Rice protein concentrate meal as a potential ingredient in practical diets for rainbow trout (Oncorhynchus mykiss)

Rice protein concentrate (RPC) is a good raw material for fish nutrition due to its high protein and lipid content. The aim of this research was to evaluate this source of nutrients as a potential substitute for fish meal in rainbow trout diets. The RPC was characterized on the basis of proximate analyses, fatty acid (FA) profile and amino acid composition and then three experimental diets were formulated to be isoproteic (CP 47%) and isoenergetic (22 MJ kg-1 DM) with increasing levels of RPC (20%, 35% and 53%) tested against a control diet without RPC (RPC0). An in vivo digestibility experiment was performed in order to determine the apparent digestibility coefficients (ADC(s)) of the diets using the indirect acid-insoluble ash (AIA) method. Statistical differences appeared among all the diets as far as the dry matter, organic matter, crude protein and gross energy ADC are concerned with a decrease as the RPC level increase. No differences appeared for the ether extract ADC, but a decreasing trend was observed when the RPC level increased. Successively, a feeding trial was performed on triplicate groups of 30 fish (mean weight: 62.4 g) fed experimental diets for 94 days. At the end of the trial, the growth performance traits were determined and somatic indexes were calculated on five fish per each replicate. The diets and dorsal muscles were analysed for the proximate composition, gross energy and FA profile. The main results of the present experimentation are that the FA composition of the fillets did not show any difference between the control group and the fish fed with the RPC diets. Moreover, RPC can be used at an inclusion level of up to 20% without a growth performance reduction in rainbow trout feedstuffs, leading to a saving on fish meal protein. Inclusion of higher levels than 20% resulted in an almost linear reduction in the ADCs of the nutrients and energy and this was mirrored in the growth performance of the fish. Although the level of lysine was marginally limiting in the RPC53 diet, the EAA profile of RPC seems quite good, relative to trout requirements, and there was no major adverse effect of RPC on feed palatability; hence, reduced growth performance was mostly a consequence of reduced digestible nutrient and energy intakes as the RPC level was increased in the diet.