Effects of enzymolytic soybean meal on the growth performance, digestive enzyme activity, some serum indexes, carcase performance and meat quality of Rex rabbits

Abstract

The purpose of this study was to investigate the effects of enzymolytic soybean meal (ESBM) on growth performance, digestive enzyme activity, serum biochemical parameters, immunity, carcase performance and meat quality of Rex rabbits. One hundred and twenty healthy weaned Rex rabbits were randomly divided into four groups (30 replicates per group and one rabbit per replicate). The control group was fed the basal diet and the ESBM treated groups (T1, T2 and T3) were fed 0.5%, 1% and 1.5% ESBM instead of the equivalent amount of soybean meal (SBM) in the basal diet, respectively. The study lasted 61 days, including a pre-feeding period of 5 days and a formal trial period of 56 days. Compared with the control group, the feed to weight ratio was significantly decreased in both T2 and T3 groups (p < 0.05). The α-amylase and trypsin activities of the caecum and serum albumin content were significantly increased in the T3 group (p < 0.05). The serum lysozyme activity was significantly increased in all treatment groups (p < 0.05). The full eviscerated slaughter ratio and half eviscerated slaughter ratio were significantly increased in the T1 group (p < 0.05). There were no significant differences in other indexes (p > 0.05). In conclusion, the replacement of SBM with ESBM (1.5%) in the diet can reduce the feed to weight ratio, increase the activity of some digestive enzymes, improve carcase performance, and enhance immunity of Rex rabbits.

HIGHLIGHTS

• ESBM improved growth performance, gastrointestinal digestive enzyme activity and carcase performance of Rex rabbits.

• ESBM increased serum lysozyme activity and albumin content of Rex rabbits.

• ESBM could be used as a new kind of feed additive for Rex rabbits.

Keywords:

• Enzymolytic soybean meal
• Rex rabbit
• growth performance
• digestive enzyme activity
• immunity

Introduction

Rex rabbits are well known as both furs and meat productive rabbits. During the weaning period, digestive tracts of Rex rabbits are not fully developed and they are subjected to multiple stresses at the same time, such as weaning, food changes and even cage transfers, which may affect their growth and development (Velasco-Galilea et al. 2020). For this reason, it is important and necessary to explore relevant products to benefit the growth and development of Rex rabbits during their weaning period.

Plant-based feeds are considered as the main nutrient sources for rabbits, and soybean meal (SBM) is the most common source of plant protein in rabbit feeds (Garcia-Santos et al. 2021). However, SBM contains a variety of anti-nutritional factors that may cause adverse effects when ingested by animals (Nguyen et al. 2018). Enzymolytic soybean meal (ESBM) can reduce the anti-nutritional factors in SBM, while producing bioactive peptides to enhance the body’s immunity and promote the digestion and absorption of the animal body (Cervantes-Pahm and Stein 2010). As stress is caused during the weaning period in Rex rabbits, the digestive system is immature at a young age and the anti-nutritional factors of SBM can affect the digestion and absorption of the body, which will be detrimental to growth and health.

The previous research of our group found that anti-nutritional factors in SBM can be degraded by enzymatic hydrolysis, and the number of small peptides can be increased, thereby improving the nutritional value of SBM. We compared the nutrient composition and biochemical characteristics of SBM and ESBM, and the comparison results are shown in Table 1. The results showed a substantial increase of small peptide and a noticeable decrease of two anti-nutritional factors (glycinin and β-conglycinin) in ESBM. Ma et al. (2019) found that ESBM improved growth performance and immunity in weaned pigs. Long et al. (2021) reported that ESBM enhanced performance via improving immune response of nursery pigs in antibiotic free diets. ESBM has been used to excellent effect on other animals, however, there is no relevant research report on rabbits. If the ESBM is used to replace the equivalent amount of SBM in Rex rabbit feed, it may be beneficial to the growth of Rex rabbits. Therefore, this study replaced the equivalent amount of SBM in Rex rabbit diets with different proportions of ESBM to investigate the effects of ESBM on growth performance, gastrointestinal digestive enzyme activity, serum biochemical parameters, immunity, carcase performance and meat quality of Rex rabbits.

Table 1. Nutrient composition and biochemical characteristics of SBM and ESBM.

