Introduction
The protein content in feed ingredients is commonly estimated by multiplying total nitrogen by the nitrogen-to-protein conversion factor of 6.25, derived from the assumption that all proteins contain 16% nitrogen (1/0.16 = 6.25) and all measured nitrogen originates from protein. However, feed ingredients also contain non-protein nitrogen, and the protein nitrogen content varies considerably depending on the amino acid composition (Jones, 1941), introducing errors of up to 20% in protein estimations (Mariotti et al., 2008). Furthermore, knowing only the true protein content is insufficient for fish feed optimization. Determination of amino acid apparent digestibility coefficients (ADCs) is also required to assess how much of each amino acid is actually available to the fish. Therefore, this study aimed to evaluate the amino acid ADCs in rainbow trout (Oncorhynchus mykiss) fed on six high quality protein ingredients (fishmeal, blood meal, wheat gluten, maize gluten, soy protein concentrate, and pea protein concentrate), and obtain more accurate nitrogen-to-protein conversion factors from the complete amino acid profiles (Ka) and from the Kjeldahl-N content (Kp).
Materials and methods
Six isoenergetic diets (25 MJ kg-1) were formulated: a fishmeal-based reference diet and five test diets where 25% of dietary protein was replaced by blood meal, wheat gluten, maize gluten, soy protein concentrate, or pea protein concentrate. Rainbow trout (~150 g) were distributed into 21 tanks (n=3) and fed the experimental diets for seven weeks. Fish were kept in a freshwater recirculating aquaculture facility (RAS) with optimal temperature, oxygen, and pH for rainbow trout. At the end of the trial, total biomass, individual weight, and fork length were recorded, and feces were collected by manual stripping. All twenty amino acids were quantified in feces, diets, and raw ingredients by UPLC to derive amino acid ADCs.
Results and discussion
ADCs of most amino acids differed significantly among ingredients. Wheat gluten showed the highest overall amino acid digestibility (98%), while soy protein concentrate showed the lowest (88%), suggesting differences in anti-nutritional factor content and protein structure among plant-based sources. Cysteine showed the greatest variation among ingredients, highlighting the importance of sulphur amino acid availability in precision feed formulation. Both Kp and Ka were considerably lower than the commonly applied conversion factor of 6.25, and while Kp did not differ significantly among ingredients, Ka did. However, although more precise, the use of Ka requires full amino acid profiling which is both costly and time-consuming. In conclusion, the study showed that the commonly applied nitrogen-to-protein conversion factor of 6.25 largely overestimates the protein content in aquafeed protein sources and that more accurate estimates can be obtained using ingredient-specific conversion factors, with Ka providing the most precise representation of true protein based on amino acid composition, while Kp offers a practical alternative for routine application when full amino acid profiling is not feasible.
Acknowledgment
This study has been carried out under the KVÆLSTOP project, co-funded by the European Union through the European Maritime, Fisheries and Aquaculture Programme (EMFAF).
References
Jones, D. B. (1941). Factors for converting percentages of nitrogen in foods and feeds into percentages of protein. United States Department of Agriculture, (183).
Mariotti, F., Tomé, D., & Patureau Mirand, P. (2008). Converting Nitrogen into Protein—Beyond 6.25 and Jones' Factors. Critical Reviews in Food Science and Nutrition, 48(2), 177–184. https://doi.org/10.1080/10408390701279749