Is Astragalus Extract Efficient in Animal Feeding?

Astragalus membranaceus is a perennial herb in the family Leguminosae. It is also known as mianqi, huangshen, and muhuangqi, and is one of the traditional Chinese herbs that are used for both food and medicine. The dried root of Huangqi contains a variety of active ingredients and trace elements, and astragalus polysaccharide (APS) is one of the main active ingredients that contribute to the efficacy of Huangqi [1-2]. Announcement No. 194 of the Ministry of Agriculture and Rural Affairs of the People's Republic of China stipulates that “the addition of feed additives containing growth-promoting drugs other than traditional Chinese medicines to feed products in China is prohibited” [3]. At the same time, the state encourages the development of traditional Chinese medicine feed additives with few toxic side effects and significant efficacy. Therefore, the research and development of feed additives that can effectively replace antibiotics has become a research hotspot in the animal husbandry industry.

Astragalus polysaccharide, as a new type of Chinese herbal medicine feed additive, can not only promote animal growth, regulate immunity and enhance the body's resistance, but also improve the quality and yield of animal products. This article reviews the structural composition, purification process and current feeding status of astragalus polysaccharide, with the aim of providing a reference for the in-depth development and application of astragalus polysaccharide in animal production.

1 Composition and physicochemical properties of astragalus polysaccharide

Astragalus polysaccharides are obtained from the dried roots of Astragalus through extraction, concentration and purification. The main components of astragalus polysaccharides are various monosaccharides such as glucose, mannose, arabinose and rhamnose[4]. The crude polysaccharide of Astragalus polysaccharide is a light brown powder that is hygroscopic; after separation and purification, a light yellow flocculent solid polysaccharide is obtained, which is soluble in water and insoluble in organic reagents. It is a kind of homogeneous polysaccharide[4]. The composition of astragalus polysaccharide will vary slightly depending on the extraction method, while the structural composition and physicochemical properties are the main factors affecting its pharmacological activity. Therefore, future efforts can be devoted to improving the pharmacological activity of astragalus polysaccharides by modifying the structure and improving the physicochemical properties.

2 Extraction process of astragalus polysaccharides

2.1 Extraction methods of astragalus polysaccharides

There are hot water extraction, alkali alcohol extraction and enzyme extraction methods for extracting plant polysaccharides. These methods are low in extraction cost and suitable for industrial-scale production. However, higher extraction temperatures or longer alkali treatment times may affect the chemical structure of the polysaccharides, and enzymes are easily inactivated at high temperatures.

In recent years, extraction methods based on physical principles such as microwave, ultrasound and homogenization-assisted negative pressure cavitation extraction have gradually attracted attention. Synergistic extraction using different methods can also complement each other's advantages and improve the extraction rate. The ideal extraction method can achieve a high polysaccharide yield and non-destructive extraction in a short extraction time. The main extraction methods for astragalus polysaccharides are shown in Table 1. Considering the extraction rate, extraction time and energy consumption, the current extraction of astragalus polysaccharides is still mainly based on the water extraction and alcohol precipitation method and the alkali alcohol extraction method. The enzyme hydrolysis time is positively and linearly correlated with the polysaccharide yield. However, the enzyme hydrolysis method has high environmental requirements. Therefore, under the premise of ensuring that the enzyme is not inactivated, extending the enzyme hydrolysis time to increase the polysaccharide yield can be a new direction for future research.

2.2 Separation and purification of astragalus polysaccharides

The isolation and purification of astragalus polysaccharides mainly involves the removal of proteins and small molecular impurities contained in crude polysaccharides. Commonly used methods for deproteinization include the enzyme-Sevage method, TCA method, hydrochloric acid method, and trichlorotrifluoroethane method. The enzyme-Sevage method is currently the main method for removing proteins from polysaccharides due to its mild enzymatic reaction conditions and the advantages of the high efficiency of the Sevage method [4].

After the crude polysaccharide of Astragalus membranaceus is treated with deproteinization and decolorization, a mixture of several polysaccharides is obtained, which needs to be further purified. Commonly used purification methods include stepwise precipitation, ion exchange chromatography, chitosan flocculation and gel column chromatography [4,12]. The gel column chromatography method is simple to operate and highly reproducible, and is currently the most widely used method for polysaccharide purification.

