Utilization of unprocessed and fibre‐reduced oilseed cakes of rapeseed and sunflower seed in rainbow trout (Oncorhynchus mykiss W.) nutrition—Evaluation of apparent digestibility and growth performance

Two digestibility trials were carried out to determine the apparent digestibility coefficients (ADCs) for crude protein (CP), crude lipid (CL) and energy of unprocessed and two differently processed fibre‐reduced rapeseed and sunflower seed oil cakes (sieved oilseed cakes and oilseed cakes produced from partially dehulled seeds) in rainbow trout (40 fish per tank; four tanks per diet). Each trial was followed by a 63‐day growth trial, wherein the respective oilseed cakes with the highest ADCs were evaluated as fishmeal substitutes, based on digestible CP. Oilseed cakes of rapeseeds and sunflower seeds had low nutrient ADCs (Trial 1). Nonetheless, the protein in rapeseed cake was able to replace up to 10% of the fishmeal protein in a diet without negatively affecting performance traits (Trial 2). Fibre reduction increased the ADCs of both types of oilseed cakes substantially (Trial 3). However, when fish were fed diets with 0%, 25% and 50% fishmeal protein replaced with dehulled rapeseed or dehulled sunflower seed cake protein, performance traits decreased with increasing substitution levels (Trial 4). Nonetheless, the amount of fishmeal needed per unit weight gain was lower for all fish fed the diets containing either one of the dehulled oilseed cakes than for the reference diets.