Rapeseed cake as a feed ingredient for Nile tilapia : responses to replacing protein from soybean meal with rapeseed cake, and fine milling and autoclaving of the rapeseed cake

Abstract

This thesis consists of two mains parts: an introduction giving a literature review of key topics relevant for design and interpretation of the experiments carried out in this thesis, and an experimental part.

Recent reports from FAO have highlighted the need for increased utilization of more low cost material in food production industries. Aquaculture industry may play an increasingly important role in providing high quality food, and approximately half of the production costs are related to feed. It is, thus, becoming increasingly important to utilize non-food and low cost ingredients in fish feed. Nile tilapia is a major farmed fish species. Moreover, it has high capacity to tolerate a wide range of environmental stresses and the presence of antinutrients compounds in diet, making it an ideal target species for upgrading low-quality ingredients to high quality food. Secondary products from rapeseed oil processing are highly abundant and represent an inexpensive source of protein. The major challenge for increased use is the presence of certain glucosinolate derivatives such as isothiocyanate and progoitrin, which previously have been reported to cause metabolic problems or reduce feed acceptability for fish. Secondary products of rapeseed also contain other antinutritional factors such as phytic acid and tannins that may represent metabolic challenges. Several of these factors can be reduced by relatively intense moist heating. The overall aim of the research in this research was to find out if simple processing such as fine milling or moist heating influenced the nutritional value of rapeseed cake (RSC), a secondary product from rapeseed oil processing. Two experiments were carried out. The first experiment aimed at defining a dietary inclusion level to which Nile tilapias were sensitive to changes in nutritional quality of RSC. The second experiment was carried out to assess the effects of fine milling or the combination of fine milling and autoclaving of RSC. The first experiment was designed on the base of a regression analysis to define the dose response of tilapia to inclusion of rapeseed cake (RSC) in diet. Five different isoenergetic and isonitrogenous, plant ingredient-based diets were produced with different level of inclusion of RSC. Crude protein from soybean meal (SBM) was gradually replaced by crude protein from RSC at 0, 25, 50, 75 and 100 % of replacement. The feeding trial was performed in two replicate tanks of Nile tilapia for each experimental diet. Each tank contained 20 tilapias with average weight of 19.9 g. Feeding was in excess, 3 times (40 min) per day. Feed was quantified on a daily basis for the first 3 weeks, and as a pooled value over the whole 6 week feeding period. The findings from Exp.1 demonstrated a decline in feed intake and growth along with increasing the level of RSC in diet. A threshold effect was observed in the regression curve near of 50% replacement. A possible explanation for these results may be the presence of bitter component in RSC which caused poor palatability of RSC containing diet and reduces feed intake. The feed conversion ratio (FCR) (g DM intake (g gain)-1) was almost close to 1 g g-1 for all levels, except for the groups feed 100% replacement of CP from RSC, where it was slightly elevated. No diet-related trends were observed apparent digestibility or retention of crude protein, mineral (Ca, P, Mg, Mn, Zn) absorption or concentration in blood plasma, energy content of whole body and energy and protein retention along with different inclusion of RSC in diet, indicating that protein from RSC and SBM had comparable availability, and that the effects of phytic acid from the two protein sources had comparable effects. Moreover, thyroid hormone (T4) in blood plasma was not markedly different for different treatments, indicating that glucosinolate derivatives may not have been a main factor in explaining the reductions in feed intake and growth. The lipid content of whole body composition decreased in treatments fed diet from 0% until 50% replacement and then increases up to 100% replacement of CP from RSC with CP from SBM. The same pattern was seen in content of DM which can be the result of lipid content of body.

It is assumed that presence of higher content of tannin in RSC on the base of feed intake pattern causes higher visceral fat deposition in fish after 50% replacement. The ration of liver weigh to body weight was increased from 0% until 50% replacement level and after that tended to reduce up to 100% replacement of CP from RSC with CP from SBM. Since the deposition of lipid in liver decreases by increasing content of tannin in diet, the pattern given from ratio of liver weight to body weight may be caused by this fact.

The aim of second experiment (Exp.2) was to assess whether fine milling and/or combination of fine milling and autoclaving the RSC applied in diet may affect the nutritional quality of feed for tilapia. Exp.2 was performed according to the results from Exp.1, on the 50% level of replacement of CP from SBM with CP from RSC in diet which causes more sensitivity in fish to nutritional quality of diet. This experiment was designed on the base of ANOVA analysis. A 3 weeks trail feeding tilapia was conducted with 3 different experimental diets. The RSC used in different experimental diets were 1mm ground (the same as Exp.1), milled to 0.5 mm of particle size, or milled to 0.5 mm and autoclaved for 10 min in 120oC. Each diet fed to tilapia in 3 replicate tanks. Each tank contained 20 fish with the average weight of 37.3 gr. Feeding and monitoring of daily feed intake was the same as Exp.1.

Feed intake and gain were significantly (P<0.05) decreased by autoclaving the RSC. FCR was close to 1 g DM g-1 gain for all treatments (P>0.05). This may be due to autoclaving having a negative effect on the palatability of RSC due to production of glucosinolates breakdown products. A linear relationship was seen between feed intake and gain which may demonstrate that the main reason of growth depression is related to decrease of FI. The finding of increased glucose concentration in the diet with autoclaved RSC, probably originating from hydrolysis of glucosinolates, supports this hypothesis. A significant decrease (P<0.05) in content of DM and crude protein, and energy and nitrogen retention in whole body was seen in fish fed the diet containing autoclaved RSC.

It can, thus, be assumed that fine milling increased availability of components that produced in fine milled and autoclaved RSC may have negative effect on tilapias metabolism.

The content of whole body lipid, ash and energy did not show any significant difference (P>0.05) among different treatments. Also the content of minerals (except zinc) in blood plasma among different treatments fed different experimental diets was the same (P>0.05). However, the content of zinc in blood plasma of tilapia fed with diet containing fine milled and fine milled and autoclaved RSC tended (0.050.05). It may prove that the certain level of inclusion of secondary compounds of RSC used in these diets may not have any goitrogenic effect on tilapias thyroid.

To conclude, this research show that presence of RSC in tilapia diet may reduce feed acceptability. However, it did not show different effects on metabolic function of fish than those caused by SBM. Fine milling did not affect the nutritional value of RSC, while autoclaving of RSC has negative effect on feed intake, energy utilization, and consequently on growth.