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Antibiotics have been used in 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 antibiotic residues in animal products, which can harm both animal health and human well-being, many countries are now shifting away from the use of traditional antibiotics. Instead, they are increasingly turning to alternatives such as tylosin and bacitracin, which are widely used across various species. Despite ongoing debates about the safety and necessity of using antibiotics in feed, it is well-established that their rational application can improve growth rates and feed conversion efficiency. Countries that have banned antibiotics in feed often experience higher production costs, reduced profitability, and environmental challenges, indicating that antibiotics remain a critical tool in modern animal farming. To maximize benefits while minimizing risks, it is essential to understand and apply these additives responsibly. This article aims to provide an overview of the role, mechanisms, and common types of antibiotic feed additives used in livestock and poultry.
The growth-promoting effects of antibiotics were first discovered by Stocktadt in the late 1940s. Since then, antibiotics like tetracyclines, macrolides, and β-lactams have been commonly added to feed to promote growth, increase weight gain, and improve feed efficiency. In 1950, the U.S. FDA officially approved the use of antibiotics in feed. Their positive impact on animal performance has been well-documented, and they have been widely adopted in the global livestock industry. The effectiveness of antibiotics varies depending on several factors: they are more effective in young animals than adults, especially in environments with poor sanitation or inadequate nutrition. They work better in single-stomach animals like pigs and chickens, but are less effective in adult ruminants. However, they may still benefit young ruminants like yaks under six months old. Prolonged use of the same antibiotic in the same environment can reduce its effectiveness over time.
The mechanism behind the growth-promoting effect of antibiotics is not fully understood, but it is generally believed to be indirect. One key aspect is the improvement of animal health. Antibiotics help control harmful pathogens and parasites, reducing disease incidence and improving immunity, especially in young animals whose immune systems are still developing. They also inhibit the growth of harmful bacteria in the gut, preventing them from consuming essential nutrients like vitamins and amino acids. Additionally, antibiotics may lower ammonia levels in the body and reduce the negative impact of certain toxins or chemicals in feed, thereby enhancing overall feed efficiency.
Another important factor is the nutritional benefits of antibiotics. By protecting the intestinal lining from microbial damage, they allow for better nutrient absorption. Some antibiotics also promote the growth of beneficial bacteria, aiding in the synthesis of essential nutrients. For example, streptomycin can stimulate yeast growth in chicks, helping them produce more nutrients. These changes support improved digestion and overall health. Some studies suggest that antibiotics can increase the length of intestinal villi, which enhances nutrient uptake. They may also stimulate appetite, increase feed intake, and even influence hormone secretion, contributing to better growth and development.
Commonly used antibiotic feed additives include peptides, phosphorylated polysaccharides, polyethers, macrolides, and aminoglycosides. These additives are designed to be effective against pathogens without disrupting the normal gut microbiota. They should be chemically stable, leave minimal residues in animal products, and have low toxicity. Many of these compounds are developed specifically for use in livestock and poultry, ensuring safety for both animals and humans.
Peptides such as bacitracin, colistin, enramycin, virginiamycin, thiopeptin, and avoparcin are widely used. Bacitracin, for instance, inhibits cell wall synthesis and is commonly used in pig and poultry feed. Colistin is effective against Gram-negative bacteria and is often combined with bacitracin for enhanced efficacy. Enramycin targets Gram-positive bacteria and is particularly useful in chicken feed. Virginiamycin improves feed efficiency and egg quality, while thiopeptin is effective against Gram-positive bacteria and mycoplasma. Avoparcin, although banned in some regions, was once widely used for its broad-spectrum activity.
Phosphorylated polysaccharides like flavomycin and queuamycin are also popular. Flavomycin improves growth and reduces stress in various animals, while queuamycin is primarily used in poultry and piglets. Polyether antibiotics, including monensin, salinomycin, and lasalocid, are known for their anti-coccidial properties and growth-promoting effects. Macrolides such as tylosin are effective against a wide range of bacteria and are often used in pig and poultry diets. Aminoglycosides like destomycin A and hygromycin B are used for their antibacterial and anthelmintic properties.
In conclusion, antibiotic feed additives continue to play a vital role in modern animal agriculture. While their use must be carefully managed to avoid resistance and residue issues, their benefits in promoting growth, improving feed efficiency, and supporting animal health remain significant. As research progresses, new and safer alternatives will likely emerge, but for now, responsible use of antibiotics remains a cornerstone of efficient livestock production.