Items	SBM	ESBM
Nutrient composition, %
Dry matter	89.47	90.90
Ash	6.20	8.04
Gross energy	19.51	19.33
Crude fibre	5.15	4.94
Crude protein	46.33	45.49
Ether extract	1.10	0.51
Calcium	0.61	0.46
Total phosphorus	0.57	0.62
Biochemical characteristics
Small peptide, %	0.68	14.94
Glycinin, mg/g	104.77	2.06
β-conglycinin, mg/g	47.12	3.35

SBM: soybean meal; ESBM: enzymolytic soybean meal.

Materials and methods

Animal care

The experimental protocols were approved by the Animal Care and Use Committee of Hebei Agriculture University (Baoding, China). The care and use of animals fully complied with local animal welfare laws, guidelines and policies.

Animals and experimental design

In the present study, the ESBM and protease (the activity of alkaline protease was 200,000 µ/g and the activity of neutral protease was 50,000 µ/g) were provided by Qihao Biotechnology Company Limited (Qinhuangdao, China). ESBM was made by crushing soybean meal, then adding enzyme preparations for enzymatic digestion, after forming, drying, crushing again, screening and packaging into products.

In this study, one hundred and twenty healthy weaned Rex rabbits with suitable weight (771.55 ± 12.30 g) at the age of 40 days were randomly divided into four groups (Each group had 15 males and 15 females, 30 replicates per group and one rabbit per replicate, each group was randomly distributed according to the average weight of male and female). The basal diet was free of antibiotics and mildew inhibitors. The control group was fed the basal diet and the ESBM treatment groups (T1, T2 and T3) were fed 0.5%, 1% and 1.5% ESBM substitute the equivalent amount of SBM in the basal diet, respectively. The rabbits were fed with the diets according to the standards of NRC (2012). The ingredients and chemical composition of the diets are shown in Table 2.

Table 2. Composition and nutrient levels of the experimental diets (air-dry basis).

Items	Control	T1	T2	T3
Ingredients, %
Corn	8.00	8.00	8.00	8.00
Wheat bran	33.00	33.00	33.00	33.00
Soybean meal	11.00	10.50	10.00	9.50
Enzymolytic soybean meal	0.00	0.50	1.00	1.50
Wheat middlings	4.00	4.00	4.00	4.00
Peanut seedling	15.00	15.00	15.00	15.00
Peanut shell	12.50	12.50	12.50	12.50
DL-methionine	0.15	0.15	0.15	0.15
Stone powder	1.05	1.05	1.05	1.05
L-Lysine Sulphate	0.30	0.30	0.30	0.30
Salt	0.50	0.50	0.50	0.50
Calcium hydrogen phosphate	0.50	0.50	0.50	0.50
Rice bran	5.00	5.00	5.00	5.00
Wormwood leaf powder	8.00	8.00	8.00	8.00
Premix^a	1.00	1.00	1.00	1.00
Nutrient levels^b
Digestible energy, MJ/kg	10.92	10.40	10.72	10.58
Dry matter, %	88.83	88.58	89.00	88.86
Crude protein, %	15.49	15.53	15.86	16.52
Ash, %	11.96	12.30	12.18	12.62
Ether extract, %	3.16	2.87	3.21	3.31
Crude fibre, %	14.75	14.53	14.18	14.17
Calcium, %	1.50	1.60	1.68	1.71
Total phosphorus, %	0.70	0.70	0.70	0.73

^aPremix provides the following (per kg of the premix): VA 10,000 IU, VB₁ 2 mg, VB₂ 6 mg, VB₃ 50 mg, VB₄ 1,000 mg, VB₅ 50 mg, VB₆ 2 mg, VB₇ 0.2 mg, VB₁₂ 0.02 mg, VD 900 IU, VE 50 mg, VK 2 mg, Cu (as copper sulphate) 20 mg, Fe (as ferric sulphate) 70 mg, Mn (as manganese sulphate) 10 mg, Zn (as zinc sulphate) 70 mg, Se (as sodium sulphate) 0.25 mg, Co (as cobalt sulphate) 0.15 mg, I (as iodine sulphate) 0.2 mg.

^bDigestible energy was a calculated value, while the others were measured values.

