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Aquaculture Nutrition 2008 14; 174–180 doi: 10.1111/j.1365-2095.2007.00517.x .............................................................................................. Growth and intestinal morphology in cobia (Rachycentron canadum) fed extruded diets with two types of soybean meal partly replacing fish meal O.H. ROMARHEIM1,2, C. ZHANG2, M. PENN1,3, Y.-J. LIU4, L.-X. TIAN4, A. SKREDE1,2, Å. KROGDAHL1,3 & T. STOREBAKKEN1,2 1 Aquaculture Protein Centre, CoE, Norway; 2 Department of Animal and Aquacultural Sciences, Norwegian University of Life Sciences, Ås, Norway; 3 Department of Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary Science, Oslo, Norway; 4 School of Life Sciences, Institute of Aquatic Economic Animals, Sun Yat-Sen University, Guanzhou, Guangdong, China Abstract Juvenile cobias, Rachycentron canadum, were fed extruded diets containing toasted defatted soybean meal (SBM) or untoasted defatted SBM [white flakes (WF)] to study growth and feed conversion, and to study if SBM induces morphological changes in the gastrointestinal (GI) tract. Three diets were produced: a fish meal-based control diet (FM diet) with 558 g FM kg)1, and two diets with 335 g FM and either 285 g SBM kg)1 (SBM diet) or 285 g WF kg)1 (WF diet). The diets were extruded at approximately 120C with 280 g kg)1 moisture. Triplicate groups of cobias (mean weight: 25.9 g) were fed the diets during 6 weeks. Feed intake of the FM and SBM diets were not significantly different, whereas the cumulative feed intake of cobias fed the WF diet was lower (P < 0.05) than that of cobias fed the FM and SBM diets after the first 21day period. Specific growth rate and feed conversion ratio were not significantly different between cobias fed the FM and SBM diets, but significantly poorer results were obtained in cobias fed the WF diet. No morphological differences in the GI tract could be attributed to the diets, and cobias fed soy did not develop enteritis in the distal intestine. KEY WORDS: cobia Rachycentron canadum, distal intestine, growth, histology, soybean meal, white flakes Received 15 September 2006, accepted 10 April 2007 Correspondence: Dr Trond Storebakken, Aquaculture Protein Centre, PO Box 5003, N-1432 Ås, Norway. E-mail: [email protected] Introduction The cobia, Rachycentron canadum, is a warm-water pelagic fish widely distributed in tropical/subtropical waters, except in the central and eastern Pacific Ocean. It is recognized for its fast growth and good meat quality, and has been intensively farmed since the 1990s (Liao et al. 2004). World fish meal (FM) production has been relatively stable during the past 20 years, except in El Niño years, whereas fish feed production has grown rapidly (Tacon 2004; Shepherd et al. 2005). Therefore, access to a stable supply of suitable, lowpriced dietary protein sources is a major challenge for further expansion of intensive fish farming. Commercial feeds for growing cobias contain approximately 450 g kg)1 protein (Liao et al. 2004) and recent studies have concluded that approximately 400 g kg)1 of the FM can be replaced by soybean meal (SBM) in diets for juvenile cobias without a reduction in growth (Chou et al. 2004; Zhou et al. 2005). Furthermore, Zhou et al. (2004) found that the apparent digestibility of protein from SBM was above 90% in juvenile cobias determined with feces collected in settling columns, which also reflected high apparent digestibility values of the individual amino acids. However, neither Chou et al. (2004) nor Zhou et al. (2004, 2005) have included measurements of antinutritional factors present in the soybean-containing diets, which may cause problems for carnivorous fishes (reviewed by Storebakken et al. 2000; Francis et al. 2001). Toasting or extrusion may inactivate most of the heat labile antinutrients in soybeans (Björck & Asp 1983; Qin et al. 1998; Clarke & Wiseman 1999; Romarheim et al. 2005). Even small amounts of protease inhibitors have been shown .............................................................................................. 