What Are the Benefits of Mannan Oligo Saccharide in Rabbit Feeding?

The overuse of feed antibiotics in aquaculture has led to a series of problems such as antibiotic residues, resistance of pathogenic bacteria and health risks for humans and animals. The development of efficient alternatives to antibiotics is an important way to solve these problems and to gradually realize antibiotic-free farming. Manno-oligosaccharides (MOS) are a new type of antigenic active substances extracted from yeast cell walls, which are safe, stable and non-toxic, and can remain active during the processing and storage of rabbit feed, and have no residue or resistance, and can to a certain extent replace antibiotics, which is a hot research topic in the field of feed additives at present. Relevant studies have shown that oligosaccharides can effectively promote the growth of rabbits, improve immunity and intestinal function (Dimitroglou et al., 2009; Rosen, 2007), and have a good application prospect in rabbit production.

1 Physicochemical properties and metabolic characteristics of manno-oligosaccharides

1.1 Physical and Chemical Properties of Mannooligosaccharides 

MOS is soluble in polar solvents such as water and insoluble in organic solvents such as ether, ethanol and acetone. The viscosity of MOS varies with the environmental pH and temperature, and remains relatively stable within pH 3-9, which helps to improve the hardness of particles and reduce the chalking rate of the product during the pelleting process, and reduces the waste of raw materials (Gu Zilin et al., 2013). The MOS is relatively stable and can maintain structural and functional integrity under the high temperature (75 ~ 85 °C) of rabbit feed conditioning and pelleting.

The total sugar content of MOS reaches 90.12%, which is sweeter than sucrose (Liang Yong et al., 2013). Rabbits have a developed sense of taste and like to eat sweet feed. Adding MOS to feed can improve palatability and increase feed intake to a certain extent. Some sources of MOS also have unique gelation properties, such as konjac MOS, which can form thermo-reversible and thermo-irreversible gels under specific conditions. Currently, most of the MOS available on the market as feed additives are enzymatically produced as mixtures of disaccharides, trisaccharides, and tetrasaccharides (Guo et al., 2012). The good physicochemical properties are the basis for the large-scale application of MOS in rabbit production.

1.2 Metabolic characteristics of glycoligosaccharides    

MOS are non-absorbable oligosaccharides composed of several mannose or mannose molecules linked with glucose, and are widely found in konjac flour, guar gum, lecithin gum, and the cell walls of many microorganisms. The structure of MOS from different sources varies, with the yeast cell wall source having the main chain connected by α-1,6 glycosidic bonds, the side chains connected by α-1,2 and α-1,3 glycosidic bonds, and the content of α-1,4 glycosidic bonds being less. Konjac MOS consists of glucose and mannose residues with a molecular ratio of 1:1.5, with the main chains linked by β-1,4 glycosidic bonds and the side chains linked by β-1,3 glycosidic bonds. Since the digestive enzymes secreted by the digestive system of rabbits mainly act on α-1,4 glycosidic bonds and have less effect on other types of glycosidic bonds, MOS can pass through the anterior part of the digestive tract to the posterior part of the digestive tract, where it can be utilized by the fermentation of some beneficial bacteria, such as bifidobacteria, and be emitted in the form of organic acids, CO2, CH4, etc., or be involved in the metabolism of energy supply (Liang et al. 2013; Flickinge et al. 2000). 2000).

2 Effects of mannan oligosaccharide on growth performance and intestinal function of rabbits

2.1 Effect of mannan oligosaccharide on growth performance of rabbits    

MOS can increase the daily weight gain and feed efficiency of rabbits by enhancing intestinal function, maintaining intestinal micro-ecological balance, and promoting the proliferation of beneficial bacteria, as well as helping to reduce mortality and improve growth performance. Mourão et al. (2006) reported that the addition of MOS to the diet of 32-day-old rabbits significantly increased feed utilization and concentration of volatile fatty acids in the cecum, and reduced mortality.Bovera et al. (2010) showed that the addition of MOS to the diet of 35-day-old Illa rabbits reduced mortality during episodes of intestinal epidemics, and increased daily weight gain and nutrient utilization.Bovera et al. (2010) showed that the addition of MOS to the diet of 35-day-old Illa rabbits reduced mortality and improved daily gain and nutrient utilization. Bovera et al. (2012) reported that MOS significantly increased the daily weight gain of weaned Illa rabbits. Yang Dejun (2008) obtained similar findings in a study on white rabbits with large ears, in which MOS increased feed utilization and reduced the feed-to-meat ratio. In a different study, Volekd et al. (2007) found that the addition of MOS to the diet of weaned Ypresa rabbits not only had no significant effect on the growth performance during the fattening period, but also reduced the digestibility of cellulose in the cecum.