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

The feeding experiment lasted for 61 days, including a pre-feeding period of 5 days and a formal trial period of 56 days. Before the feeding experiment, the experiment equipments (including cages, water nipples and feeders etc) were disinfected. During the feeding experiment, the Rex rabbits were given free access of feed and water, and each rabbit lived in a single cage. The natural light and ventilation were maintained as well as the rabbits were immunised according to rabbit vaccination regulations.

Growth indices

During the feeding period, Rex rabbits were weighed on the morning before the meal on the first and last day respectively. The feed intake was counted, the average daily gain (ADG), average daily feed intake (ADFI) and feed to weight ratio (feed/gain) were calculated.

Carcase traits and meat quality

At the end of the feeding experiment, after the Rex rabbits were fasted for 12 h, we randomly selected six rabbits from each group. After each rabbit was euthanized, the rabbit was weighed before slaughter. The carcase weight, full eviscerated slaughter weight and half eviscerated slaughter weight were recorded after each rabbit was slaughtered to calculate the slaughter ratio, full eviscerated slaughter ratio and half eviscerated slaughter ratio. The carcase weight was defined as the weight of the Rex rabbit after the blood, fur, limbs, tail, gastrointestinal and genitourinary tract were removed. The full eviscerated slaughter weight was the carcase weight with the head and all internal organs removed. The half-eviscerated slaughter weight was taken to be the full eviscerated slaughter weight with the addition of the heart, liver and kidneys. Formulas were calculated as follows: the slaughter ratio (%) = (the carcase weight/the pre-slaughter weight) × 100; the full eviscerated slaughter weight ratio (%) = (the full eviscerated slaughter weight/the pre-slaughter weight) × 100; the half-eviscerated slaughter weight ratio (%) = (the half-eviscerated slaughter weight/the pre-slaughter weight) × 100.

The longest back muscle was taken from both sides after each rabbit was slaughtered and the pH was measured at 45 min and 24 h after slaughtering with a testo 205 pH metre (Testo Company, Lenzkirch, Germany). The meat lightness (L*), redness (a*) and yellowness (b*) were measured using a WR-18 precision colourimeter (Wave Optoelectronics Technology Company Limited, Shenzhen, China). Rabbit meat samples were put into plastic bags and placed in a water bath at 80 °C for 2 h and cooled under running water for 30 min. Meat samples of 1.5, 1.0 and 0.5 cm in length, width and thickness were cut respectively in a direction parallel to the muscle fibres, and the shear force was measured using a C-LM3B digital muscle tenderness metre (Tongde Business Technology Company Limited, Beijing, China). Meat samples were weighed and put into plastic bags, then placed in a water bath at 80 °C for 1 h and cooled under running water for 30 min. Wiped off the surface and weighed, the cooking loss was calculated using the following formula: the cooking loss (%) = [(the weight before cooking - the weight after cooking)/the weight before cooking] × 100.

Determination of digestive enzymes

After slaughtering, the contents of 5 mL each from the stomach, jejunum and caecum were collected in sterile cryopreservation tubes and stored at −80 °C. The corresponding samples were cut and weighed, then added with phosphate buffers (PBS), milled at a low temperature and centrifuged, the supernatant was taken and stored at −20 °C for testing. To detect the activities of α-amylase, trypsin and lipase in the jejunum and caecum, as well as pepsin and caecum cellulase, by using commercial Elisa kits (Boruichangyuan Technology Company Limited, Beijing, China), following the kit instructions.

Blood sampling

At the end of the feeding experiment, six Rex rabbits were randomly selected from each group. The blood was taken from the ear vein and collected from rabbits, centrifuged at 3500 x g for 10 min, and the supernatant was taken as the serum sample and stored at −20 °C. The content of serum total protein (TP), albumin (ALB), immunoglobulin A (IgA), immunoglobulin G (IgG), immunoglobulin M (IgM), complement-3 (C3), complement-4 (C4) and the lysozyme activity was measured using commercial Elisa kits (Boruichangyuan Technology Company Limited, Beijing, China), following the kit instructions.

