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Antibiotics have been added to animal feed for nearly 60 years to enhance the growth and development of livestock and poultry. However, due to concerns over the emergence of drug-resistant strains and the risk of drug residues in animal products, which can harm both animal and human health, there has been a growing shift toward using alternative antibiotics such as tylosin and bacitracin. These additives are widely used across various species. Despite ongoing debates about their use, it is well-established that when applied properly, antibiotic feed additives can significantly improve growth rates and feed conversion efficiency. Countries that have banned antibiotics in feed often face increased production costs, reduced profitability, and challenges related to environmental impact and international trade. As a result, antibiotics remain extensively used in the industry. To fully harness their benefits while minimizing risks, it is essential to understand their mechanisms and apply them responsibly. This article aims to provide an overview of the role, growth-promoting mechanisms, and commonly used antibiotics in animal feed.
The growth-promoting effects of antibiotics were first observed by Stocktadt et al. in the late 1940s. Since then, antibiotics like tetracyclines, macrolides, and β-lactams have been widely used to enhance growth, increase weight gain, and improve feed efficiency. In 1950, the U.S. Food and Drug Administration (FDA) officially approved the use of antibiotics in feed. The positive and consistent effects on livestock and poultry performance have made these additives a staple in many countries’ agricultural practices. Common characteristics of these effects include greater efficacy in young animals, stronger results under poor sanitation conditions, and varying effectiveness depending on the type of animal. For example, antibiotics tend to work better in single-stomach animals like pigs and chickens than in adult ruminants, though they may still be effective in younger ruminants such as calves and lambs.
The mechanism behind the growth-promoting action of antibiotics is not yet fully understood, but it is generally believed to involve indirect effects rather than direct stimulation. One key aspect is the improvement of animal health. Antibiotics can inhibit or kill certain pathogens, reducing disease incidence and enhancing immune function, especially in young animals with underdeveloped immune systems. They also help prevent the overgrowth of harmful bacteria in the gut, which can interfere with nutrient absorption. Additionally, antibiotics may reduce ammonia levels in tissues and environments, and mitigate the negative effects of toxins or other unfavorable factors in feed, thereby improving overall feed efficiency.
Another important aspect is the nutritional impact of antibiotics. By protecting the intestinal lining from microbial damage, antibiotics allow for better nutrient absorption. Some studies suggest that antibiotics can promote the growth of beneficial bacteria, enhance the structure of the gut lining, and even stimulate appetite and hormone secretion. These combined effects contribute to improved growth and performance in livestock and poultry.
Commonly used antibiotic feed additives today include peptides, phosphorylated polysaccharides, polyethers, macrolides, and aminoglycosides. These additives are designed to be effective against pathogens without disrupting the natural gut microbiota, have low toxicity, and leave minimal residues in animal products. They also resist resistance development and do not cross-react with other antibiotics.
Peptides such as bacitracin, colistin, enramycin, virginiamycin, thiopeptin, and avoparcin are widely used. Each has specific applications and dosing guidelines based on the species and age of the animals. For instance, bacitracin is commonly used in pigs and chickens, while colistin is effective in preventing bacterial infections in calves and pigs. Enramycin is known for its strong activity against Gram-positive bacteria, and virginiamycin improves feed utilization in broilers. Thiopeptin is effective against Gram-positive bacteria and mycoplasma, but should be used cautiously due to potential liver toxicity.
Phosphorylated polysaccharides, including flavomycin and queuamycin, are another class of antibiotics used in feed. Flavomycin, for example, promotes growth in various animals and helps reduce stress. It is often used in combination with polyethers to improve meat quality in poultry.
Polyether antibiotics, such as monensin, salinomycin, and lasalocid, are primarily used as anticoccidials but also have broad-spectrum antibacterial properties. Monensin is widely used in cattle, while salinomycin is effective in pigs and poultry. Lasalocid is particularly useful in ruminants and is considered safe for horses.
Macrolides, such as tylosin, are effective against Gram-positive bacteria, mycoplasma, and spirochetes. They are safer than some other antibiotics but can develop resistance more easily. Aminoglycosides like destomycin A and hygromycin B are used for their broad-spectrum antibacterial and anthelmintic properties, and are often combined with other additives to enhance their effectiveness.
In conclusion, while the use of antibiotics in feed remains controversial, their benefits in promoting growth, improving feed efficiency, and reducing disease prevalence cannot be ignored. Proper management, responsible use, and continued research into alternatives are essential to ensure the long-term sustainability of this practice.