in methionine by inclusion of high levels of dehulled, solvent-extracted soybean meal and with no fish meal (methionine level was 0.45% of diet as fed, 1.5% of total amino acids). Three test diets were manufactured based on the control diet but supplemented with L-methionine, DL-methionine, or an analog of methionine (2-hydroxy-4-methylthio butanoic acid [HMTBA], calcium salt) at the level of 0.5% as fed (equivalent to 1.5% of total amino acids). Cystine was constant among these diets at 0.5% of diet as fed. A commercial feed (30% crude protein) was included as a reference. The culture vessels were flat-bottomed, cylindrical, black fiberglass tanks (1.5-m diameter) initially filled with 1000 L seawater and supplied with aeration and seawater from a well at a rate of 2 L/min. At the end of the trial, all surviving shrimp were counted and weighed. The trial was run outdoors, with a mean water temperature of 25.9 C. The mean survival, final weight, growth rate, and feed efficiency of shrimp in the control group were 95.9%, 10.8 g, 0.93 g/wk, and 0.44 g/g, respectively. The mean values of all the test treatments were 96.9%, 11.6 g, 1.00 g/wk, and 0.47 g/g, respectively. The final weight and growth rate of shrimp fed the control diet were significantly lower than those of shrimp fed the methionine-supplemented diets, indicating that this species has a requirement for dietary methionine. There were no significant differences (P > 0.05) in any of the parameters among the test treatments, indicating that L-methionine, DL-methionine, and HMTBA can be used to meet the methionine requirement of this species under these experimental conditions. It is concluded that the three sources of methionine examined in this trial, L-methionine, DL-methionine, and HMTBA, are similar in their ability to meet the methionine requirement of this species.