LAUSR

Abstract The impact of feed processing on the digestibility of two major grain proteins fed to post-smolt Atlantic salmon (Salmo salar) was examined using a simple factorial experimental design of two protein types (wheat and corn gluten) and two feed processing methods (hot [cooking] extrusion and cold extrusion). A series of diets were each prepared from the same basal mash, including two basal diets (each diet was prepared using the same processing methods as the test diets) were also included in the experiment. Triplicate tanks of salmon (286 ± 20.9 g) were fed each diet for one week and then stripped of faeces, which was repeated two more times. Faeces, diets and ingredients were assessed for protein, amino acid, and energy digestibilities and by derivation the ingredient digestibilities for these parameters were also determined. Ingredient protein digestibility was affected by both ingredient type and feed processing method, with an interaction effect between parameters also present. Protein digestibilities were highest in diets that were cold extruded, with corn gluten protein digestibility being significantly affected by feed processing method, but wheat gluten comparatively less affected. Energy digestibilities typically reflected the protein digestibilities, but the interaction effect was not evident. The digestibility of the sum of amino acids in corn gluten was negatively affected by hot extrusion but this processing method had limited impact on wheat gluten digestibility. The digestibility of individual amino acids was highly variable between ingredients and processing methods, though was typically consistent with the digestibility observations of crude protein and sum of amino acids. Wheat gluten had superior digestibility characteristics than that of corn gluten across most parameters. This study demonstrates that the feed processing method has a clear influence on the nutritional value of corn gluten but in contrast has negligible effect on the digestibilities of wheat gluten.