Maintenance threonine requirement and efficiency of its use for accretion of whole-body threonine and protein in Atlantic salmon (Salmo salar L.) fry

Eighteen groups of seventy Atlantic salmon (Salmo salar L.) fry (initial mean body weight 0·8 (sd 0·01) g) were fed on semi-purified diets containing graded levels of l-threonine (Thr) in 15 litres aquaria at a temperature of 14·5±1°C. Doses of Thr represented 1, 31, 41, 51, 62, 72, 83 and 93% of its ideal level for optimumprotein deposition. Indispensable amino acids other than Thr were included in the same proportion (on a g/16g N basis) as in the Atlantic salmon fry whole-body carcass. Following 36d of feeding and a 36h fast, fry were killed for whole-body protein and amino acid analysis. Weight gain (r2 0·98), protein accretion (r2 0·97), and Thr accretion (r2 0·97) were linear (P<0·01) functions of Thr intake. Slope of the Thr accretion regression line showed that the efficiency of Thr utilisation above maintenance was 76%. At zero Thr intake, fry lost 5·4mg Thr/kg body weight0·75 per d. The Thr maintenance requirement was 7·2mg/kg body weight0·75 per d and the Thr requirement for growth was 66mg for 1g protein deposition. Increasing doses of Thr resulted in increased (P<0·05) concentrations of histidine and lysine, and decreased concentrations of isoleucine in whole-body protein. The maintenance need for Thr represented 13·4% of the total need for Thr. The data suggest that efficiency of Thr utilisation above maintenance is constant at all levels of Thr intake between 1 and 93% of the level required for optimum protein deposition.