Effects of Dietary Replacement of Fish Meal by Raw and Fermented Soybean Residues on Growth Performance, Biological Parameters and Nutrient Digestibility in Red Tilapia Fish (Oreochromis sp.)

Hung Phuc Nguyen1*, Doan Van Thuoc2*, Nguyen Thi Trung Thu1,

Huong Tran Thi Mai3, Nguyen Tran Khanh Hoa1, Nguyen Thi Tuyet Nhi1, Nguyen Phuong Thao1, Tran Thi Loan2 and Nguyen Thi Huyen My2

1Department of Human and Animal Physiology, Faculty of Biology, Hanoi National University of Education, Caugiay 11310, Hanoi 10000, Vietnam

2Department of Biotechnology and Microbiology, Faculty of Biology, Hanoi National University of Education, Caugiay 11310, Hanoi 10000, Vietnam

3Centre for Aquaculture Biotechnology, Research Institute for Aquaculture No.1, Tuson 16352, Bacninh 16000, Vietnam

ABSTRACT

This study was conducted to examine the effects of dietary replacement of fish meal (FM) by raw soybean residue (SR) and fermented soybean residue (FSR) on growth performance, biological parameters and nutrient apparent digestibility coefficients (ADCs) of red tilapia (Oreochromis sp.). FSR was obtained by fermenting SR with Bacillus subtilis V37. Five isonitrogenous and isoenergetic diets were formulated to replace 35% or 50% of FM by SR or FSR. The diets were denoted as follows: FMD, SR35D, FSR35D, SR50D, and FSR50D. The FMD (the reference diet) contained FM as a main source of dietary protein. A total of 300 fingerling red tilapia with an initial body weight (BW) of 13.7 g were randomly distributed into 15 tanks (20 fish/tank, 3 tanks/dietary treatment) and fed the experimental diets twice daily, for 8 weeks. Results showed that fish fed SR35D and SR50D had significantly lower final BW, weight gain (WG) and specific growth rate (SGR), but higher feed conversion ratio (FCR), than fish fed FMD (P < 0.05). Meanwhile, no significant differences in growth performance and feed utilization between the FSR35D and FMD groups were detected. Fish fed FSR-included diets showed significantly better growth performance and FCR than those fed SR-included diets (P < 0.05). Feeding the fish with SR35D and SR50D reduced digestive enzyme activity, bile juice secretion, and nutrient ADCs. These parameters of the experimental fish were markedly improved by feeding with FSR35D and FSR50D, and no statistical differences were observed between FSR35D- and FMD-fed fish. The results of the current study indicated that SR interfered with digestive enzyme activity, bile juice secretion, nutrient ADCs, and growth and feed performances. The enhancements of these parameters in fish fed FSR-included diets suggested that fermentation of SR with Bacillus subtilis V37 benefited digestive physiology, growth performance and feed utilization of red tilapia fed soybean protein-based diets.

Article Information

Received 15 October 2021

Revised 27 April 2022

Accepted 11 May 2022

Available online 27 July 2022

(early access)

Published 28 July 2023

Authors’ Contribution

HPN, DVT, NTTT and HTTM involved in conception and design of study, performed experimental work and drafted the manuscript. NTKH, TNTN, NPT, TTL and NTHM involved in sampling, analysis and interpretation of data.

Key words

Soybean residue, Fermentation, Growth, Digestion, Red tilapia

DOI: https://dx.doi.org/10.17582/journal.pjz/20211015111004

* Corresponding author: hungnp@hnue.edu.vn, thuocdv@hnue.edu.vn

0030-9923/2023/0005-2065 $ 9.00/0

Copyright 2023 by the authors. Licensee Zoological Society of Pakistan.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Introduction

Fish meal (FM) is commonly used as the main protein source in aquafeeds; however, the limited supply of this ingredient and the increase of aquaculture industry require alternatives for replacement of FM (Olsen and Hasan, 2012). Soybean residue (SR), so called okara, is a by-product left from ground soybean after extracting water-soluble components used to produce soymilk or soybean curd (Noguchi, 1987). SR is relatively rich in nutrients with large quantities produced every year (O’Toole, 1999), therefore, this meal can be a potential alternative to replace FM in aquafeeds. On dry matter basis, contents of crude protein, crude lipid, crude fiber, nitrogen free extract, and ash of SR range from 15.2 to 32.2%, 6.9 to 10.9%, 9.1 to 18.6%, 27.5 to 46.3%, and 3.0 to 4.5%, respectively (Bourne, 1976; Van der Riet et al., 1989; O’Toole, 1999; Li et al., 2012).

