Antibiotics are used in livestock for three main reasons: treating sick animals, preventing disease outbreaks in herds or flocks, and promoting faster growth. In 2020, global antibiotic use in cattle, sheep, chickens, and pigs totaled roughly 99,500 tonnes of active ingredient, representing about 73% of all antibiotics used worldwide. That massive share has made livestock antibiotic use one of the most debated topics in public health.
Treating and Preventing Disease
The most straightforward use of antibiotics in livestock is the same as in humans: treating bacterial infections. When an animal gets sick, a veterinarian prescribes antibiotics to clear the infection. In cattle, the biggest target is bovine respiratory disease complex, a collection of bacterial lung infections that causes significant economic losses. In poultry, common targets include colibacillosis (caused by E. coli), necrotic enteritis, and secondary bacterial infections that follow viral illnesses like infectious bronchitis. Pigs face their own set of bacterial threats, including Streptococcus suis, Mycoplasma infections, and various causes of pneumonia.
Prevention, or prophylaxis, is where things get more complicated. In modern intensive farming, thousands of animals live in close quarters, which creates ideal conditions for disease to sweep through an entire flock or herd. Rather than waiting for animals to show symptoms, producers have historically added low doses of antibiotics to feed or water to keep infections from taking hold in the first place. This is especially common when animals face known stress points, like young cattle arriving at feedlots after transport or chicks in dense broiler houses. When the U.S. poultry industry started raising more birds without antibiotics (growing from under 10% in 2014 to nearly 50% by 2018), farms saw a notable rise in intestinal and respiratory diseases, illustrating how dependent the current production model has become on preventive antibiotic use.
Growth Promotion and Feed Efficiency
The most controversial use of antibiotics in livestock has nothing to do with disease. For decades, farmers added low “subtherapeutic” doses of antibiotics to animal feed because it made animals grow faster on less food. An animal receiving these growth-promoting antibiotics converts feed into body weight more efficiently, which lowers the cost of producing meat and eggs.
The exact biological mechanism behind this effect is still being studied, but research points to changes in the gut microbiome. Antibiotics reshape the bacterial community in an animal’s intestines in ways that reduce how many nutrients the bacteria consume for themselves, leaving more for the animal to absorb. Other proposed mechanisms include suppressing low-grade gut infections the animal might not visibly show, reducing inflammation in the intestinal lining, and altering how the animal digests and stores fat. A 2022 study published in Nature’s Communications Biology found that antibiotic-treated chickens had altered nitrogen metabolism in their gut bacteria, suggesting the drugs shift the balance of nutrient competition between microbes and host.
Subtherapeutic antibiotic use has essentially been a management tool, making it possible to raise animals profitably in the confined, high-density housing that dominates modern agriculture.
Which Antibiotics Are Used
The major antibiotic classes used in livestock overlap heavily with those used in human medicine. Tetracyclines are the most widely used, followed by penicillins, macrolides (related to erythromycin), sulfonamides, and fluoroquinolones. This overlap is a core part of the problem: many of these drugs are classified as “medically important” for treating human infections, meaning their effectiveness in people is directly threatened when resistance develops in animal agriculture.
Antibiotics are delivered through feed, drinking water, or individual injections depending on the situation. Feed and water delivery makes it easy to dose entire groups of animals at once, which is practical for prevention and growth promotion but means healthy animals receive antibiotics alongside sick ones.
How This Drives Antibiotic Resistance
When bacteria are repeatedly exposed to low doses of antibiotics, the ones that survive are those with genetic traits that help them resist the drug. These resistant bacteria multiply and can share their resistance genes with other bacterial species through a process called horizontal gene transfer, which accelerates the spread.
Resistant bacteria from livestock reach humans through several routes. The most direct is consuming contaminated meat, milk, or eggs that haven’t been fully cooked or pasteurized. Farm workers face daily exposure through direct contact with animals and their environment. But the reach extends further: antibiotic-resistant bacteria shed in animal waste contaminate soil and waterways, entering the broader ecosystem. Heavy antibiotic use in livestock has been linked to the emergence of drug-resistant strains of MRSA, E. coli that produce enzymes breaking down common antibiotics, colistin-resistant bacteria, and fluoroquinolone-resistant Salmonella.
Regulatory Changes in the U.S.
The FDA has taken significant steps to curb the riskiest uses of livestock antibiotics. Starting in January 2017, it became illegal in the United States to use medically important antibiotics for growth promotion or feed efficiency in food-producing animals. All 31 drug applications that included growth-promotion uses either voluntarily aligned with the new guidelines or had their approvals withdrawn. Any remaining therapeutic use of these drugs in feed or water now requires authorization from a licensed veterinarian for a specifically identified disease.
The results have been mixed. Overall U.S. sales of medically important livestock antibiotics in 2024 were 27% below their peak in 2015, reflecting meaningful progress. But the most recent data showed a 16% jump in sales between 2023 and 2024, breaking a period of generally stable numbers since the 2017 rules took effect. The FDA noted this increase as a departure from recent trends, and its stewardship plan for 2024 through 2028 focuses on further aligning antibiotic use with stewardship principles and improving monitoring of resistance patterns.
The Push Toward Alternatives
The World Health Organization recommends that healthy animals should not receive antibiotics to prevent disease unless the disease has already been diagnosed in other animals in the same group. Sick animals should ideally be tested to identify the specific infection before choosing an antibiotic, rather than using broad-spectrum drugs as a default.
Replacing routine antibiotic use requires a combination of strategies rather than a single substitute. Better vaccines are a major piece: if more effective vaccines existed for viral diseases like infectious bronchitis and infectious bursal disease in poultry, the secondary bacterial infections that currently require antibiotics would drop significantly. Improved biosecurity (controlling what enters and leaves a farm), lower stocking densities, better ventilation, and probiotics or other feed additives that support gut health are all being used to varying degrees. None of these individually replaces the convenience and cost savings of adding antibiotics to feed, which is why the transition has been gradual and, in some cases, accompanied by increased disease on farms making the switch.
The fundamental tension remains: modern livestock production was built around the availability of cheap, routine antibiotics. Removing them requires rethinking housing, genetics, nutrition, and veterinary care simultaneously, not just swapping one feed additive for another.