Biochemical analysis

Dry matter, ash, ether extract, calcium, total phosphorus of soybean meal, ESBM and the diets were analysed following the procedures of the AOAC (2000). Gross energy was determined by the YX-ZR automatic calorimeter (Youxin Instrument Manufacturing Company Limited, Changsha, China). Crude fibre was measured by the A2000I automated fibre analyser (ANKOM Technology, New York, NY). Crude protein was determined by the Kjeltec 8400 fully automated Kjeldahl analyser (Foss Company, Hillerød, Denmark). The contents of small peptide were analysed according to the method using electrophoresis in sodium dodecyl sulphate-polyacrylamidegels (Fling and Gregerson 1986). The concentrations of glycinin and β-conglycinin were analysed using commercial Elisa kits (Longkexinyu Biotechnology Company Limited, Yantai, China), following the kit instructions.

Immunity status

After slaughtering, the spleen, thymus and sacculus rotundus were taken out, the attached tissue was removed, and the blood was wiped off before weighing, and the immune organ index was calculated using the following formula: the immune organ index (g/kg) = the immune organ weight (g)/the pre-slaughter weight (kg).

Statistical analysis

Excel 2019 and SPSS software package (version 20) were used for data statistical analysis (for growth performance analysis, n = 30; for gastrointestinal digestive enzyme activity, serum biochemical and immune indexes, immune organ indexes, carcase performance and meat quality analysis, n = 6). Duncan’s method was used to conduct multiple comparisons, and one-way analysis of variance (ANOVA) was used to test the significant difference among the groups. When the significant difference was observed, the difference among the groups was assessed (p < 0.05).

Results

Growth performance

The effect of ESBM on the growth performance of Rex Rabbits are shown in Table 3. There were no significant differences in body weight among all groups in experimental period (p > 0.05). There were no significant differences in ADFI and ADG among all groups (p > 0.05). Compared with the control group, the feed to weight ratio was significantly decreased in the T2 and T3 groups (p < 0.05), the T1 group was not significantly different (p > 0.05).

Table 3. Effect of enzymolytic soybean meal on the growth performance of Rex rabbits (n = 30).

Items	Control	T1	T2	T3	SEM	p-Value
Initial body weight, g	784.22	777.95	770.52	753.37	12.30	0.851
Final body weight, g	1931.50	2000.00	2046.43	2042.79	22.64	0.276
ADFI, g	107.64	112.83	111.55	111.69	1.28	0.544
ADG, g	20.49	21.82	22.78	23.03	0.35	0.052
Feed/gain	5.28^a	5.21^ab	4.96^bc	4.87^c	0.05	0.018

ADFI: average daily feed intake; ADG: average daily gain; Feed/gain: the feed to gain ratio.

^a,b,cMeans in a row without the same superscript are different significantly (p < 0.05).

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Gastrointestinal digestive enzyme activity

The effect of ESBM on gastrointestinal digestive enzyme activity of Rex Rabbits are shown in Table 4. Compared with the control group, the pepsin activities in treatment groups increased by 37.13–50.68%, but the difference did not reach a significant level (p > 0.05). There were no significant differences in the activities of α-amylase, trypsin and lipase activities of jejunum among all groups (p > 0.05). Compared with the control group, the α-amylase and trypsin activities of caecum was significantly increased in the T3 group (p < 0.05). There were no significant differences in the lipase and cellulase activities of caecum among all groups (p > 0.05).

Table 4. Effect of enzymolytic soybean meal on gastrointestinal digestive enzyme activity of Rex rabbits (n = 6).

Items	Control	T1	T2	T3	SEM	p-Value
Pepsin, U/mL	14.76	20.48	22.24	20.24	1.72	0.437
Jejunum
α-Amylase, U/mL	262.87	289.65	323.21	317.65	15.95	0.549
Trypsin, U/mL	279.72	271.90	277.84	296.89	8.86	0.805
Lipase, U/L	610.61	652.91	585.23	678.22	22.28	0.505
Caecum
α-Amylase, U/mL	387.96^b	389.12^b	402.89^b	438.17^a	7.06	0.025
Trypsin, U/mL	281.61^b	288.81^b	307.91^b	357.45^a	8.14	<0.001
Lipase, U/L	555.45	557.66	568.35	696.56	21.40	0.063
Cellulase, U/mL	262.28	270.05	266.97	296.90	6.70	0.207

^a,bMeans in a row without the same superscript are different significantly (p < 0.05).

