Availability and utilisation of free lysine in rainbow trout (Oncorhynchus mykiss). 1. Effect of dietary crude protein level.

Abstract It was investigated whether the availability and utilisation of free lysine are affected by crude protein concentration in rainbow trout diets. Wheat gluten and free amino acids were used as sole sources of amino acids to formulate two basal diets differing in crude protein (350 and 550 g/kg dry matter) but equal in lysine concentration (9 g/kg dry matter). Lysine was supplemented at the expense of l -alanine to achieve further lysine levels of 13 and 17 g/kg dry matter. l -lysine·HCl was used as the supplemental lysine source at both crude protein levels. Diets were offered to three groups of 20 trout each over a period of 56 days. Amount of feed was restricted to about 16 g dry matter/kg BW daily in order to allow for an equal and complete intake in all groups. Trout had an initial body weight of 50 g/fish. Digestibility was determined using acid-insoluble ash as a marker. Faecal samples were collected via a sedimentation system attached to each tank. Retention was determined by comparative whole body analysis. Faecal lysine excretion was unaffected by lysine supplementation irrespective of dietary crude protein level. Hence, l -lysine·HCl can be regarded 100% available to trout. In spite of comparable levels of DE intake, growth and gain/feed ratio increased with lysine supplementation. In gained body mass, protein concentration was increased and lipid concentration was decreased with increasing lysine supplementation. Amino acid pattern of gained body protein was changed by lysine supplementation as well. Lysine utilisation, determined as Δretained/Δdigested×100, was 55% on the low and 71% on the high dietary crude protein levels, respectively. This difference was close to significance, as judged by comparative regression analysis ( P =0.11). Dose–response studies with a wider range in lysine concentration at different crude protein levels are necessary to identify the consequences regarding lysine allowances for practical type trout diets.