The use and selection of alternative antibiotics in pig feed
The irrational use of antibiotics in pig production has led to increasingly serious problems such as antibiotic residues in pork and the emergence of superbugs, causing severe losses to the pig industry and posing a serious threat to food safety and public health. Therefore, ensuring pork safety and promoting antibiotic-free farming have received high attention from relevant national departments. Developing and finding green and efficient alternatives to feed antibiotics to protect pig health and increase production capacity is urgently needed. According to relevant research, the main antibiotic alternatives currently used in production include antimicrobial peptides, phage preparations, plant extracts, microecological preparations, lysozyme, organic acids, traditional Chinese medicine, enzyme preparations, trace elements, and oligosaccharides. To help pig producers utilize these alternatives scientifically and rationally, they are introduced below:
- Antimicrobial peptides
Organisms possess both an innate and an acquired immune system, and antimicrobial peptides (AMPs) are a component of the innate immune system. AMPs are a collective term for small peptides with potent antibacterial, antifungal, and antiviral activity, typically composed of 12–50 amino acid residues, and are widely found in nature. Antimicrobial peptides achieve their bactericidal effect by disrupting bacterial osmotic pressure homeostasis through membrane permeation mechanisms; this membrane permeation mechanism is less prone to inducing drug resistance.
Antimicrobial peptides possess advantages such as high solubility, low molecular weight, no induction of drug resistance, low toxicity to eukaryotic cells, reduction of the risk of intestinal pathogen infection, and significant growth-promoting and disease-preventing effects. They can be produced through in vitro fermentation of genetically engineered strains, resulting in low cost and great application potential. However, they also exhibit some instability; factors such as temperature, digestive proteases, and salt concentration can inactivate them. Furthermore, some types of peptides may cause cell membrane damage in animals, leading to toxicity.
Adding 0.5% and 1.0% antimicrobial peptides to the basal diet of weaned piglets significantly improved their growth performance and immune function.
- Plant extracts
Plant extracts refer to plant products in which one or more Active Ingredients from plant raw materials are separated and purified using physical, chemical, and biological methods. These mainly include alkaloids,polysaccharides, tea polyphenols, flavonoids, and volatile oils. Some natural plant components extracted through appropriate methods possess characteristics such as naturalness, low toxicity, no drug resistance, and multifunctionality. Their mechanisms of action are mainly divided into direct antibacterial effects and immune-enhancing effects. They also exhibit antiviral, antibacterial, anti-stress, immune-boosting, and growth-promoting effects.
The main advantages of plant extracts lie in their rich and unique chemical diversity, their wide global distribution and easy accessibility, and their various antibacterial effects and clinical efficacy. Their disadvantages include complex composition and unclear material basis. When used alone, many plant extracts (allicin, berberine, quercetin, etc.) have poor therapeutic effects, and some extracts also have drawbacks such as toxicity, high volatility, instability, and irritating odor.
Adding tea polyphenols to feed can improve the immunity and antioxidant capacity of weaned piglets, increase the content of unsaturated fatty acids in the body, increase the digestibility of various nutrients, and improve the composition of the main intestinal microbiota of piglets.
- Probiotic preparations
Microecological preparations are live bacterial preparations made from one or more beneficial microorganisms that contribute to the balance of the gastrointestinal microecology in animals, based on the principles of microecology. Microecological preparations can maintain intestinal flora colonization and balance, promote the proliferation of beneficial bacteria, and inhibit the proliferation of harmful bacteria, thereby maintaining the normal function of the gastrointestinal tract and indirectly enhancing the body's immunity and resistance. Microecological preparations mainly include three types: probiotics, prebiotics, and synbiotics. Lactic acid bacteria, Bacillus, and Bifidobacterium are the most common types of probiotics, and my country has explicitly permitted the use of more than 30 types of probiotics as feed additives. Prebiotics refer to oligosaccharides that can promote the growth or activity of beneficial bacteria in the intestines. Synbiotics refer to the combined use of probiotics and prebiotics to achieve a synergistic effect.
Microecological preparations have advantages such as no drug resistance, no toxic side effects, and no residues. While improving animal growth performance and immunity, they can also improve feed conversion efficiency. However, as live bacteria preparations, they require strict storage conditions, and their activity is significantly affected by conditions (acidity, temperature, and humidity). Different animal groups have different digestive systems, and the types of bacteria they require also vary. Studies have found that adding chitosan oligosaccharides to feed can significantly improve the growth performance and health of weaned piglets.