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Serum biochemical and immune indexes

Table 5 shows the results of ESBM on serum biochemical and immune indexes of Rex rabbits. There was no significant difference in the TP content among the groups (p > 0.05). Compared with the control group, the ALB content was significantly increased in the T3 group (p < 0.05), but no significant difference was observed in the T1 and T2 groups (p > 0.05). There were no significant differences in the content of serum IgA, IgG, IgM, C3 and C4 among all groups (p > 0.05). The activity of serum lysozyme was significantly increased in all treatment groups compared to the control group (p < 0.05).

Table 5. Effect of enzymolytic soybean meal on serum biochemical and immune indexes of Rex rabbits (n = 6).

Items	Control	T1	T2	T3	SEM	p-Value
TP, μg/mL	744.21	793.65	787.63	781.10	13.72	0.625
ALB, ng/mL	140.42^bc	148.76^b	124.76^c	171.48^a	4.95	<0.001
IgA, μg/mL	2542.74	2567.72	2280.96	2440.54	104.46	0.791
IgG, g/L	30.59	24.24	28.53	31.10	1.28	0.269
IgM, μg/mL	869.26	761.45	962.52	901.31	53.42	0.683
C3, μg/mL	237.20	231.86	213.83	254.33	10.12	0.588
C4, μg/mL	342.12	393.56	430.06	386.45	21.33	0.630
Lysozyme, U/L	11.23^b	17.59^a	17.30^a	18.77^a	0.89	<0.001

TP: total protein; ALB: albumin; IgA: immunoglobulin A; IgG: immunoglobulin G; IgM: immunoglobulin M; C3: complement 3; C4: complement 4.

^a,b,cMeans in a row without the same superscript are different significantly (p < 0.05).

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Immune organ indexes

The effect of ESBM on immune organ indexes of Rex rabbits are shown in Table 6. There were no significant differences in spleen index, thymus index and sacculus rotundus index among all groups (p > 0.05).

Table 6. Effect of enzymolytic soybean meal on immune organ indexes of Rex rabbits (n = 6).

Items	Control	T1	T2	T3	SEM	p-Value
Spleen index, g/kg	0.52	0.35	0.51	0.39	0.05	0.530
Thymus index, g/kg	1.89	1.71	2.15	2.21	0.14	0.600
Sacculus rotundus index, g/kg	1.31	1.11	1.15	1.14	0.05	0.567

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Carcase performance

Table 7 shows the effect of ESBM on the carcase performance of Rex Rabbits. There was no significant difference in slaughter ratio among all groups (p > 0.05). Compared with the control group, the full eviscerated slaughter ratio and half eviscerated slaughter ratio of Rex Rabbits were significantly increased in the T1 group (p < 0.05), the full eviscerated slaughter ratio and half eviscerated slaughter ratio was also increased in the T2 group, but the difference was not statistically significant (p < 0.05). The slaughtering rate of whole viscera and half viscera of Rex rabbits in T3 group was the same as that in control group (p > 0.05).

Table 7. Effect of enzymolytic soybean meal on the carcase performance of Rex rabbits (n = 6).

Items	Control	T1	T2	T3	SEM	p-Value
Pre-slaughter weight, g	1748.33^b	1900.00^a	2003.83^a	1890.83^a	26.02	0.001
Slaughter ratio, %	68.68	69.73	68.96	67.25	0.34	0.058
Full eviscerated slaughter ratio, %	53.87^b	56.54^a	55.65^ab	53.85^b	0.41	0.029
Half eviscerated slaughter ratio, %	57.32^b	59.72^a	58.31^ab	57.01^b	0.37	0.022

^a,bMeans in a row without the same superscript are different significantly (p < 0.05).

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Meat quality

Table 8 shows the effect of ESBM on the meat quality of Rex Rabbits. No significant differences were observed in the pH at 45 min and 24 h of post-slaughter, cooking loss, shear force and meat colour (L*, a* and b*) among all groups (p > 0.05).

Table 8. Effect of enzymolytic soybean meal on the meat quality of Rex rabbits (n = 6).

