Dietary protein level can be REDUCED WITHOUT affecting amino acid utilization efficiency for growth of Atlantic salmon post-smolts
Introduction
Crude protein (CP) is rarely a true reflection of the true protein and amino acid composition of the diet; representing nothing more than a mixture of different nitrogen (N) containing compounds in different proportions. Fish, like other animals, require a specific quantity and balance of the dietary essential amino acids (EAA) rather than CP per se. The development of more sustainable and cost-effective aquafeeds requires the optimization of dietary amino acid (AA) profile; thus increasing the efficiency with which the AA are used for synthesis of protein and reducing N release into the environment. This study constitutes a first attempt to evaluate the feasibility of reducing protein level in salmon post-smolts diets using the Requirements by Ration Level technique (Helland et al 2010) to evaluate two diets differing in CP content but similar in EAA/Lys ratios. The efficiencies of AA utilization for gain above maintenance were determined by feeding low (LP; 44%) or high (HP; 50%) CP diets with increasing ration levels.
Material and methods
Two isoenergetic diets (23.6 kJ/g feed) having low (44%) or high (50%) CP levels (as fed) were fed at different ration levels to Atlantic salmon post-smolts at Nofima, Averøy, Norway. EAA were added to each diet to keep similar EAA/Lys ratios between diets. Yttrium oxide was included for digestibility determinations. Groups of 233g post-smolts were weighed and randomly allocated to sixteen 125-m3 cages. The fish was fed using automatic feeders, once per hour during daylight, for 7 weeks. Feed intake was estimated daily based on collection of waste feed from each cage. For each diet, triplicate groups were fed in excess (100% ration) to estimate maximum intake, single cages were fed at levels approximating 40, 50 and 75% of the intake of the full-fed groups, and two other groups were fed at levels approximating 25% of the intake of the full-fed groups. Differences between treatments were considered significant at the P<0.05 level.
Results
At the end of the 7 weeks feeding trial, full-fed groups had doubled their body weights. Reduction of dietary protein level from 50 to 44% did not affect final body weight, specific growth rate, protein efficiency ratio or nutrient gain, including protein and EAA. But when compared with HP, LP diet resulted in increased feed intake and poorer feed conversion. Linear relationships were found between digestible protein intake and weight gain, protein gain and EAA gain, including lysine and methionine+cysteine (Figure 1). At similar EAA/Lys ratios between diets reduction of dietary protein concentration had no negative effective on the utilization efficiencies of protein (0.65 vs 0.64), lysine (0.74 vs 0.67), methionine+cysteine (0.73 vs 0.70) or any other EAA.
Discussion and conclusion
Due to the increasing cost of protein-rich feedstuffs and environmental issues and regulations associated with animal production, considerable efforts have been focused on the possibility of reducing CP levels in diets for livestock and more recently in diets for fish and shrimp. Although aquaculture seems more eco-efficient than pig or poultry production in providing nutrients for human consumption (Ytrestøyl et al. 2012), the move towards a lower N output is a major sustainability drive in fish farming, both for environmental and economic reasons. The fact that final body weight, specific growth rate, protein efficiency rate or nutrient gain, including protein and EAA gain, were not affected by reducing CP from 50 to 44%, confirm that salmon as other animals have a requirement for EAA and not for dietary protein level. At similar EAA/Lys ratios between diets, reduction of dietary protein concentration had no negative effect on the utilization efficiencies of protein, lysine, methionine+cysteine or any other EAA. Our results are in general agreement with studies with rainbow trout showing that diets supplemented with all the limiting EAA allow for the reduction of CP without compromising performance and even improving protein retention efficiency (Yamamoto et al. 2005; Gaylord and Barrows 2009). These achievements would not have been possible if free AA were not utilized as efficiently as protein-bound AA in meeting the requirements of fish. But the exact application of ideal protein concept when formulating practical diets, containing low levels of high quality and AA-balanced raw materials such as fish meal, and protein-rich raw material limiting in one or more EAA, and providing others in excess, is at the present time very difficult or even impossible. The next step will be to evaluate the possibility of reducing CP in practical diets for salmon by using only commercially available, free AA to balance the dietary AA profile.
References
Gaylord, T. G. and F. T. Barrows (2009): Multiple amino acid supplementations to reduce dietary protein in plant-based rainbow trout, Oncorhynchus mykiss, feeds, Aquaculture 287: 180-184.
Helland, S. J., B. Hatlen, B. and B. Grisdale-Helland (2010): Energy, protein and amino acid requirements for maintenance and efficiency of utilization for growth of Atlantic salmon post-smolts determined using increasing ration levels, Aquaculture 305: 150-158.
Yamamoto, T., T. Sugita, and H. Furuita (2005): Essential amino acid supplementation to fish meal-based diets with low protein to energy ratios improves the protein utilization in juvenile rainbow trout Oncorhynchus mykiss, Aquaculture 246: 379-391.
Ytrestøyl, T., T. S. Aas, E. Skontorp Hognes, F. Ziegler, V. Sund, B. Hatlen, B. Ruyter, G. M. Berge, M. Thomassen, M. Sørensen, and T. Åsgård (2012): Eco-efficiency and resource use in Norwegian salmon farming, XV International Symposium on Fish Nutrition and Feeding, June 4-7, Molde, Norway: O2.
The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 262336