174 2007 Blackwell Publishing Ltd Cobia fed two types of soybean meal to influence protein digestibility and intestinal proteolytic activities in rainbow trout, Oncorhynchus mykiss (Krogdahl et al. 1994), and work should therefore be carried out to establish the acceptance level of protease inhibitors in the cobia. In salmonid fish, another major concern with soybean products are the morphological alterations found in the distal intestine (DI) (van den Ingh et al. 1991; Rumsey et al. 1994; Baeverfjord & Krogdahl 1996). The morphological changes in salmonids are probably caused by alcohol-soluble components (van den Ingh et al. 1996; Bureau et al. 1998) that have yet to be conclusively identified. The main objectives of the present study were to: (1) evaluate performance of juvenile cobias fed extruded high-fat diets with inclusion of either SBM or white flakes (WF), and (2) find out if inclusion of soybean products induces morphological changes in the intestine of the cobia. Materials and methods Diets The protein feedstuffs utilized in this study were FM (Norse LT-94; Norsildmel, Bergen, Norway), conventional defatted and toasted SBM (Deno-Soy F; Denofa, Fredrikstad, Norway) and untoasted defatted SBM (WF, Prosam F; The Solae Company LLC, Esteio, Brazil). The control diet (FM diet) contained high-quality FM as the only protein feedstuff. In the two other diets, 400 g kg)1 of the total amino acids from FM were replaced by either SBM (SBM diet) or WF (WF diet) (Table 1). Substitution was made on the basis of the analysed total amino acid contents of the FM, SBM and Table 1 Formulation and analysed chemical composition of diets FM diet SBM diet WF diet )1 Main ingredients, g kg Fish meal Soybean meal White flakes Wheat Fish oil Vitamin and mineral mixture1 Dry matter (DM), g kg)1 Chemical composition, g kg)1 DM Crude protein Crude fat Starch Ash Gross energy, MJ kg)1 DM Trypsin inhibitor activity2, mg g)1 DM Phytic acid, mg g)1 DM 1 2 558 – – 237 200 4.97 917 335 285 – 176 200 4.97 922 335 – 285 176 200 4.97 916 442 293 146 94 24.3 0.3 1.5 446 279 109 80 23.2 0.7 3.2 433 279 102 79 22.7 2.5 3.8 WF (presented by Romarheim et al. 2005). Fish oil (Silfas, Karmsund, Norway) was added in the same amount to all diets, whereas the inclusion of wheat was reduced in the SBM and WF diets to compensate for the lower content of total amino acids in SBM and WF compared with FM. The chemical composition (g kg)1 DM) of the diets were: 433– 446 g kg)1 crude protein, 279–293 g kg)1 crude fat, and 146, 109 and 102 g kg)1 starch in the FM, SBM and WF diets, respectively. The diets were extruded at the Center for Feed Technology, Ås, Norway, during December 2003. The diets were produced at approximately 120C with 280 g kg)1 moisture, dried, coated with fish oil and stored cold until feeding (see Romarheim et al. 2005 for details). Fish, rearing conditions and feeding Juvenile cobias, R. canadum, were supplied by a local fry dealer in Zhanjiang, Guangdong, China. The cobias were cultured in floating sea-cages, then moved to circular tanks where they were fed a commercial extruded feed (Evergreen Feed Company, Zhanjiang, Guangdong, China) during the adaptive period prior to the experiment. Two weeks before the start of the experiment, the fish were transferred to 1-m3 circular tanks on land. They were further allocated into groups of 24 fish (average weight: 25.9 g) per tank (mean total weight per tank ± SD, 622.0 ± 34 g) 6 days before the start of the experiment. The feeding trial started on 16 June 2004, and each diet was fed to triplicate groups of fish for 42 days. The tanks were supplied with flow-through seawater (10 L min)1) at 28–30C and 32.5 g L)1 salinity. Oxygen saturation was kept at approximately 7 mg L)1 in the outlet water. A 5-day pre-trial revealed that cobias (average weight: 13.7 g) fed twice per day had nearly twice the weight increase of fish fed only once per day. Thus the fish were fed by hand two meals per day, starting at 09:00 and 16:00, which also was consistent with previous feeding experiments by Chou et al. (2001, 2004). Each meal consisted of two to three feeding rounds to satiety. Weighing, sampling and chemical analyses For details, see Romarheim et al. (2006). Inhibited bovine trypsin. .............................................................................................. 