In the future, efforts can be made to explore the coordinated use of multiple methods, with a view to achieving complementary advantages and improving the purification rate.

3 Application of astragalus polysaccharides in animal production

3.1 Application of astragalus polysaccharides in poultry production

Adding an appropriate amount of astragalus polysaccharides to the feed can promote animal growth and enhance the body's resistance. Duan Junying [13] added 0.1% astragalus polysaccharide to the broiler diet and found that the daily weight gain of broilers was significantly improved, and the immune function of broilers was also improved. Wang Yandong et al. [14] conducted a feeding trial on broilers with astragalus particles (homemade), and found that the optimal addition amount of astragalus particles was 1.74%, and the quality of chicken meat was optimal. The villus length and crypt depth in the intestinal tract of poultry can be used to determine the absorption and digestion capacity of the small intestine. The higher the villus height/crypt depth (V/C) ratio, the healthier the intestinal system of the organism.

Shan et al. [15] added a certain amount of Astragalus polysaccharide, and found that the V/C ratio of the chicks was significantly increased, indicating that astragalus polysaccharide can enhance the immune function of the intestinal mucosa of chicks. Zhang et al. [16] and Wang et al. [17] also obtained similar research results in their feeding trials on chickens. Astragalus polysaccharide injection can also act as an immune enhancer during the vaccination process against Newcastle disease in chickens, with an optimal injection volume of 1.0 mL/bird [18]. Adding astragalus polysaccharide to the diet of male parent chickens can induce an endotoxin tolerance-like immune response in the spleen of the chicks, indicating that astragalus polysaccharide has a certain transgenerational effect and nutritional epigenetic effect on the liver immunity of male parent chickens [19].

A combination of astragalus polysaccharide and Clostridium butyricum in the diet of ducklings can significantly increase the average daily weight gain and reduce the feed conversion ratio [20]. Li Guiqin et al. [21] added 0.5 g/kg astragalus polysaccharide to the basal diet of meat ducks to improve the growth performance of ducks immunized against viral hepatitis, enhance the body's immune system, and relieve the body's immune stress. Astragalus polysaccharides can also effectively stimulate mucosal immune function by improving intestinal morphology and repairing damage to the small intestinal mucosal immune barrier in ducks infected with infectious bursal disease virus[22]. Bu Weidong et al. [23] added different doses of astragalus polysaccharide to the diet of breeders at intervals and found that after 25 days of feeding, the growth performance of the breeders improved to varying degrees. The test group with 0.005 g/d astragalus polysaccharide had the best effect, with a significant increase in the weight of the breeders and an increase in their Newcastle disease HI antibody levels.

Adding astragalus polysaccharides to the feed has a certain effect on improving the intestinal microecology of poultry, which can promote the absorption of nutrients by the body, thereby promoting the growth and immunity of livestock, and also improving the quality of poultry products. However, attention should be paid to the purity of the polysaccharide and the amount of the additive, as a high amount may have side effects on the body.

3.2 Application of astragalus polysaccharides in livestock production

Wang Yicui et al. [24] found that adding 5–10 g/head of astragalus polysaccharide to the diet of early weaned heifers can significantly increase the feed intake and daily weight gain of weaned heifers. After feeding milk cows with astragalus polysaccharides, the respiratory rate and serum globulin count of stressed milk cows can be reduced, which has a relieving effect on the stress response of milk cows [25]. Astragalus polysaccharides can also promote the reproductive capacity of cows and increase the survival rate of their sperm [26]. Adding 0.2 g/kg astragalus polysaccharides to the diet of weaned lambs for 30 days significantly improved the animals' growth performance and further enhanced their immunity [27]. It can also regulate the intestinal microbial flora system [28]. Supplementing a lamb diet with 0.1% astragalus polysaccharide for 47 days was found to significantly improve both the lambs' immunity and daily weight gain [29]. Xu Duanhong et al. [30] also obtained similar results. However, there are some differences in the optimal amount of astragalus polysaccharide added, which may be related to the breed of sheep or the purity of the polysaccharide.