Soybean reportedly contains anti-nutritional factors, such as β-conglycinin, glycinin, trypsin inhibitors, stachyose, raffinose, saponins, lectins, and phytate, which negatively influence growth performance, feed utilization, and physiological conditions of aquatic animals, including fish species (Francis et al., 2001; Krogdahl et al., 2003; Iwashita et al., 2008). Since SR is the residue obtained after removal of water-soluble fractions from soybean, this ingredient may contain substances which induce detrimental effects on growth and feed performances and physiological conditions in fish. It has been suggested that fermentation is an effective method to decrease the amount of anti-nutritional factors and improve the nutritional quality of soybean meal (Hong et al., 2004; Feng et al., 2007). Some studies have demonstrated that feeding fish with fermented soybean meal enhances growth, FCR, nutrient ADCs, and digestive physiology as compared to unfermented soybean meal (Yamamoto et al., 2010; Nguyen et al., 2015; Wang et al., 2019; He et al., 2020). These findings suggest that fermentation of SR and inclusion of fermented SR (FSR) in aquafeeds may be beneficial to body growth, feed utilization and physiological conditions of fish.

Red tilapia (Oreochromis sp.) is one of the most preferred and produced fish in aquaculture in Asia due to its fast growth rate, good meat quality and color, and high market demand (Boerlage et al., 2017). To date, there have been no studies to replace FM by SR or FSR in diets for this fish. Therefore, the current study aimed to examine the effects of dietary replacement of FM by SR or FSR on growth and feed performances, biological parameters, and nutrient ADCs of red tilapia.

Materials and Methods

Soybean residue fermentation

Commercially available SR (crude protein content 17%) was purchased from Vinasoy Corp (Tu Son, Bac Ninh, Vietnam). This meal was fermented with Bacillus subtilis V37 provided by Department of Biotechnology and Microbiology, Hanoi National University of Education, Hanoi, Vietnam (Thuoc and Hung, 2021) to produce FSR. Briefly, the SR was autoclaved at 121°C for 30 min. After cooling, the SR (moisture content, 70%) was inoculated with 7.5% (v/w) of a culture broth of Bacillus subtilis V37 containing 108 CFU/ml. After through mixing under sterile conditions, the inoculated SR was incubated in a chamber at 37°C for 18 h. The processed materials were then dried in an oven at 60°C for 48 h. Finally, the FSR was ground to below 400 μm mesh size and stored at -20oC until use. The SR was also dried, ground and stored at the same conditions as the FSR. The proximate composition and total spore count of the SR and FSR were analyzed and presented in Table I.

 

Table I. Proximate composition of the SR and FSR (g/kg, dry matter basis).

	SR1	FSR2
Crude protein	169	161
Crude lipid	49	61
Ash	41	43
Crude fiber	206	128
Nitrogen free extract3	535	607
Total spore count (106 CFU/g)	0	55.5

 

1Soybean residue; 2Fermented soybean residue; 3Nitrogen free extract (NFE)=100 - [crude protein (%) + crude lipid (%) + ash (%) + crude fiber (%)].

 

Experimental diets

Five isonitrogenous and isolipitic diets were formulated to replace 35% or 50% of fish meal by SR or FSR (Table II). The diets were named as follows: FMD (FM diet, the reference), SR35D (35% SR diet), FSR35D (35% FSR diet), SR50D (50% SR diet), and FSR50D (50% FSR diet). Because the content of methionine in SR was relatively low (Li et al., 2012), this amino acid was added in the SR35D and FSR35D at the level of 7 g/kg diet and in the SR50D and FSR50D at the level of 10 g/kg diet in order to equalize its content to FMD. All of the experimental diets were supplemented with chromium oxide (5 g/kg diet) as an inert marker for determinations of nutrient ADCs. All of the ingredients were thoroughly mixed, then water was added to produce a dough. Finally, diets were pelleted with a laboratory mincer and stored at -20°C until use.