2.2 The effect of glycooligosaccharides on the intestinal function of rabbits    

The small intestine is an important digestive site in rabbits, and the digestibility of the terminal ileum in rabbits can be as high as 80% to 100% of the total digestibility of food starch and amino acids (Carabaño et al., 2009). MOS can maintain intestinal integrity, improve intestinal morphology, enhance intestinal function, and increase nutrient utilization.Mourão et al. (2006) reported that the addition of MOS to rabbit diets increased the length of intestinal villi, increased the absorptive surface area of the intestinal villi, improved the relationship between villi length and crypt depth, and resulted in more regular intestinal villi structure.Pinheiro et al. (2004) showed that the intestinal villi in rabbits were more stable than those in rabbits. Similar findings were reported by Pinheiro et al. (2004) in weaned rabbits. Yang Dejun (2008) reported that MOS could bind to the intestinal mucosal receptor of rabbits, reduce the stimulation of the intestinal mucosa by harmful flora, protect the intestinal villi and the intestinal wall, and effectively maintain the integrity of the intestinal tract.

3 Effect of mannan-oligosaccharides on disease resistance in rabbits

MOS can improve the disease resistance of rabbits by regulating the intestinal micro-ecological balance of rabbits and enhancing their immunity. Mortality and morbidity are important indicators of disease resistance in animals. Bovera (2010) and Mourão (2006) reported that MOS could reduce the mortality of rabbits.

3.1 Effect of glyco-oligosaccharides on the non-immune defense system of rabbits    

Intestinal microecological balance is one of the foundations of efficient intestinal function, and the interaction between nutrients and intestinal microorganisms determines animal health, performance and meat quality. Abnormalities in gut-microbe interactions lead to pathogen susceptibility, disruption of intestinal epithelial integrity, atrophy of the intestinal villi and inflammatory reactions in the intestinal mucosa.

MOS can regulate the microecological balance of the intestinal tract and improve the disease resistance of rabbits by inhibiting the colonization of harmful bacteria and promoting the proliferation of beneficial bacteria.Ian-Naccone et al. (2013) showed that MOS can improve the intestinal environment of rabbits and reduce the colonization of pathogenic bacteria, and the effect is obvious on rabbits at the late weaning stage. In addition to reducing the colonization of pathogenic bacteria, MOS also dislodged pathogens that had been adsorbed to the intestines. Ofek et al. (1977) showed that MOS could dislodge pathogenic bacteria already adsorbed to the intestinal epithelium and excreted in the feces. Increasing the concentration of volatile fatty acids in the intestinal tract and lowering the pH of the intestinal tract, indirectly inhibiting acid-sensitive pathogens such as E. coli, is also one of the ways in which MOS can inhibit bacteria (Zduńczyk et al., 2004). MOS can also be selectively utilized as a specific nutrient substrate for beneficial bacteria such as bifidobacteria to promote their proliferation (Yue Wenbin et al., 2002). Diarrhea is very harmful to rabbits, and intestinal microecological imbalance is one of the main causes of diarrhea in rabbits. Maintaining intestinal microecological balance is an important way to reduce the incidence of diarrhea.

3.2 The effect of glycooligosaccharides on the immune defense system of rabbits

3.2.1 Regulation of the non-specific immune system    

MOS has the effect of enhancing intrinsic immune cells and intrinsic immune factors.Kim et al. (2000) reported that MOS could enhance the phagocytosis of monocytes and macrophages, and increase the levels of interferon IFN, interleukin IL-2, and serum lysozyme.MOS could also increase the level of intestinal mucosal immunity. Li Peng et al. (2007) reported that galactomannan MOS could significantly increase the number of lymphocytes and cup cells in the epithelium of various intestinal segments of rabbits, and the number of both of them could reflect the level of mucosal immunity to a certain extent.

3.2.2 Regulation of the specific immune system 

MOS can exert immunomodulatory effects through protein receptors on the surface of enterochromaffin cells or lymph nodes and epithelial cells in the lamina propria of mucous membranes, and the strength of the effect is related to the number of acetyl groups and the degree of phosphorylation of MOS. MOS has immune-activating properties and can act as an immune adjuvant to delay antigen uptake and increase antigenic potency, and the auxiliary activity of MOS is derived from the linear side chains of lipopolysaccharide (LPS).Torrecillas et al. (2015) reported that MOS can also increase the levels of immunoglobulins, the major histocompatibility complex class II (MHC II), and the T-cell receptor.3 In the anticoccidial context, MOS has been shown to enhance the immunomodulatory effects of MHC II and the T-cell receptor. MOS has also been shown to have a positive effect in the fight against coccidia, with G6mez et al. (2009) reporting that MOS can enhance the resistance of neonatal animals to infection by Eimeria cepacia. The causative agent of rabbit coccidiosis is Eimeria coccidioides, which has a wide prevalence and high infection rate, and the infection rate of young rabbits can reach 100%, and the mortality rate of diseased young rabbits can be as high as 80%, so improving the resistance of rabbits to infection and doing a good job of group prevention is the key to reducing the harm of this disease (Gu Zilin et al., 2013).