Adding astragalus polysaccharide to the diet of pregnant sows 7 days before parturition significantly increased the levels of immunoglobulin G and immunoglobulin M in the colostrum of the sows after giving birth, and also increased the blocking rate of the swine fever virus [31]. Astragalus polysaccharides can improve the growth performance of piglets, reduce liver dysfunction and immune stress caused by lipopolysaccharides, and improve the intestinal barrier function of the body [32]. Wang et al. [33] found that using Astragalus polysaccharide to dilute the vaccine for live pigs at a ratio of 1:1 can reduce the stress response of pigs to the vaccine, improve the immune effect, and prolong the duration of the peak antibody level. Astragalus polysaccharide and probiotic compound have a certain interactive effect when fed to pigs, which improves the immune function of the body and promotes the body to maintain an oxidatively balanced state [34].

Therefore, astragalus polysaccharide not only promotes the growth of livestock, improves their antioxidant capacity and stress levels, but also enhances their immunity by promoting the development of immune organs and tissues, and promotes the development of the intestinal tract, which helps to improve the utilization rate of feed and has high economic benefits.

3.3 Application of astragalus polysaccharide in aquaculture

Astragalus polysaccharide also has certain applications in aquaculture. Studies have shown that astragalus polysaccharide can promote the growth of young crucian carp and big yellow croaker and improve their immunity. However, the optimal additive amount of the two is different, which may be related to the species and environmental conditions [35-36]. Adding a certain amount of astragalus polysaccharide can improve the non-specific immune function of grass carp and its resistance to pathogens. The optimal dosage is 10-30 mg/kg of fish body weight [37]. Zhou Zhiyu et al. [38] found that astragalus polysaccharides not only significantly enhanced the immune capacity of grass carp females, but also transmitted immune factors between generations. Li et al. [39] added astragalus polysaccharides to the feed of zebrafish and found after 3 weeks of feeding and found that the weight of the test group of zebrafish increased significantly; and the addition of 0.02% astragalus polysaccharide had a negative impact on the liver health of the test group, while a low dose of astragalus polysaccharide (0.01%) could exert a variety of beneficial effects while maintaining liver health.

Astragalus polysaccharides can improve the growth performance of turbot, enhance the antioxidant capacity of the liver, and stimulate the immune stress response of turbot. The optimal addition amount is 0.15 g/kg [40]. Liu Jinhai et al. [41] also pointed out that the addition of 0.6 g/kg astragalus polysaccharide to the diet can significantly increase the activity of various enzymes in the mucus of the half-smooth tongue sole and enhance the body's non-specific immune capacity.

Adding the right amount of astragalus polysaccharide to the feed of white leg shrimp can significantly increase its survival rate and maintain the high activity of various immune-related enzymes in the body for a long time. 0.2 g/kg is the optimal amount [42]. After injection of astragalus polysaccharide, the gene expression level of immune-related factors in the body of the river crab was increased [43]. Astragalus polysaccharide can also stimulate the production of non-specific immune responses in sea cucumbers, significantly improve the regulatory effect of their intestinal microecology, and the optimal addition amount of astragalus polysaccharide is 0.8 g/kg [44]. Due to the large quantity of aquaculture, astragalus polysaccharides are generally added to the feed as an immune enhancer to regulate the intestinal microbial flora system, promote the development of the body's immune organs, and improve the body's immune capacity.

Astragalus polysaccharides can promote the growth of fish, shrimp, crabs and sea cucumbers at the right dosage, improve the intestinal digestion ability of the body, enhance immunity and antioxidant capacity, and reduce the occurrence of stress-related diseases.

4 Outlook

Astragalus polysaccharides can be used to a certain extent as a herbal feed additive to replace antibiotics. However, more research is needed to promote the use of astragalus polysaccharides in animal production. First, there are many different purification processes for astragalus polysaccharides, each with its own advantages and disadvantages. Further improvements and exploration of new processes are needed to obtain polysaccharides with higher purity. The structure of astragalus polysaccharides needs to be modified and the advanced structure of astragalus polysaccharides studied to enhance and expand its biological activity.

Second, astragalus polysaccharides are mainly used orally as a feed additive for traditional Chinese medicine. There have been few studies on the mechanism of action of astragalus polysaccharide injections, and the optimal dosage for animal production applications also needs further research. Finally, research on the combined use of astragalus polysaccharides and other additives should also be strengthened, with a view to developing more effective and cost-saving compound additives for large-scale production and application in animal husbandry. With the optimization of the polysaccharide purification process and further in-depth research on the application of astragalus polysaccharides in animal production, astragalus polysaccharides will have even broader application prospects.

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