Fish rearing conditions

The feeding trial was carried out at the Faculty of Biology, Hanoi National University of Education (Hanoi, Vietnam). Fingerling red tilapia were acclimated to the experimental conditions for 10 days by feeding with FMD. A total of 300 fingerling red tilapia (mean BW 13.7 g) were randomly distributed into 15 tanks (20 fish/tank, 3 tanks/dietary treatment). For 8 weeks, the fish were fed the test diets to apparent satiation twice daily (09:00 am and 16:00 pm).

 

Table II. Formulation and proximate composition of the experimental diets.

	FMD	SR35D	FSR35D	SR50D	FSR50D
Ingredients (g/kg)
FM1	250	162.5	162.5	125	125
SR	0	350	0	500	0
FSR	0	0	350	0	500
Soybean meal2	200	200	200	200	200
Corn gluten meal3	100	100	100	100	100
Pollock liver oil4	50	40	40	35	35
Cellulose5	80	40	40	0	0
Corn starch6	293	73.5	73.5	3	3
Vitamin and mineral mixture7	12	12	12	12	12
DL-Methionine8	0	7	7	10	10
CMC-Na9	10	10	10	10	10
Chromium oxide10	5	5	5	5	5
Proximate composition (g/kg, dry matter basis)
Crude protein	308	307	306	305	303
Crude lipid	76	72	77	74	79
Ash	74	66	67	61	64
Crude fiber	97	124	101	162	121
Nitrogen free extract	445	431	449	398	433

 

1Minh Tam Co. Ltd., Rachgia, Kiengiang, Vietnam; 2Bresur S.A., Corrientes, Buenos Aires, Argentina; 3Pangoo Biotech Hebei Co.,Ltd., Cangzhou, Hebei, China; 4Pesquera pacific star S.A., Calle Ruta, Puerto Montt, Chile; 5,6Shanghai Richem Internation Co., Ltd., Shanghai, China; 7Hinter Biotechnology Group Co., Ltd., Zhuhai, Guangdong, China. Vitamin and mineral mixture (IU or mg/kg mixture): thiamine HNO3, 1030; riboflavin, 3070; pyridoxine HCl, 1390; cyanocobalamin, 8.1; vitamin C (L-ascorbate-2-monophosphate), 18100; vitamin A acetate, 485000; vitamin D3 (cholecalciferol), 172000; vitamin E (DL-α-tocopherol acetate, 7010; vitamin K3 (menadione sodium bisulfite), 1850; folic acid, 550; nicotinamide, 5200; D-calcium pantothenate, 4250; D-biotin, 16.5; inositol, 15400; ZnSO4, 2700; MnSO4, 1730; CuSO4, 1310; FeSO4, 6250; CoSO4, 156; potassium iodide, 175; sodium selenate, 38.1; 8 TRInternational, Inc., Seattle, WA, USA; 9Sodium carboxymethyl cellulose, SINOCMC Chemical Co. Ltd., Qingdao, China; 10Sigma-Aldrich Corp., St. Louis, MO, USA.

 

Abbreviations: FM, fish meal; FMD, fish meal diet; SR, soybean residue; SR35D, 35% soybean residue diet; SR50D , 50% soybean residue diet; FSR, fermented soybean residue; FSR35D, 35% fermented soybean residue; FSR50D, 50% fermented soybean residue.

Sample collection

At the end of the experiment, all fish were fasted for 24 h, then anesthetized with 400 ppm 2-phenoxyethanol. The fish were individually determined the final BW and measured the total body length. Three fish from each tank were randomly selected for whole body composition analysis. Blood samples were collected with heparinized syringes from the caudal veins of five fish in each tank for determinations of hematological parameters. These fish were then dissected to collect gallbladders to measure gallbladder somatic index (GBSI). The remaining fish were further fed the experimental diets for fecal collection through the method described by Kaushik et al. (2004). The fecal samples were used for measurements of nitrogen free extract (NFE), protein and lipid ADCs. Finally, six fish in each tank were dissected at 4 h after feeding to collect gallbladders and anterior intestinal digesta for determinations of GBSI and digestive enzyme activity, respectively. In each tank, the intestinal digesta samples from three fish were pooled. The intestinal tract was sectioned as described by Tengjaroenkul et al. (2000), and the anterior intestinal digesta were taken from the whole straight region. All samples were maintained at -20°C until analysis.