2007 Blackwell Publishing Ltd Aquaculture Nutrition 14; 174–180 The fish were weighed at the start of the experiment, at day 21 and at the end of the experiment (day 42). One tank with cobias fed the WF diet and two tanks with cobias fed the SBM diet were affected by insufficient water supply the night before completion of the experiment. All fish in the affected WF diet tank died, whereas eight and 22 fish died in the SBM diet tanks. The fish that died were weighted, 175 176 O. H. Romarheim et al. and those that survived were kept until sampling at the termination of the experiment the day thereafter. All fish were anaesthetized with 60 mg MS 222 L)1 prior to weighing. Random samples of the diets were collected and analysed. At the end of the experiment, fish from each tank were taken and samples from the gastrointestinal (GI) tract were taken for histological evaluation. The sample numbers for histology were: FM, n = 9; SBM, n = 8 and WF, n = 6 as the third replicate SBM tank only had two live fish left for sampling and the third replicate WF tank had none. The fish were opened and a 5 · 5 mm tissue sample was taken from the following GI sections (Fig. 1): stomach (ST), central part of the pyloric caecum (PC), proximal mid-intestine (MI1, 1=3 the distance from the pyloric caeca to the intestinal constriction), mid-intestine (MI2, 2=3 the distance between the pyloric caeca and the intestinal constriction), DI (from the constriction to the anus). Samples were fixed in buffered formalin. The tissues were subsequently dehydrated in ethanol, equilibrated in xylene and embedded in paraffin according to standard histological techniques (National Veterinary Institute of Norway, Oslo, Norway). Sections of approximately 5 lm were cut and stained with haematoxylin and eosin (H&E) and examined by light microscopy. The intestinal morphology was evaluated according to criteria described by Baeverfjord & Krogdahl (1996) for SBMinduced enteritis in Atlantic salmon, Salmo salar. The criteria included (1) widening and shortening of the intestinal folds; (2) loss of the supranuclear vacuolization in the absorptive cells (enterocytes) in the intestinal epithelium; (3) widening of the central lamina propria within the intestinal folds, with increased amounts of connective tissue and (4) infiltration of a mixed leucocyte population in the lamina propria and submucosa. The diets were analysed for dry matter, crude protein (Kjeldahl N · 6.25) and ash according to the methods of the AOAC (1990), crude fat after hydrolysis with petroleum Figure 1 Illustration of sites of tissue sampling for histological analysis. ST, stomach; PC, pyloric ceca; MI1, proximal mid-intestine; MI2, distal mid-intestine; DI, distal intestine. ether on an Accelerated Solvent Extractor (ASE200) from Dionex (Sunnyvale, CA, USA), starch (Total Starch Assay Kit [AA/AMG]; Megazyme International Ireland Ltd, Wicklow, Ireland) as total glucose after hydrolysis with glucosidase, and gross energy (Parr 1281; Parr Instruments Company, Moline, IL, USA). The trypsin inhibitor activity was analysed according to Hamerstrand et al. (1981), and phytic acid was determined by HPLC according to the method of Carlsson et al. (2001). Calculations and statistical analyses Feed intake was estimated by subtracting feed collected at the outlet water from the amount of feed fed into each tank. Feed waste was calculated by counting the number of uneaten pellets multiplied by the average pellet weight of the diet. Specific growth rate (SGR) was calculated as SGR ¼ 100 ðln BW1 ln BW0 Þ d 1 where BW0 and BW1 represent the initial and final body weights, respectively, and d is the days of feeding. Feed conversion ratio (FCR) was calculated as FCR ¼ DMfeed ðBW1 BW0 Þ1 where DMfeed is the consumption of dry matter from the feed. Statistical analyses were carried out using one-way analysis of variance by the general linear model procedure in SAS (SAS 1988). The results for cumulative feed intake, growth and feed conversion are presented as least-square mean (lsmean) ± pooled standard error of the means (SEM), and significant (P < 0.05) differences among means were ranked by DuncanÕs multiple range method. Results Feed intake and growth Fish fed the FM and SBM diets had similar cumulative feed intake, growth and feed conversion throughout the experiment, whereas cumulative feed intake, growth and feed conversion of fish fed the WF diet were lower in both periods (Table 2). Cobias fed the WF diet were also observed to frequently reject pellets after tasting. Fish fed the FM and SBM diet increased initial weight by 365 and 346%, respectively, and fish fed the WF diet increased their initial weight by 201% during the whole experimental period. The SGRs in fish fed the FM and SBM diets in the second period were roughly half those of the first period, whereas in fish fed .............................................................................................. 2007 Blackwell Publishing Ltd Aquaculture Nutrition 14; 174–180 Cobia fed two types of soybean meal Table 2 Least-square means for cumulative feed intake, growth and feed conversion in juvenile cobia, Rachycentron canadum Diet FM diet Cumulative feed intake, g per tank 0–21 days 1156b 22–42 days 1476b 0–42 days 2632b Mean weight increase, g per fish 0–21 days 47.1b 22–42 days 48.8b 0–42 days 95.9b Specific growth rate 0–21 days 4.9b 22–42 days 2.4b 0–42 days 3.6b Feed conversion ratio, g DM intake 0–21 days 0.9a 22–42 days 1.2a SBM diet WF diet Pooled SEM P-value 1226b 1463b 2690b 975a 978a 1953a 33 87 93 0.005 0.011 0.002 49.4b 43.3b 92.6b 32.0a 17.4a 49.4a 5.0b 4.0a 2.1b 1.3a b 3.6 2.6a (g body weight gain))1 1.0a 1.2b 1.4a 2.3b 1.6 4.7 5.7 0.001 0.007 0.002 0.17 0.17 0.14 0.009 0.009 0.004 0.04 0.10 0.005 0.001 Different superscript letters within a line indicate significant (P < 0.05) differences among dietary treatments. the WF diet it was reduced to 1=3. FCRs increased by 32 and 43% in the second period in fish fed the FM and SBM diets, respectively, whereas it increased by 93% in the second period for fish fed the WF diet. One cobia fed the WF diet died during the first period, and two cobias fed the SBM diet died during the second period in addition to those that died in connection with the incident described previously. Intestinal histology H&E stained tissue sections from the GI tract were evaluated blindly, and morphological changes were noted. No changes in the GI tissues resembling those described for SBM-induced enteritis in the posterior intestine of Atlantic salmon were observed (Fig. 2). However, incidental findings that could not be related to diets were noted. Areas of focal leucocyte infiltration were noted in the submucosa/propria of the intestinal tissues, but did not appear similar to the diffuse infiltration of the submucosa and lamina propria seen in soybean enteritis. The width of the lamina propria of intestinal mucosal folds exhibited slight variation, but the increases in width did not appear to be a result of cellular infiltration or hyperplasia, but rather appeared non-cellular. The blood vessels within the lamina propria of intestinal mucosal folds also appeared more prominent than seen in salmonids. Vacuolization of intestinal epithelial cells, ranging from absent to highly vacuolated, varied between individuals. Discussion The lack of significant differences in growth and FCR when substituting 400 g kg)1 of the amino acids from FM by .............................................................................................. 