Fat encapsulation and supplementation with free amino acids cannot compensate for negative effects from dietary rapeseed protein isolate on growth performance of rainbow trout (Oncorhynchus mykiss)

Abstract There are still restrictions when using rapeseed protein products as fish meal alternatives in diets for rainbow trout. Digestibility of a particularly purified rapeseed protein isolate (protein content: 98.7 g 100g−1) was determined in rainbow trout (initial body weight: 178.2 ± 20.9 g) and it was used to exchange fish meal gradually (0 %, 66 % and 100 %) in diets based on commercial trout diets. To overcome potential limiting factors on growth performance from rapeseed protein isolates, duplicates of these experimental diets were encapsulated with stearic acid and one diet devoid of fish meal included rapeseed protein isolate as well as a mixture of free amino acids. The highest recorded crude protein digestibility (mean = 99.8 % ± standard deviation = 1.6 %) of rapeseed protein was reached using the stripping method. Daily feed intake (DFI) of the dietary group devoid of fish meal (DFI: 2.1 ± 0.06) was significantly reduced in comparison with control (DFI: 2.6 ± 0.07) at the end of the experiment. Similarly, feed conversion ratio (FCR) was significantly impaired when comparing control (FCR: 0.9 ± 0.01) to the dietary group with complete exchange of fish meal (FCR: 1.1 ± 0.02). Neither encapsulation of diets nor supplementation with free amino acids could overcome negative effects on growth performance from dietary rapeseed protein isolate inclusion. Results suggest that even highly purified rapeseed protein products can impair growth performance of rainbow trout and that factors other than ANFs could determine the effectiveness of rapeseed protein products as fish meal alternatives.