Analytical methods and calculations

Hematocrit and plasma constituents were measured using an automatic analyzer (Architect c16000, Abbott, IL). Activities of digestive enzymes in the intestinal digesta samples were determined according to the method described by Murashita et al. (2007). Viable spore count was evaluated by colony count after drying SR and FSR at 60oC for 48 h. The samples were serially diluted with 0.9% NaCl solution, and then 100 µl of the diluted samples were spread on LB (Luria Bertani) plates. After 48 h of cultivation at 35oC, the colonies were counted, statistically analyzed, and expressed as colony-forming unit per gram (CFU/g). The proximate compositions of SR, FSR, experimental diets, feces, whole body, and the inner marker were analyzed in accordance with the Association of Official Analytical Chemists standard methods (AOAC, 2005).

The WG, SGR, feed intake (FI), FCR, condition factor, GBSI, and nutrient ADC were calculated using the following formulas:

WG (%) = 100 × [final average BW (g) - initial average BW (g)]/initial average BW (g)

SGR (%BW/day) = 100 × [ln final average BW (g) - ln initial average BW (g)]/feeding days

FI (%BW/day)= 100 × total dry FI (g)/[total initial BW (g) + total final BW (g)]/2/ feeding days

FCR = total dry FI (g)/ [total final BW (g) - total initial BW (g)]

GBSI = 100 × individual gallbladder weight (g)/wet individual BW (g)

Condition factor (CF) = 100 × individual BW (g)/individual total body length3 (cm)

Nutrient ADC (%) = 100 × [100 - Cr2O3 in diet (%)/Cr2O3 in feces (%) × nutrient in feces(%)/nutrient in diet(%)]

Statistical analysis

Data were analyzed using one-way analysis of variance (ANOVA). Statistical differences between groups were assessed with the Tukey-Kramer test, and the significance was based on a 5% level of probability.

Results

Growth and feed performances

The growth and feed performances of red tilapia fed the test diets are presented in Table III. The experimental diets did not affect survival rate and FI of the experimental fish. The final BW, WG, and SGR of fish fed SR35D and SR50D were significantly lower than those fed FMD (P < 0.05). Meanwhile, there were no significant differences in growth performance between the FSR35D and FMD groups. FCR values of SR35D- and SR50D-fed fish were similar and significantly higher than those of FMD- and FSR35D-fed fish (P < 0.05). Feeding with FSR-included diets resulted in significantly better growth performance and FCR as compared to SR-included diets (P < 0.05).

Hematological parameters

As presented in Table IV, total cholesterol levels of fish fed SR35D and SR50D were the lowest among the treatments, and the significant differences were found among these groups and the FMD and FSR35D groups (P < 0.05). Hematocrit, total protein, glucose, and triglyceride levels did not differ among the treatment groups.

Condition factor, whole body composition and GBSI

The condition factor and whole-body composition of red tilapia fed the test diets are shown in Table V. The experimental diets did not alter condition factor and whole body composition of the fish. Before feeding (starving), no significant differences in the GBSI were observed among the treatments (Fig. 1). However, at 4 h after feeding, the FSR35D and FSR50D groups had significantly lower GBSI values than the SR35D and SR50D groups, respectively (P < 0.05).

 

Table III. Growth performance and feed utilization of red tilapia fed the experimental diets1.

Dietary groups	Initial BW (g)	Final BW (g)	WG (%)	SGR (%BW/day)	FI (%BW/day)	FCR	Survival (%)
FMD	13.7 ± 0.2	90.1 ± 3.5d	555.9 ± 16.9d	4.09 ± 0.06d	4.75 ± 0.19	1.42 ± 0.02ab	97.5 ± 3.5
SR35D	13.6 ± 0.1	65.9 ± 1.6b	383.7 ± 9.0b	3.43 ± 0.03b	4.94 ± 0.34	1.74 ± 0.09c	100.0 ± 0.0
FSR35D	13.8 ± 0.2	94.5 ± 2.1d	583.7 ± 27.3d	4.18 ± 0.09d	4.28 ± 0.25	1.30 ± 0.06a	97.5 ± 3.5
SR50D	13.7 ± 0.1	55.4 ± 2.8a	304.6 ± 23.9a	3.04 ± 0.13a	4.76 ± 0.15	1.82 ± 0.11c	100.0 ± 0.0
FSR50D	13.7 ± 0.2	76.6 ± 2.3c	458.5 ± 14.8c	3.77 ± 0.04c	4.32 ± 0.21	1.51 ± 0.07b	100.0 ± 0.0

 

1Values are presented as means ± standard deviations of three replicates. The values in the same column with different superscripts are significantly different (P < 0.05).