Items	Control	T1	T2	T3	SEM	p-Value
pH45min	6.67	6.58	6.66	6.67	0.03	0.789
pH24h	5.57	5.57	5.57	5.59	0.01	0.751
Cooking loss, %	28.83	27.59	28.58	26.71	0.41	0.307
Shear force, kg·f	2.89	2.78	2.82	2.43	0.11	0.504
L*	24.26	25.91	24.72	23.38	0.50	0.388
a*	4.40	3.76	4.44	4.85	0.51	0.911
b*	2.09	2.82	2.78	3.03	0.27	0.680

pH_45min: pH at 24 h post slaughter; pH_24h: pH at 45 min post slaughter; L*: lightness; a*: redness; b*: yellowness.

T1 means 0.5% enzymolytic soybean meal addition group, T2 means 1.0% enzymolytic soybean meal addition group, and T3 means 1.5% enzymolytic soybean meal addition group.

Discussion

Growth performance

A number of anti-nutritional factors are present in SBM, which leads to poor tolerance of SBM in animals and also plays an important role in inhibiting growth and health (Kim et al. 2015). In recent years, ESBM has been reported in numerous studies on the effects of growth performance in animals. Ghazi et al. (2002) found that SBM treated with protease at pH 4.5 significantly increased the feed intake and live weights of chicks compared to untreated SBM, which improved the growth performance of chicks. In the present study, the feed to gain ratio of Rex rabbits was significantly decreased when the replacement level of ESBM reached 1% and 1.5% compared to the control group, which improved the growth performance. It was reported that protease treated SBM significantly decreased the feed to gain ratio and improved growth performance in broilers compared to untreated soybean meal (Ghazi et al. 2003), which was similar to the present study. The ESBM in the present study was produced by protease hydrolysis and contained fewer anti-nutritional factors and more easily digestible proteins, thereby making it easily absorbed. Li et al. (2021) confirmed that the reason that ESBM improved the growth performance of pig was due to the enhanced nitrogen utilisation. In addition, our study found that ESBM enhanced the gastrointestinal digestive enzyme activity of Rex rabbits, making it easier for the body to digest and absorb nutrients, which might also be responsible for improving the growth performance of Rex rabbits.

Gastrointestinal digestive enzyme activity

The gastrointestinal environment of Rex rabbits plays a key role in maintaining the well-being of the body (Liu et al. 2019). The digestive enzyme activity includes α-amylase, lipase, trypsin, cellulase and pepsin, etc., which reflects to a certain extent the degree of digestion and utilisation of feed nutrients by the body, and there is a positive relationship between body growth and digestive enzyme activity. In this study, the addition of ESBM increased the pepsin activity and the activities of α-amylase, lipase, trypsin and cellulase of caecum among all treatment groups. When the replacement level of ESBM was 1.5%, the differences of α-amylase and trypsin activities of caecum from the control group reached a significant level. Protein and starch in mammalian food are mainly digested by trypsin and α-amylase, and a small amount is digested by intestinal bacteria (Switzar et al. 2013). Therefore, the replacement of soybean meal with ESBM in the diet improved the digestive capacity of Rex rabbits by increasing the activities of α-amylase and trypsin of caecum and the pepsin activity, which was more beneficial to the growth and development of Rex rabbits.

Serum biochemical indexes

Serum biochemical indexes reflect the body condition and the changes caused by internal and external factors. The change of TP content reflects the metabolism and synthesis of proteins in animals (Xie et al. 2021), and the dysfunction of protein synthesis in the liver can contribute to decrease the content of serum TP. In the present study, no significant differences were observed in the serum TP contents of Rex rabbits, indicating that protein synthesis in the liver was functioning normally in all treatment groups. ALB is the abundant protein of serum, which is synthesised by the liver and associated closely with inflammation (Jin et al. 2019). In addition, the serum ALB is a major plasma protein target of oxidative stress with important antioxidant activity (Roche et al. 2008). In the present study, when the replacement level of ESBM was 1.5%, the serum ALB contents of Rex rabbits were significantly increased compared to the control group, it can be seen that ESBM is beneficial to enhance the resistance of Rex rabbits and protect body’s health.

