Increasing feed allowance in low-fish meal diets allows for a reduction in dietary methionine for juvenile Litopenaeus vannamei raised in green-water tanks

A 10-week study was conducted to evaluate the effect of feed allowance and graded levels of dietary methionine (Met) on growth performance of Litopenaeus vannamei. Juvenile shrimp of 1.83 +/- 0.14 g were stocked in 42 outdoor green-water tanks of 1 m(3) under 120 shrimp m(-2). Animals were fed under two feed allowances, regular and 30% in excess. Five diets with 30 g kg(-1) fishmeal were designed to contain 318 +/- 2 g kg(-1) crude protein and a minimum amount of protein-bound Met. To achieve graded levels of dietary Met, a control diet with 4.6 g kg(-1) Met or 8.9 g kg(-1) methionine + cysteine (M+C) was supplemented with 1.2, 2.2, 3.2, and 4.2 g kg(-1) of DL-methionyl-DL-methionine to result in total dietary Met of 5.6, 6.9, 7.9, and 9.2 g kg(-1) (10.0, 11.2, 12.1, and 13.5 g kg(-1) M+C, respectively). A final survival of 86.5 +/- 3.6% was reached with no significant influence from feed allowance or dietary Met. Feed inputs significantly affected apparent feed intake, weekly shrimp growth, final body weight (BW), and gained yield. Larger meals and a higher dietary Met had no impact on feed conversion ratio. There was a significant interaction between feed allowance and Met over shrimp BW. By feeding animals in excess, BW was enhanced at 6.9 g kg(-1) Met. A dietary Met of 7.9 g kg(-1) was required to achieve a maximum BW under a regular feed allowance. Thus, shrimp required less amounts of dietary Met to maximize BW when higher feed inputs were delivered. Our findings demonstrate a sparing effect of dietary Met for L. vannamei when a higher feed allowance is adopted. Shrimp farmers should consider adjusting feed allowance to dietary Met to maximize shrimp growth performance and yield.