For details of groups, see Table II.

 

Table IV. Hematological parameters of red tilapia fed the experimental diet1.

Dietary groups	Hematocrit (%)	Total protein (g/100ml)	Glucose (mg/100ml)	Total cholesterol (mg/100ml)	Triglyceride (mg/100ml)
FMD	33.7 ± 2.3	2.8 ± 0.5	114.8 ± 6.3	170.4 ± 10.8b	52.6 ± 3.6
SR35D	32.2 ± 1.9	2.7 ± 0.4	102.5 ± 8.6	132.8 ± 6.5a	46.8 ± 4.0
FSR35D	34.4 ± 2.8	2.9 ± 0.5	108.1 ± 9.2	168.6 ± 8.2b	48.5 ± 3.8
SR50D	31.6 ± 2.1	2.6 ± 0.7	110.4 ± 11.8	118.3 ± 9.6a	45.3 ± 2.4
FSR50D	32.9 ± 2.7	2.7 ± 0.3	107.5 ± 7.3	144.7 ± 7.2ab	50.6 ± 2.9

 

1Values are presented as means ± standard deviations (n = 15). The values in the same column with different superscripts are significantly different (P < 0.05).

For details of groups, see Table II.

 

Table V. Condition factor and whole body composition of red tilapia fed the experimental diets1.

Parameters	Dietary groups
	FMD	SR35D	FSR35D	SR50D	FSR50D
Condition factor	2.0 ± 0.2	1.8 ± 0.2	1.9 ± 0.1	1.7 ± 0.2	1.8 ± 0.1
Whole body composition (g/100g, wet matter basis)
Moisture	73.8 ± 0.6	74.9 ± 0.9	73.7 ± 1.5	74.8 ± 2.1	76.9 ± 2.3
Protein	15.6 ± 0.4	15.2 ± 0.6	15.8 ± 0.3	15.5 ± 0.8	15.0 ± 0.5
Lipid	7.1 ± 0.5	6.8 ± 0.6	7.2 ± 0.4	6.7 ± 0.5	6.8 ± 0.3
Ash	4.3 ± 0.3	4.0 ± 0.3	4.2 ± 0.1	3.9 ± 0.2	4.1 ± 0.4

1Values are presented as means ± standard deviations (condition factor, n = 60; whole body composition, n = 9). The values in the same raw with different superscripts are significantly different (P < 0.05).

For details of groups, see Table II.

 

Digestive enzyme activity in anterior intestinal digesta

As shown in Figure 2, the SR35D and SR50D groups showed lower digestive enzyme activities in the anterior intestine than the FMD group, and significant differences were recorded for amylase and trypsin activities (P < 0.05). Meanwhile, fish fed FSR35D had similar digestive enzyme activities as compared to those fed FMD. At each dietary replacement level of FM, feeding the experimental fish with FSR-included diets resulted in higher intestinal amylase and trypsin activities as compared SR-included diets (P < 0.05).

 

Nutrient ADC

The nutrient ADCs of red tilapia fed the experimental diets are shown Figure 3. The SR35D and SR50D groups had significantly lower protein, lipid and NFE ADCs than the FSR35D and FSR50D groups, respectively (P < 0.05). No statistical differences in NFE, protein and lipid ADCs were detected between fish fed FMD and FSR35D.