2007 Blackwell Publishing Ltd Aquaculture Nutrition 14; 174–180 conventional SBM is consistent with previous findings with cobias by Chou et al. (2004) and Zhou et al. (2005). The lower feed intake of cobias fed the WF diet compared with cobias fed the FM and SBM diets was obviously the main reason for the poorer growth, and the low feed intake probably had a negative effect on the FCR as well. SGRs vary widely among experiments with cobia depending on factors such as size, rearing conditions and genetic variation. For example, cobias growing from approximately 6 to 126 g have obtained SGRs from 4.2 (Resley et al. 2006) to 6.6–6.7 (Wang et al. 2005). Cobias growing in the interval from approximately 12 to 86 g have obtained SGRs from 3.6 (Zhou et al. 2006) to 4.2 (Lunger et al. 2006), whereas others have obtained maximum SGRs of 2.4–2.6 (Chou et al. 2001) and 2.7 (Chou et al. 2004) for cobias growing from approximately 32 to 140 g. The FCRs for cobias fed the FM and SBM diets during the first period and the overall period were in line with several previous studies (Wang et al. 2005; Resley et al. 2006; Zhou et al. 2006), and slightly higher than obtained by Chou et al. (2001, 2004). Comparison of growth results with previous experiments with cobias is complicated as there often are considerable dissimilarities regarding genetic stock, rearing facilities, stocking density and feed composition. Growth and FCR in the present study were acceptable through the first 21 days, but rather poor during the last period of 21 days, in particular in cobias fed the WF diet. Higher age and larger size may explain some of the poor progress during the second period, but probably not all. The dietary crude protein level of 433–446 g kg)1 of DM was similar to the protein level regarded as optimal for maximum weight gain in juvenile cobias (Chou et al. 2001) and the level commonly used in commercial feeds for cobias in Taiwan (Liao et al. 2004). The fat level of 279–293 g kg)1 177 178 O. H. Romarheim et al. (a) (b) Figure 2 Distal intestinal histology (H&E) of cobia, Rachycentron canadum, fed (a) FM and (b) SBM diets (·40). C, circular muscular layer; S, submucosa; LP, lamina propria; MF, mucosal folds. of DM was close to twice the usual level in commercial cobia feeds in Taiwan (Liao et al. 2004). Chou et al. (2001) did not find any significant difference in growth, FCR or protein conversion ratio of juvenile cobia fed 54, 84, 123 or 189 g kg)1 crude lipids (on DM basis). However, Wang et al. (2005) reported significantly poorer feed intake and growth, but better FCR and protein efficiency ratio in juvenile cobia fed a diet with 261 g kg)1 crude lipids compared with 51 or 170 g kg)1 crude lipids (on DM basis). The high level of fat in our study may therefore have lead to a suboptimal digestible protein to digestible energy ratio (DP/DE), and thereby contributed to rather low feed intake and growth. Nevertheless, the crude protein and gross energy levels were relatively similar among the diets, and the internal ranking of the diets were therefore probably not affected by dietary fat level or DP/DE. Substitution of FM by SBM will decrease the dietary methionine level as well as increase the level of phytic acid, and may thereby impair growth and feed utilization (reviewed by Storebakken et al. 2000). Information about the requirement of essential amino acids for cobias is limited, but some studies have reported the requirement of methionine. Chou et al. (2004), using broken line analysis, estimated the dietary requirement of sulphur-containing amino acids (methionine + cysteine) to be 2.66 g 16 g)1 N for juvenile cobias. A more recent study by Zhou et al. (2006) estimated the optimum methionine level for maximum growth of juvenile cobias to be as high as 11.9 g kg)1 dry diet in the presence of 6.7 g kg)1 cysteine, which is equal to approximately 4.2 g 16 g)1 N sulphur-containing amino acids. The levels of sulphur-containing amino acids in our study where 3.31, 3.07 and 3.19 g 16 g)1 N for the FM, SBM and WF diets, respectively, and may therefore be limiting for maximum growth according to Zhou et al. (2006). A recent study by Denstadli et al. (2006) revealed that inclusion of 4.7 g kg)1 dietary phytic acid had no direct