3.3 Regulation of antioxidant and immune stress processes    

Immune stress can lead to high levels of inflammatory factors, causing metabolic changes in the organism that ultimately affect nutrient requirements and feed utilization.MOS can regulate the levels of inflammatory mediators and diminish the effects of immune stress.Che et al. (2012) reported that yeast MOS can regulate immune stress and over-immune responses by promoting the secretion of anti-inflammatory factors such as IL-10.MOS also plays a role in the antioxidant process of the organism. MOS can also play a role in the antioxidant process of the body. Wang et al. (2007) reported that supplementation of mice with 1~2 g/kg of MOS increased the total antioxidant capacity of mouse liver, scavenging reactive oxygen species (ROS), and significantly increased the activity of enzymes such as glutathione peroxidase (GSH-Px), superoxide dismutase (SOD) and catalase (CAT). The pathway of its action may be to scavenge ROS through the reduction reaction, and activate SOD and GSH-Px through the in vivo metabolic pathway.

4 Effect of mannan-oligosaccharides on feed quality

MOS can bind mycotoxins in feeds through physical adsorption or direct binding to improve feed quality and reduce the risk of mycotoxins to rabbits.Zaghini et al. (2005) showed that the addition of 0.11% MOS to the diet of laying hens could reduce the intestinal absorption of aflatoxin B1 in laying hens and decrease the content of aflatoxin B1 in their liver tissues.Raju et al. (1998) found that the in vitro adsorption of MOS to aflatoxin, zearalenone and ochratoxin was 82% in broiler digestive tract simulated in vitro test. (1998) found that the in vitro adsorption rates of MOS to aflatoxin, zearalenone and ochratoxin were 82.5%, 26.4% and 51.6%, respectively, in an in vitro test on a simulated broiler digestive tract. Digestive tract diseases accounted for about 1/2 of the total number of diseases in rabbits, and were especially harmful to rabbits before and after weaning, with a high lethality rate. Inadequate feed quality and moldy and spoiled raw materials are one of the main factors causing digestive diseases. Improving feed quality and reducing the harm of mycotoxins are of great significance to the healthy growth of rabbits.

5 Factors affecting the effect of mannan oligosaccharides and the appropriate additive amount    

5.1 Factors affecting the effect of mannan-oligosaccharides    

The breed, age, and physiological status of rabbits all affect the action effect of MOS. Li Yuxin (2015) reported that the effect of MOS on young animals is more obvious than that on adult animals. Different feeding management levels and rabbit hutch environments are also factors affecting the utilization efficiency of MOS in rabbits, and the effect of MOS is more pronounced in rabbit hutches under rough feeding conditions or with a history of epidemics. At the present stage of China's rabbit raising industry, the main form of rabbit raising is the farmhouse model, which accounts for about 40%~50% of the total number of rabbits, and the research and application of MOS is in line with the needs of China's rabbit raising industry at the present stage. Different sources of MOS have different contents of active ingredients and chemical structures, which may affect the biological activity of MOS. The synergistic and antagonistic effects between nutrients and MOS, as well as the types and contents of antinutritional factors in different feed compositions will also affect the utilization efficiency of MOS.

5.2 Appropriate levels of glycollo-oligosaccharides   

The amount of MOS added must reach a certain concentration in order to exert its biological function efficiently. If the amount of MOS is not enough, it will have little effect on the growth performance and disease resistance of rabbits; if it is added in excess, it may cause negative effects such as imbalance of the intestinal microecology and overstimulation of immunity, which may increase the cost of production. Therefore, the study of the appropriate amount of MOS added to rabbit production and the factors affecting its effects is particularly important for the large-scale application of MOS in rabbit production. Bovera et al. (2010) added 0.5, 1, and 1.5 g/kg of yeast MOS to the diets of weaned Illa rabbits, and found that MOS had a positive effect on the reduction of rabbit mortality, the improvement of rabbit growth and the digestibility of nutrients. Bovera et al. (2012) reached a similar conclusion in their study on the intestinal tract of weaned Ehrlich rabbits, and found that 1 g/kg of yeast MOS was more effective when added at 0.5, 1, and 1.5 g/kg. Mourão et al. (2006) reported that 1 g/kg of yeast MOS was comparable to 1 g/kg of yeast MOS in the reduction of pathogenic bacteria in the cecum of weaned rabbits. Mourão et al. (2006) reported that 1 g/kg yeast MOS was as effective as 0.1 g/kg zinc bacitracin in reducing cecum pathogens in weaned rabbits.

6 Summary

As a safe and efficient feed additive, the application of MOS avoids a series of problems such as superbugs and secondary infections brought about by the excessive use of antibiotics. With the new normalization of China's rabbit breeding industry, the improvement of people's health care awareness and the expansion of rabbit products' export business, the application of MOS in rabbit production has a broad prospect. However, in order to expand the scale of application in rabbit production, it is also necessary to study the optimal dosage for different breeds of rabbits, different stages of production, and different production conditions; the mechanism by which MOS affects the rabbit's immune system; the development of new production processes and new dosage forms of MOS, the control of production costs, and the further enhancement of utilization efficiency; and the differences and similarities in the structure, composition, and function of different sources of MOS, as well as synergistic and antagonistic effects with antibiotics and other prebiotics. In-depth study on the synergistic and antagonistic effects of MOS with antibiotics and other probiotics.

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