 

Discussion

In the current study, fish fed SR35D and SR50D showed significantly lower final BW, WG, and SGR, but higher FCR, than those fed FMD. These results indicated that the replacement of FM by SR at the rate of 35% in the diet impaired growth and feed performances of red tilapia. Fish fed FSR35D and FSR50D showed significantly better growth performance and FCR than those fed SR35D and SR50D. Moreover, fish fed FSR35D had similar final BW, WG and FCR as compared to those fed FMD. These observations suggested that SR fermentation benefited growth performance and feed utilization of red tilapia, and FSR could replace 35% of FM in red tilapia diets. Dietary FSR inclusion promoted growth and FCR of the fish, however, these parameters of the FSR50D group were inferior to those of the FSR35D and FMD groups. Some studies have demonstrated that growth and feed performances of fish decrease as dietary soybean protein proportion increases (Refstie et al., 2005; Wang et al., 2016; He et al., 2020). Thus, the excessive inclusion level of FSR in the diet could be the reason causing the poor growth and feed performances of fish fed FSR50D in the current study.

Feeding fish with soybean protein-based diets reportedly reduces intestinal bile acid level and digestive enzyme activity in fish (Nguyen et al., 2011b, 2015; Murashita et al., 2008, 2018). Low secretions of bile acids from the gallbladder and digestive enzymes from the pancreas are suggested to be responsible for the inferior bile acid concentration and low digestive enzyme activity in the intestine (Nguyen et al., 2017, 2021; Murashita et al., 2019). In the present study, there were no significant differences in GBSI before feeding (24 h fasted fish) among the treatment groups, however, fish fed SR35D and SR50D showed significantly higher GBSI values than those fed FMD at 4 h after feeding (Fig. 1). These results implied that the secretion of bile juice was inhibited in fish fed SR35D and SR50D. Bile acids are synthesized in the liver, then stored in the gallbladder. After meal, bile acids are secreted from the gallbladder into the intestine for functioning in the lipid digestion process, such as lipid emulsification and lipase activation (Tuchweber et al., 1996). Thus, the impairment of bile secretion from the gallbladder in fish fed SR-included diets might lead to the insufficiency of bile acid level in the intestine, thereby reducing dietary lipid digestion and absorption. In addition to the inferior bile secretion from the gallbladder, trypsin, lipase and amylase activities in the intestinal digesta of SR35D- and SR50D-fed fish were significantly lower than FMD-fed fish (Fig. 2). The reduced intestinal bile level and digestive enzyme activity of fish fed SR35D and SR50D suggested that feeding of SR negatively affected digestive physiology of red tilapia, and these adverse effects might interfere with dietary nutrient digestion and absorption. In the current study, bile juice secretion and intestinal digestive enzyme activity were markedly improved by feeding with FSR35D and FSR50D as compared to SR35D and SR50D. Moreover, FSR35D-fed fish showed similar bile juice release and intestinal digestive enzyme activity as compared to FMD-fed fish. These results indicated that fermentation of SR was beneficial to digestive physiology of red tilapia, and FSR could be included at a rate of 35% in the low FM diet without any negative effects on bile juice release and digestive enzyme activity. The positive effects of FSR on digestive physiology in the present study supported earlier findings in rainbow trout (Yamamoto et al., 2010), yellowtail (Nguyen et al., 2015, 2017), and pompano (Nguyen et al., 2021; Mai et al., 2021), where biliary bile acid secretion and intestinal digestive enzyme activity of the fish were elevated by including fermented soybean meals in diets. Cholecystokinin (CCK) plays important roles in stimulating the release of bile juice from the gallbladder (Shiratori et al., 1986; Einarsson et al., 1997) and secretion of digestive enzymes from the pancreas (Ramirez and Farrar, 1970). In this study, feeding of SR inhibited bile secretion from the gallbladder and lowered pancreatic digestive enzyme activity in the intestine. Thus, these adverse effects might have been caused by low CCK production and/or secretion. In agreement with this suggestion, feeding yellowtail with soybean protein-based diets reportedly decreases CCK mRNA levels (Furutani et al., 2012; Nguyen et al., 2017).