effects on feed intake, growth and feed conversion, or on the digestibility of the main nutrients in juvenile Atlantic salmon during an 80-day feeding trial. Both the SBM diet and the WF diet had phytic acid levels well below 4.7 g kg)1, and unless cobia is more sensitive than salmonids, growth performance or feed conversion in our experiment should not be influenced by phytic acid. Moreover, the level of phytic acid was similar in the SBM and WF diets. The poorer feed intake on the WF diet was most likely the result of low palatability or a negative physiological reaction as cobias fed the WF diet were observed to frequently reject caught pellets. The main difference between SBM and WF was that SBM, as opposed to WF, had been heat-treated (toasted) prior to extrusion. Although the heat treatment applied during the extrusion reduced the trypsin inhibitor activity in the SBM and WF diets to 0.7 and 2.5 mg g)1 DM, respectively, the WF diet contained 3.6 times as high trypsin .............................................................................................. 2007 Blackwell Publishing Ltd Aquaculture Nutrition 14; 174–180 Cobia fed two types of soybean meal inhibitor activity as the SBM diet. Most cultured fish species have a tolerance limit for trypsin inhibitor activity below 5 mg g)1 (reviewed by Francis et al. 2001). However, no studies have established the tolerance for trypsin inhibitor activity in the cobia, and the WF diet might exceed the tolerance for dietary trypsin inhibitor activity in this species. The results of the present study suggest that defatted SBM can be included in the diet with up to 285 g kg)1 without inducing morphological changes in the GI tract of juvenile cobias. This conclusion is consistent with studies of Atlantic halibut, Hippoglossus hippoglossus, fed 360 g kg)1 full-fat SBM (Grisdale-Helland et al. 2002), mangrove red snapper, Lutjanus argentimaculatus, fed 480 g kg)1 defatted SBM (Catacutan & Pagador 2004), and Atlantic cod, Gadus morhua, fed 250 g kg)1 defatted SBM (Refstie et al. 2006). In contrast, SBM gave mild morphological changes in the anterior intestine of Asian seabass, Lates calcarifer, fed 210–285 g kg)1 SBM (Boonyaratpalin et al. 1998) and mild loss of supernuclear vacuolization in the DI of channel catfish, Ictalurus punctatus, fed 450 g kg)1 defatted SBM (Evans et al. 2005). Salmonids are more sensitive and Atlantic salmon (S. salar) developed moderate changes in the DI with 76 g kg)1 defatted SBM and severe changes with 117 g kg)1 defatted SBM (Krogdahl et al. 2003), classified after Baeverfjord & Krogdahl (1996) as noninfectious subacute enteritis. Thus, the sensitivity to dietary SBM seems to be subject to interspecies variation. The sensitivity of cobia with regard to soy antigens remains to be elucidated. In conclusion, toasted SBM can be included in extruded diets for cobia with up to 285 g kg)1 diet without affecting feed intake, growth and feed conversion negatively, whereas WF promoted poorer results provided the present extrusion temperature. In contrast to salmonids, cobia seems not to develop SBM-induced enteritis in the DI. Acknowledgements The authors want to thank Guadong Evergreen Company for providing the experimental facilities, Solae (Esteio-RS, Brazil) for kindly donating the white flakes, and Mr Konrad Münz from Bühler (Uzwil, Switzerland) for expertise and help during the feed production. References AOAC (1990) Official Methods of Analysis, 15th edn. 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Zhou, Q.-C., Mai, K.-S., Tan, B.-P. & Liu, Y.-J. (2005) Partial replacement of fish meal by soybean meal in diets for juvenile cobia (Rachycentron canadum). Aquacult. Nutr., 11, 175–182. Zhou, Q.-C., Wu, Z.-H., Tan, B.-P., Chi, S.-Y. & Yang, Q.-H. (2006) Optimal dietary methionine requirement for juvenile cobia Rachycentron canadum. Aquaculture, 258, 551–557. .............................................................................................. 2007 Blackwell Publishing Ltd Aquaculture Nutrition 14; 174–180