Content of digestible energy in fish feed ingredients determined by the ingredient-substitution method

Abstract

The content of digestible energy (DE) in various fish meals, soya protein concentrate and soya bean meal was determined in compound diets for rainbow trout ( Oncorhynchus mykiss). The determinations were performed using casein with a known digestibility in trout, as a reference compound. In the determination of DE of fish feed ingredients, the test sample was exchanged with casein in the test diet. Cr 2O 3 was used as an inert marker, and faeces was obtained by stripping. The determination differentiated between various qualities of fish meal. Digestibility coefficients for energy in fish meals of `high quality' (five samples), `medium quality' (three samples) and `fair average quality' (FAQ) (one sample) were determined to be 97.3, 94.1 and 85.3%, respectively. Corresponding values for apparent digestibility coefficient (ADC) for protein were 89.3, 86.4 and 76.7%, respectively. The directly determined values for DE were in good agreement with values obtained by calculation based on the chemical composition of the ingredient, the ADC for protein, fat and carbohydrate and the respective combustion values for these nutrients. Absolute values of soya products could be questioned due to decreased digestibility of carbohydrate with increased inclusion level and due to possible nutrient interference. An average value for DE based on five different samples of special-quality Norwegian fish meal (Norse-LT94®) was determined to be 20.5±0.3 MJ kg −1 dry matter (dm). One sample of soya protein concentrate and extracted soya bean meal had values of 12.6±1.0 and 11.0±0.9 MJ kg −1 of dm. Digestibility coefficients for energy and protein were found to be higher in fish reared in freshwater compared to saltwater.