The Application Value and Mechanism of Yeast Cell Wall in Livestock Farming

1. Introduction
      Yeast Cell Wall (YCW) is a functional feed material made from brewing yeast (Saccharomyces cerevisiae) through enzymatic hydrolysis, separation, drying, and other processes. It is rich in active substances such as β - glucan and mannan oligosaccharides, and has multiple effects such as enhancing immunity, promoting growth, improving intestinal health, and adsorbing mycotoxins. It is widely used in pigs, poultry, ruminants, and aquaculture, and has become one of the important choices for antibiotic substitution under the background of "prohibition".

2、 Structural composition
      The yeast cell wall accounts for about 20% -30% of the dry weight of yeast and is divided into three layers: the inner layer is the β - glucan layer, the middle layer is the protein layer, and the outer layer is the mannan layer. By dry weight, mannan accounts for about 30%, β - glucan accounts for about 30%, glycoproteins and chitin account for about 20%, and the rest are proteins, lipids, etc.

      Among them, β - glucan is mainly composed of β -1,3-glucan as the main chain β-1, 6-glucan is branched and forms a three-dimensional network structure; Mannan is mainly composed of α -1,6-mannose with α -1,2 and α -1,3 side chains. The above structure is the basis for yeast cell walls to perform physiological functions.

3、 Core mechanism of action
（1） Immune regulation
      β - glucan is a core active substance in immune regulation. Its unique spiral configuration can be recognized by Dectin-1 receptors on the surface of intestinal immune cells, activating macrophages, NK cells, and T/B cells through the MAPK/NF - κ B pathway, promoting the secretion of cytokines such as TNF - α, IL-6, IL-12, enhancing phagocytic function, and IgM antibody production. MOS, as a prebiotic, selectively promotes the proliferation of lactic acid bacteria and bifidobacteria, inhibits pathogenic bacteria such as Escherichia coli and Salmonella, and dual regulates immune and microbial balance.

（2） Intestinal protection
      The yeast cell wall protects the intestine through multiple mechanisms: firstly, the MOS structure is similar to the adhesion receptors of pathogenic bacteria on the intestinal wall, which can competitively bind to surface lectins of pathogenic bacteria, block colonization, and promote their excretion; Secondly, promote the proliferation of goblet cells and secretion of mucin, enhance the intestinal epithelial barrier function, and prevent intestinal leakage; Thirdly, upregulate the expression of tight junction proteins such as claudin and occludin to further enhance barrier integrity.

（3） Fungal toxin adsorption
      The three-dimensional network structure of β - glucan contains a large number of hydroxyl groups, which can bind to aflatoxin, zearalenone, vomitoxin, etc. through hydrogen bonding with van der Waals forces, forming polysaccharide toxin complexes that prevent intestinal absorption. Especially for zearalenone, it has specific adsorption capacity, and as an organic adsorbent, its non selective adsorption of nutrients such as vitamins and minerals in feed is significantly lower than that of inorganic adsorbents.

（4） Antioxidant and stress resistance
      β - glucan can activate the Nrf2 pathway, upregulate the expression of antioxidant enzyme genes such as superoxide dismutase (SOD) and glutathione peroxidase (GSH Px), and eliminate free radicals; MOS enhances overall antioxidant capacity by increasing plasma antioxidant enzyme activity. Meanwhile, yeast cell walls can enhance the tolerance of livestock to environmental stresses such as heat stress and hypoxia.

4、 Application effect in various animal breeds
（1） Pig
      Adding 0.05% yeast cell wall to the diet of weaned piglets can increase average daily weight gain and dry matter digestibility (P<0.05), reduce diarrhea frequency (P=0.083), and significantly decrease serum TNF - α and IL-1 β levels (P<0.05). The expression of barrier related genes such as occludin and MUC1 in the ileum is upregulated, and the abundance of beneficial bacterial genera (Prevotella and Rosella) increases. Adding yeast cell wall to sow diet can increase feed intake and improve milk quality. In production, yeast cell walls can replace 30% -50% of plasma protein powder, reducing costs while maintaining the same growth performance.

（2） Poultry
      Adding 250-500 g/t yeast cell wall to broiler feed can improve feed conversion rate (from 1.74 to 1.70, P ≤ 0.01), increase the number of intestinal goblet cells, and reduce CD45+immune cell density (P<0.001), indicating significant alleviation of intestinal inflammation. Adding 800 g/t yeast cell wall and 100 g/t probiotics to the diet of laying hens resulted in a 5.60% increase in egg production rate (P<0.05) and a 6.67% decrease in feed egg ratio (P<0.01). Replacing 250 g/t of enrofloxacin with yeast cell wall in yellow feathered broiler chickens resulted in a 1.3% increase in slaughter weight at 42 days of age, and significant increases in serum globulin and Newcastle disease antibody levels (P<0.05). Actual production shows that yeast cell wall can reduce the diarrhea rate of broiler chickens by 20% -50%, and reduce the feed to weight ratio by 3% -5%.

（3） Ruminant animals
      The mechanism of yeast cell wall in the rumen has certain specificity: some polysaccharides bind with mycotoxins to prevent excessive levels of aflatoxins in milk; Partially degraded by rumen microorganisms into digestible sugars, promoting microbial protein synthesis and increasing milk protein content; The remaining polysaccharides enter the posterior gut and exert immunomodulatory effects. Adding 0.40% yeast cell wall to the diet of calves resulted in an average daily weight gain increase of 11.70%, a decrease in feed to weight ratio of 11.30%, a significant increase in the apparent digestibility of neutral detergent fiber, acidic detergent fiber, and calcium (P<0.05), an increase in serum IgG and IgA levels, and a significant decrease in fecal score (P<0.05). Adding yeast cell wall to dairy cow diet can increase milk protein content by 0.1% -0.3% and help stabilize rumen pH.

（4） Aquatic animals
      Adding 0.1% yeast cell wall to aquatic feed can significantly increase the weight gain rate of crucian carp and reduce the feed coefficient (P<0.05), without adverse effects on muscle nutrition. Yeast β - glucan can activate macrophages and monocytes in aquatic animals, enhance phagocytic function and cytokine production ability. Its natural fragrance also has a food attractant effect, which can increase feeding speed and appetite.

5、 Value as a substitute for antibiotics
      Starting from July 1, 2020, China has completely banned the addition of growth promoting antibiotics to feed, and the feed industry has entered the era of "ban on antibiotics". Yeast cell wall, as a green, safe, and efficient immune regulator and growth promoter, has become an important choice for antibiotic substitution. It is non-toxic, residue free, and does not develop drug resistance. It has multiple functions such as improving growth performance, regulating intestinal microbiota, enhancing immunity, resisting pathogens, and adsorbing toxins. In the challenge test of Clostridium perfringens in broiler chickens, there was no significant difference in growth performance, feed to meat ratio, and mortality rate between the yeast cell wall group and the antibiotic group, and the lesion score was significantly reduced. In actual production, yeast cell walls have successfully partially or completely replaced antibiotic growth promoters, plasma protein powder, and high-dose zinc oxide, with significant economic benefits.

6. Conclusion
      The yeast cell wall regulates livestock health systematically from four dimensions: immune regulation, intestinal protection, toxin clearance, and antioxidant and stress resistance through the synergistic effect of β - glucan and oligosaccharides. It has been proven to have significant effects in pig, poultry, ruminant, and aquaculture. Further research is needed in the future to determine the appropriate dosage and compatibility schemes for different animal breeds, growth stages, and breeding conditions, in order to provide more comprehensive technical support for the green, efficient, and sustainable development of animal husbandry.