Dietary inclusion of crystalline D- and L-methionine: effects on growth, feed and protein utilization, and digestibility in small and large Atlantic salmon (Salmon salar L.)

Use of crystalline methionine to optimize amino acid profiles in feeds for Atlantic salmon (Salmo salar L.) was studied in three experiments. A basic experimental diet with 25% of dietary protein from marine origin and the re maining 75% from soya protein concentrate was used either with or without s upplementation Of L-methionine, D-methionine or DL-methionine. A fishmeal-b ased diet was used as control. In experiment 1, growth performance of 100 g salmon in seawater was improved by supplementation with the different meth ionine products. Growth performance was, however, inferior to that of fish fed the control. Feed conversion ratios (FCR) exhibited similar trends to s pecific growth rate (SGR) with methionine supplementation, but again not as the control. Protein retention, measured as productive protein value (PPV) , was significantly better in the control and D-methionine groups as compar ed with either L-methionine or the nonsupplemented group. These trends were confirmed in experiment 2, using fish of the same size, where an absorptio n and digestibility study of D- and L-methionine clearly indicated that D-m ethionine was absorbed as well as L-methionine. Results indicated that race mization Of D- into L-methionine occurs in liver and kidney in Atlantic sal mon. Ornithine decarboxylase (ODC) activity in liver was significantly affe cted by dietary treatment and there was a good correlation with growth and feed utilization data. In experiment 3, a growth study with large salmon (I kg) was performed using increasing supplementation Of DL-methionine to the basic diet. Results indicated a methionine requirement for maximum growth of 9.2 g kg(-1) feed dry matter (DM) kg-1 growth. This is slightly higher t han that reported in the literature. The present study clearly shows that o ptimizing the amino acid profile by inclusion of a single crystalline amino acid (crystalline methionine) does not give the same protein utilization a s that of a diet in which the amino acids are mainly protein bound. D-methi onine was found to result in better protein utilization than L-methionine w hen measured as PPV.