- Lysozyme
Lysozyme is a protease that specifically hydrolyzes the β-1,4-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in the bacterial cell wall, breaking down high-molecular-weight cell wall proteins into low-molecular-weight cytokines. This leads to cell wall perforation and bacterial cell membrane lysis, ultimately causing bacterial lysis and death. Lysozyme is an important innate defense factor of the host, abundant in epithelial secretions and phagocytes, exerting its bactericidal effect through antimicrobial peptide activity and peptidoglycan hydrolysis activity.
Lysozyme has the advantage of being naturally non-toxic, and its safety has been recognized by the World Organisation for Animal Health (WHO) and the Food and Agriculture Organization of the United Nations (FAO). It can effectively fight bacteria and improve the immune level and growth performance of animals. The disadvantages of lysozyme are that the extraction cost is relatively high, the storage and transportation conditions are strict, and its activity is conditionally biased, so it has not yet been widely used in the feed industry.
Lysozyme has been widely used in animal husbandry. In transgenic animals (including chickens, pigs, and mice) expressing the human lysozyme gene, it has been shown to improve intestinal morphology and enhance immunity. Egg white lysozyme promotes growth performance, intestinal morphology, and immunity in piglets. Chemically synthesized lysozyme improves growth performance and small intestinal morphology in finishing pigs.
- Organic acids
Organic acids are acidic organic compounds that can lower the pH of the gastrointestinal tract, inhibiting and killing bacteria such as Escherichia coli, Salmonella, Staphylococcus, and Clostridium. They can also regulate the intestinal flora structure and promote growth and digestion. Common organic acids in feed include tartaric acid, fumaric acid, citric acid, malic acid, benzoic acid, lactic acid, acetic acid, sorbic acid, butyric acid, and propionic acid. Their effects on animals can be summarized as follows: first, the antibacterial activity of non-dissociated organic acids; second, lowering the pH of digesta, especially in the stomach, to aid protein digestion; third, reducing gastric emptying rate; and fourth, stimulating the excretion and activity of enzymes (pancreatic enzymes) in the small intestine.
Organic acids are widely used worldwide due to their antibacterial activity. They can lower the pH of the gastrointestinal tract, which is conducive to the growth of acid-loving beneficial bacteria and plays a positive role in preventing digestive diseases. The problem is that the decrease in gastrointestinal pH caused by organic acids may affect animal feed intake, their effectiveness may be limited by duodenal metabolism, and reduced effective absorption may affect their ability to regulate the intestinal flora.
Studies have found that adding 0.3% of compound organic acids (formic acid, lactic acid, citric acid, and lauric acid) to the diet of weaned piglets can improve their growth performance, enhance their antioxidant and immune capabilities, maintain the stability of the intestinal barrier, and improve intestinal health. Feeding pigs with 2% benzoic acid reduced NH3 emissions by 60%–70%. This reduction in malodorous gases may be related to a decrease in the number of pathogenic bacteria in the gastrointestinal tract, increased activity of beneficial microorganisms, and enhanced nutrient digestibility. Tannic acid, as a natural feed additive, has significant effects, high safety, and high application value in preventing diarrhea and improving intestinal health.
- Oligosaccharides
Oligosaccharides, also known as functional oligosaccharides or oligosaccharides, are a general term for small polymers consisting of 2 to 10 monosaccharides linked by glycosidic bonds. After oral administration into the animal's intestines, these sugars promote the proliferation of beneficial bacteria while inhibiting the growth of harmful bacteria. Their mechanism of action is twofold: First, they maintain the balance of the intestinal microecology, significantly promoting bacterial growth. They are not digested and absorbed in the intestines but are decomposed and fermented by the gut microbiota, acting as growth factors for many probiotics, including Bifidobacteria. Oligosaccharides have a very significant effect on maintaining the intestinal microecology. Second, they help maintain blood sugar levels. Oligosaccharides can be used as a base material in many functional foods to replace some sucrose, thus significantly inhibiting the burden on the pancreas and glucose tolerance, and therefore having a significant effect on blood sugar maintenance. Currently, oligosaccharides used as feed additives include: fructooligosaccharides, lacto-oligosaccharides, xyloo-oligosaccharides, galactooligosaccharides, isomaltooligosaccharides, manno-oligosaccharides, and soybean oligosaccharides.