Immunity

Immunoglobulins, including IgA, IgG and IgM, are the important immune products of the response to antigenic substances in the body. As an essential component of the immune regulation in the body, C3 is critical for the activation of the entire complement system, C4 is the major protein in the classical cascade reaction (Kasperska-Zajac et al. 2013). Lysozyme activity is known to activate non-specific immunity which is closely associated with the health of animals (Huang et al. 2020), lysozyme also has the ability to fight against harmful bacteria and reduce inflammation. Therefore, the increase of lysozyme activity is considered to be beneficial to the health of animals. In the present study, there were no significant differences in the contents of serum IgA, IgG, IgM, C3 and C4 in Rex rabbits due to ESBM, and serum lysozyme activity was significantly increased in all treatment groups compared to the control group. It was reported that no significant differences were observed in the ESBM treatment groups on the contents of plasma IgG and IgM in weaned piglets compared to the control group (Li et al. 2021). Li et al. (2020) reported that enzyme-treated dehulled soybean meal significantly increased the serum lysozyme activity of largemouth bass and improved the immunity of the body, which was similar to the present study. More excitingly, a study also confirmed that soybean peptides obtained by enzymatic hydrolysis of soybeans can modulate cellular immune system, thereby enhance the immunity of the body and protect it from destruction (Yimit et al. 2012).

Carcase performance

Carcase performance is linked closely to the economic efficiency of animals (Li et al. 2017), full eviscerated slaughter ratio and half eviscerated slaughter ratio are both important indicators of carcase performance. If the carcase performance can be increased, it will help to improve the economic efficiency of Rex rabbits. In the present study, compared with the control group, the full eviscerated slaughter ratio and half eviscerated slaughter ratio of Rex rabbits in the T1 and T2 groups were increased. The changes of carcase weight and body structure might be related to the growth speed of animals (Combes et al. 2010), we speculate that the improvement of carcase performance of Rex rabbits may be associated with the ability of ESBM to improve their growth performance.

Meat quality

The meat pH is related closely to the shelf-life stability of meat and is one of important indicators for assessing meat quality (Kasperska-Zajac et al. 2013). In the present study, there was no significant difference in pH of rabbit meat of the ESBM treatment groups at both 45 min and 24 h after slaughter compared to the control group, and we were able to observe a pH lower than 45 min at 24 h after slaughter. This is due to the fact that with the passage of post-slaughter storage time, the myocytes were still performing various biochemical reactions and large amounts of lactic acids were produced by glycolysis, which led to a reduction in pH. The ability of the muscle to retain water can be indicated by the cooking loss to some extent, in relation to the colour, juiciness and so on of the meat. The shear force is an important parameter that directly influences the tenderness of meat (Jin et al. 2019), which is dependent on the content of myogenic fibres, connective tissues and so on in the muscle. The meat colour is an important characteristic that plays a crucial role in the freshness and quality of meat (Liu et al. 2018). In the present study, no significant differences were observed in the cooking loss, shear force and meat colour of the meat between the treatment groups and the control group, indicating that the addition of ESBM to the Rex rabbit diet had no obvious effects on the water retention, tenderness, and freshness of rabbit meat. The reduction of meat pH led to poor protein solubility, which resulted in less water retention as well as worse texture in meat products (Lesiów and Xiong 2013), therefore it was possible that the absence of significant difference in rabbit meat pH in the present study was associated with the fact that ESBM did not influence the meat quality of Rex rabbits.

Conclusion

The present study showed that the replacement of SBM with ESBM in the diet of Rex rabbits improved growth performance, immunity and carcase performance as well as increase the activity of some digestive enzymes and the content of serum ALB. It can be seen that ESBM could be used as a new kind of feed additive for Rex rabbits. Under this experimental condition, the recommend replacement level was 1.5%.

Ethical approval

The protocol used in this experiment approved by the Institutional Animal Care and Use Committee of Institute of Animal Science and Veterinary Medicine, Hebei Agricultural University (Baoding, Hebei, China). The authors confirm that the procedures imposed on the animals were carried out to meet the Directive 2010/63/EU of the European Parliament and the Council of 22 September 2010 on the protection of animals used for scientific purposes.

Acknowledgments

Thank you to all those who contributed to this experiment.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the Modern Agriculture Industry Technology System of Rabbit [CARS-43-B-2] and the Science and Technology Program of Inner Mongolia Autonomous Region [2021GG0247].