NATURAL FOOD AND STOCKING DENSITY AFFECTS THE SUPPLEMENTATION OF DL-METHIONYL-DL-METHIONINE (AQUAVI® Met-Met) IN PRACTICAL DIETS FOR THE WHITELEG SHRIMP, Litopenaeus vannamei, FARMED IN GREEN-WATER CONDITIONS
This study evaluated the effect of stocking density and natural food availability on the dietary supplementation of DL-Methionyl-DL-methionine (AQUAVI® Met-Met) to juvenile L. vannamei. Diets with 50 g/kg of fish meal were supplemented with Met-Met at 0, 0.11, 0.21, 0.31 and 0.41% of the diet (as is basis) to reach a dietary crude protein of 36.1 ± 0.53% and a total methionine (Met) content of 0.48, 0.62, 0.72, 0.81 and 0.94% (dry matter basis), respectively. First, to evaluate the effect of natural food, shrimp of 1.93 ± 0.13 g were raised under 70 animals/m2 for 63 days in 50 outdoor tanks of 1.0 m3 (five replicate tanks per treatment). In order to simulate different levels of naturally available food in water, each dietary treatment was evaluated under two water regimes, flow-through (FL, water exchange of 14.4% a day) and static green-water (GR, 10-20% a week). In another study, juvenile shrimp of 1.98 ± 0.14 g were raised for 70 days under 50, 75 and 100 shrimp/m2, in 75 outdoor tanks in green-water (five tanks per dietary treatment and stocking density).
The availability of natural food significantly influenced final shrimp survival, feed intake and feed conversion ratio (P<0.05). The FL system promoted a higher survival (95.4 ± 1.1%) for L. vannamei compared to the GR system (86.4 ± 1.7%). In the FL system, shrimp performance responded linearly to increasing doses of Met-Met, indicating a requirement of Met+Cys to be above 1.28% diet (1.40% on a dry matter basis). In GR system, shrimp performance responded exponentially to increasing doses of Met-Met, with a dietary Met+Cys content of 1.05% (1.15% dry matter) being required to reach 95% of maximum body weight.
Stocking density and dietary Met content had a significant effect on shrimp weekly growth and final body weight. Up to 0.72% Met, shrimp weekly growth was reduced from 1.49 ± 0.11 to 1.36 ± 0.06 g when density exceeded 50 shrimp/m2. Under 50 and 100 shrimp/m2, shrimp final body weight increased progressively up to 0.72% Met (maximum of 16.80 ± 1.95 g). Under 75 shrimp/m2, the highest final body weight was observed with a dietary Met of 0.81% (16.59 ± 1.88 g). Dietary Met beyond these levels did not result in a higher shrimp body weight. The increase in shrimp stocking densities from 50 to 75 and 100 shrimp/m2 responded to increasing dietary Met levels. Regardless of the density used, under green water and with the nutrient dietary profile adopted, the optimal level of methionine to achieve maximum growth of L. vannamei juveniles was between 0.72 and 0.81% of the diet (DM basis).
Altogether, data indicates that in systems where natural food is scarcer or stocking densities are higher, an increase of about 20% of dietary Met+Cys in a total basis (FL= 1.28 vs GR= 1.05), might be required to sustain maximum growth of L. vannamei. Although the present data is not sufficient to determine the impact of natural food and stocking density on the Met+Cys recommendations for shrimp, it supports the idea of adjusting Met and Met+Cys content of shrimp feeds according to the specific production conditions of the farm.