Oligosaccharides offer advantages beyond just antibacterial effects; they also possess prebiotic properties and exhibit a degree of stability within the digestive tract, making them easier to incorporate into animal feed on a large scale. However, their antibacterial mechanisms and production processes require further investigation, and their high cost necessitates optimization of production processes to expand their application.
Adding it to feed can improve the growth performance of weaned piglets, while reducing diarrhea and mortality rates and improving fecal characteristics.
- Enzyme preparations
Enzyme preparations refer to a class of substances with enzymatic properties extracted from organisms. They can be divided into single enzyme preparations and compound enzyme preparations. Single enzyme preparations mainly include amylase, protease, cellulase, phytase, etc., while compound enzyme preparations include transglutaminase, pentosanase, stearase, etc. Their mechanism of action is mainly to effectively improve the feed conversion efficiency and production performance of livestock and poultry by decomposing plant cell walls in feed, supplementing the lack of their own digestive enzymes, and eliminating components such as tannins, daidzein, phytic acid, NSP, and polyphenols.
The advantages of enzyme preparations are high efficiency and specificity, no chemical pollution, direct digestion of feed or reduction of feed viscosity, increased feed utilization, and the production of functional oligosaccharides, which improves the gut microbiota. Their disadvantage is that appropriate probiotics need to be added to maximize their benefits.
Adding exogenous compound enzyme preparations to the basal diet of piglets can improve the growth performance of weaned piglets, reduce the rate of diarrhea and feeding costs, and improve economic benefits. Li Yang et al. discovered a new type of phytase whose enzymatic properties and application effects are superior to ordinary phytase. It has high enzymatic activity against phytic acid in feed and can reduce the amount of dicalcium phosphate used in the diet.
- Traditional Chinese medicine
Traditional Chinese medicine is based on traditional Chinese medicine and veterinary medicine. It is used scientifically in feeding to promote animal growth and development, and enhance the immunity, antibacterial and antiviral capabilities, and resistance to environmental stress of livestock and poultry.
Traditional Chinese medicine (TCM) has proven its effectiveness over a long period and can be taken long-term as a substance to improve animal immunity without leaving any residues or causing drug resistance. However, its drawbacks include the large variety of herbs, complex mechanisms of action, and diverse processing methods. Its efficacy is significantly affected by the quality of the raw materials, necessitating strict control over the quality of the herbs and production standards.
Studies by Jiang Lu et al. have shown that adding herbal feed additives to the basal diet can improve the immune performance, antioxidant capacity, and promote the synthesis and secretion of growth hormones in fattening pigs.
- Trace elements
Trace elements are important nutrients that make up body tissues. They are components of most metabolic enzymes and mainly exist in the body in the form of proteins and amino acids. Their main functions are: 1. to improve the quality and stability of feed; 2. to have intestinal barrier function; 3. to improve the body's antioxidant capacity; 4. small peptide trace elements have antibacterial properties; and 5. to improve the disease resistance of pigs.
Micronutrient supplementation can effectively improve livestock and poultry production performance and immunity, and enhance the quality of livestock and poultry products (meat, eggs, milk, etc.), offering unique advantages. However, it also has drawbacks: micronutrients have high requirements for content and quality. Inorganic micronutrients have low absorption and utilization rates, are prone to absorbing moisture, clumping, and oxidation, and contain large amounts of heavy metal impurities, easily causing pollution. Organic micronutrients have high production costs and are currently mainly used as additives in feed for young and mother livestock.
Antibiotic substitution in pig production is a complex issue, as there is a wide variety of antibiotic alternatives available, each with its own characteristics. Differences exist between pig breeds and different growth stages, resulting in variations in the growth-promoting and immunomodulatory effects of different alternatives. Therefore, in actual production, it is crucial to emphasize the development of antibiotic substitution programs, conduct thorough research and evaluation of alternative products, and adhere to a "one-plant-one-policy" approach, objectively analyzing and scientifically judging to select targeted and highly beneficial antibiotic alternatives tailored to local conditions.