Threonine requirement for maintenance and efficiency of utilization for threonine accretion in Atlantic salmon smolts determined using increasing ration levels

A nine-week study was conducted to determine the maintenance requirement and efficiency of utilization of digestible threonine for threonine gain above maintenance in Atlantic salmon smolts. Two diets were studied; one containing threonine (Thr) at the previously determined requirement level (1.33%; anhydrous form), and the second, a deficient diet containing 1.11% Thr. The fish (39g) were held in fresh water the first week and then the water supply was switched to seawater (11°C, 32.5ppt). Triplicate groups were fed in excess to estimate maximum intake. In addition, three groups were fed at approximately 15% and single groups fed at 40, 50 and 75% of the intake of the full-fed groups. In the full-ration groups, feed intake and growth rates were significantly higher in fish fed the 1.33% Thr diet compared with those fed the deficient diet, but feed efficiency was not affected by diet composition (P≥0.05). For both diets, linear relationships were found between intake of digestible Thr and Thr gain in the fish. The efficiency of utilization of digestible Thr for gain above maintenance was 107% for the deficient diet and 93% for the diet containing 1.33% Thr. The fish fed the 1.11 and 1.33% Thr diets had maintenance requirements for digestible Thr of 9.6 and 9.5mgkg−0.7d−1, respectively. Protein accretion as a function of digestible Thr intake (mgkg−0.7d−1) was linear in fish fed the deficient diet and 33mg Thr intake was required per 1g protein gain. In contrast, a second-order polynomial was shown for those fed the 1.33% Thr diet. Despite the fact that the full-fed fish in this experiment had the highest recorded growth for salmon in this phase of life, the results indicate that another nutrient may be marginal in the Thr-supplemented diet. A consequence of this is that the Thr requirement for Atlantic salmon smolt we earlier measured (Helland and Grisdale-Helland, 2011) might be an underestimate. ► Utilization of digestible threonine (kdThr) determined in Atlantic salmon smolts ► kdThr 1.07 for Thr-deficient diet; 0.93 for diet at the Thr requirement level ► No effect of Thr level on the maintenance requirement for digestible Thr ► 33mg Thr intake required per 1g protein gain