Results of their study, published in the journal Aquaculture, showed the inclusion of rapeseed acid and re-esterified oils along with a 5% of fish oil in aqua feeds does not seem to have negative effects on fat and fatty acid digestibility, growth, plasma parameter or morphology of liver and intestine in rainbow trout.
“However, before recommending their use, further studies regarding their effects on the final composition and quality of fillets should be carried out,” said the authors, based at Skretting ARC in Norway and the University of Barcelona.
Several vegetable oils have been suggested for use to replace fish oil, researchers said. However, competition for plant-based oils, like rapeseed oil, means that other “suitable and economically interesting” alternative oils are needed.
The authors said there is, as a result, growing interest from the feed industry in the by- and co-products generated during crude vegetable oil (VO) processing: “Acid oils from the chemical refining of [vegetable oil] VO, a free fatty acid (FFA)-rich by-product, were found to be quite promising for feeding use,” said the researchers.
Good results in fat absorption and growth performance in broiler chicks and chickens have been obtained when including re-esterified VO in diets (Vilarrasa et al., 2014 and Vilarrasa et al., 2015), noted the team.
While the digestibility of acid and re-esterified oils has been evaluated previously in rainbow trout, growth performance and productive parameters have not been, said the team.
One of the objectives of this study then was the evaluation of the growth performance and the feed utilization of rainbow trout fed acid and re-esterified rapeseed oils in comparison with those of fish fed native rapeseed oil.
Because diet composition could induce changes in specific plasma haematological and biochemical parameters, the team also assessed the effects of the inclusion of these oils on plasma biochemical parameters and the morphology of the liver and the intestine in trout.
The 72-day feeding trial used 576 rainbow trout and eight experiments diets, said researchers. All diets were 45% protein and 21% lipid and had the same ingredients except for the lipid source, researchers said.
Three different types of rapeseed oil — native (RNO), re-esterified (REO) and acid (RAO) — were included in the diets alone — RN, RE or RA — or blended in graded levels — diet RE/RA: 66% RE-33% RA; diet RA/RE: 66% RA-33% RE; diet RN/RA: 66% RN-33% RA and diet RA/RN: 66% RA-33% RN —in a proportion of 15%.
And 5% of commercial fish oil (FO) was included in all the experimental diets.
A control diet (F) included 20% fish oil.
Daily feed intake was noted, researchers said. On day 60, fish were measured, weighed and a fecal sample was collected for analysis of yttrium oxide, total fat, fatty acid composition and gross energy.
On day 72, blood samples were taken and five fish per tank were euthanized and weighed. Liver and viscera were examined for weight and a histological examination was done.
The diets were accepted and had apparent digestibility coefficients (ADC) of fat and total fatty acids at or above 90%, higher than fishmeal, said researchers.
The RAO diet had the lowest ADC for the single lipid diets, however diets including both RAO and RNO gave similar values, falling between those for RAO and RNO, researchers said. “Similarly, no differences in total fatty acid ADC were observed as REO was replaced by RAO (RE/RA, RA/RE),” they added.
“No significant differences were obtained among the final weights of fish fed RN (393.7 ± 6.1 g), RA (375.9 ± 2.9 g) and RE (381.5 ± 11.1 g),” they said. “Those of RA and RE were, in turn, significantly lower than that of F (411.1 ± 3.3 g).”
Results for both weight gain and the conversion factor followed the same trend, they said. There was no clear trend regarding the level of triacylglycerol found in plasma, and fish getting the RE/RA diet had higher levels than those getting the control diet.
“Fish fed diets RN and RE had significantly lower LDL-cholesterol levels (114.67 ± 4.30 mg dl− 1 and 125.48 ± 10.98 mg dl− 1, respectively) than those fed F (201.67 ± 16.36 mg dl− 1),” they added.
There were no differences observed in the morphology of liver or intestine for any of the diets, noted the researchers.
But they said further work is needed to see how the diets would influence fish development over a longer period of time.
The authors concluded: “Results from the present study indicate that rainbow trout fed diets including RAO and REO showed acceptable fat and fatty acid digestibility, with no relevant changes in plasma parameters or in the morphology of liver and intestine.
However, growth of fish fed these two diets did not reach that obtained in fish fed F, while growth of fish fed diets including a blend of RAO and REO improved when combined with REO at both 33% and 66% levels of inclusion. Therefore, the rapeseed acid oil, which is the most economically advantageous, yields better growth results when blended with the re-esterified oil.
It has to be taken into account that the inclusion of these oils should be done with a minimum proportion of 5% of FO in diets.”
Title: Acid and re-esterified rapeseed oils as alternative vegetable oils for rainbow trout diets: Effects on lipid digestibility and growth
DOI: published online ahead of print: 10.1016/j.aquaculture.2015.09.021
Authors: C. Trullàs, R. Fontanillas, A. Tres, A.C. Barroeta, R. Sala