Feeding fish with soybean protein-based diets has been known to reduce nutrient ADCs (Biswas et al., 2007; Nguyen et al., 2015, 2017, 2020; Mai et al., 2021). In the current study, fish fed SR35D and SR50D resulted in lower ADCs of NFE, protein and lipid than fish fed FMD, indicating that SR decreased dietary nutrient digestion and absorption in red tilapia. In contrast, the FSR35D and FSR50D groups presented significantly higher NFE, protein and lipid ADCs than the SR35D and SR50D groups, respectively. Moreover, similar nutrient ADCs were found in the FSR35D group as compared to the FMD group. These observations, together with bile release and digestive enzyme activity in the anterior intestine, suggested that fermentation of SR enhanced dietary nutrient digestion and absorption through the improvements of bile secretion and intestinal digestive enzyme activity of red tilapia. The positive effects of FSR on nutrient digestion and absorption in the present study are in agreement with previous findings, where diets containing fermented soybean meals enhance nutrient ADCs of fish (Refstie et al., 2005; Yamamoto et al., 2010; Nguyen et al., 2015, 2020; Yuan et al., 2017).

Soybean reportedly contains several components which negatively influence growth performance, feed utilization, and physiological conditions of fish (Francis et al., 2001; Krogdahl et al., 2003; Iwashita et al., 2008). High molecular weight soybean proteins, such as β-conglycinin and glycinin (Fukushima, 2011), have been reported to decrease intestinal digestive enzyme activity, nutrient ADCs, and growth performance (Nguyen et al., 2011a; Li et al., 2017a, b; Han et al., 2019). Saponins, lectins, and oligosaccharides in soybean have been shown to inhibit the secretions of biliary bile acids and pancreatic digestive enzymes, lower nutrient ADCs, cause intestinal morphological changes, and interfere with growth and feed performances (Olli and Krogdahl, 1995; Refstie et al., 1998; Iwashita et al., 2009; Chikwati et al., 2012; Nguyen et al., 2011b, 2017, 2021). Some studies have demonstrated that soybean fiber also decreases growth and feed performances in fish (Dioundick and Stom, 1990; Amirkolaie et al., 2005; Lekva et al., 2010; Zhonga et al., 2020). Fermentation has been suggested to be an effective method to improve the nutritional quality of soybean meal by reducing the amount of oligosaccharides, fiber, soy antigens, trypsin inhibitors, phytate, and breaking down β-conglycinin into smaller peptides (Hong et al., 2004; Feng et al., 2007; Li et al., 2012; Nguyen et al., 2020). It has been reported that growth performance, feed utilization, and physiological conditions of fish are elevated by feeding with fermented soybean meal-included diets as compared to unfermented soybean meal-included diets (Azarm and Lee, 2014; Nguyen et al., 2020; Mai et al., 2021). Moreover, dietary supplementation of bacteria, such as Lactobacillus sp. and Bacillus sp., promotes growth performance, feed utilization and digestive physiological conditions in fish (Al-Dohail et al., 2009; Apun-Molian et al., 2009; Truong Thy et al., 2017; Liu et al., 2018; Abarike et al., 2018). In the present study, FSR was obtained by fermenting SR with Bacillus subtilis V37, which resulted in a lower content of crude fiber, but a higher content of NFE, in comparison with SR (Table I). On the other hand, FSR contained a relatively high amount of bacterial spores. Therefore, the improvement of nutritional quality and the presence of Bacillus subtilis V37 spores in FSR could be the factors contributing the enhancements of nutrient digestion and absorption, thereby promoting growth and feed performances of the fish fed FSR-included diets. Apart from positive effects on digestive physiology, growth performance and feed utilization, fermentation reportedly improves the intestinal histology and microbiota in fish fed soybean protein-based diets (Refstie et al., 2005; Yamamoto et al., 2010; Wang et al., 2019; He et al., 2020). In the present study, morphology and microorganism population in the intestine were not examined. For this reason, further studies are required to evaluate whether FSR benefit such parameters in red tilapia fish.

Conclusion

In conclusion, SR reduced bile juice secretion, intestinal digestive enzyme activity, nutrient digestion, and growth performance of red tilapia. Fermentation of SR with Bacillus subtilis V37 was beneficial to digestive physiology and growth and feed performances of fish fed soybean protein-based diets. The findings of the current study suggested that FSR could replace 35% of FM in the diet without any adverse effects on digestive physiological conditions, growth performance and feed utilization in red tilapia fish.

Acknowledgements

This study was funded by Ministry of Education and Training under grant number B2020-SPH-07. We express special thanks to the staff members of Faculty of Biology, Hanoi National University of Education, Vietnam for their support during the experiments.

Statement of conflict of interest

The authors have declared no conflict of interest.

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