Marcia S. Carlson and Thomas J. Fangman
Department of Animal Sciences

The term antibiotic means "against life" or "destructive to life." An antibiotic is a natural compound synthesized by a living organism that inhibits the growth of another living organism.

Nearly all antibiotics are produced by bacteria or molds. Many microorganisms produce antibiotics that inhibit or kill other organisms, a process called antibiosis. Some antibiotics are bacteriostatic; they prevent growth of bacteria. Others, like penicillin, are bactericidal; they destroy bacteria. Groups of antibiotics based on their action are as follows:

  • Agents that act on the cell wall of bacteria, such as penicillin and bacitracin.
  • Agents that have a detergent effect on the cell membrane, such as polymyxin and novobiocin.
  • Agents that interfere with protein synthesis, such as tetracyclines and streptomycin.
  • Agents that affect nucleic acid metabolism, such as griseofulvin.

Another way to compare antibiotics is to look at their scope of effectiveness.

  • Narrow-spectrum antibiotics have an antibacterial effect on a relatively small number of species.
  • Broad-spectrum antibiotics are active against a variety of organisms. In general, where no specific disease is isolated, the antibiotic with the broader spectrum — that is, effective against the greater number of detrimental organisms — is preferred.
Figure 1. Proper use of feed additives can help ensure swine health and growth performance.

Antibiotics have a short history. In 1929, Alexander Fleming discovered penicillin by accident when a small amount of penicillin fungus landed on a petri dish loaded with staphylococci. During the 1930s, penicillin could be obtained only from government sources and was used to treat soldiers with infections or gangrene. Penicillin was first used in the U.S. general public in 1942 to combat staphylococci among victims of a nightclub fire. Over the next 50 years, scientists and health professionals learned to make use of the therapeutic and prophylactic qualities of many antibiotics as they became available. In swine production, the development of a large number of antibiotics and other additives has helped increase gain and reduce the feed required per unit of gain.

At least 11 antibacterial or antifungal compounds or groups of compounds are widely used in swine feeds. These compounds include various salts of bacitracin, chlortetracycline, dynafac, mycostatin, oxytetracycline, oleandomycin, penicillin, streptomycin, bambermycins, tilmicosin and tylosin.

Chemotherapeutics and anthelmintics

Chemotherapeutics are organic compounds with bacteriostatic or bactericidal properties similar to those of antibiotics. But, unlike antibiotics, these compounds are produced chemically rather than microbiologically. Anthelmintics, or dewormers, are also organic compounds added to swine diets generally for short intervals to help control worms in growing-finishing swine and the breeding herd.

Other additives with potential for swine feeds are also commonly used, such as carbadox and sulfas. New products are likely to continue to be developed and approved.

Mode of action

The referring veterinarian commonly uses feed-grade antibiotics at therapeutic levels to treat an acute disease outbreak following a diagnosis. In this way, the antibiotic sensitivity of the bacterial pathogen causing the observed clinical signs is treated with an antibiotic to which the pathogen has a demonstrated susceptibility. This mode of specific pathogen treatment results in a bactericidal or bacteriostatic effect; that is, the treatment either kills the pathogen or prevents it from spreading.

In many instances, antibiotics are used to promote growth, or weight. It is generally accepted that antibiotics aid in the promotion of quality pork growth through a direct metabolic effect, by sparing use of nutrients or by controlling low levels of endemic pathogens.

1. Nutrient sparing effect

The effects of antibiotics on nutrition have been the subject of extensive research since the early 1950s. In hogs, the action of antibiotics in a nutrient sparing role is confined to the effect on the microbial population of the intestinal tract, particularly the large intestine and the lower regions of the small intestine. Microbial activity affecting nutrition in the stomach is very limited.

Antibiotics can influence nutrition in the following ways:

  • Enhancing the growth of intestinal organisms that synthesize nutrients required by the animal. Such organisms may provide vitamins and amino acids. They may also digest cellulose (fiber) to end products that are useful to the animal. Pigs, like other monogastric animals, cannot digest fiber.
  • Depressing growth of organisms that compete with the host animal for nutrients. Such organisms include those which break down glucose, degrade amino acids and alter fatty acids and bile salts.
  • Reducing the intestinal wall thickness, implying the potential for improved absorption.

2. Disease control effect

By suppressing disease-causing organisms, including toxin producers, in the animal's environment, antibiotics may reduce the incidence of clinical and subclinical diseases that hinder performance. Producers may see these disease control effects in a greater response to antibiotics in the following situations:

  • In environments that are not intensively managed for cleanliness.
  • In hogs that are gaining at slower rates.
  • In buildings where mixed-age swine are present.
  • In older buildings.

3. Metabolic effect

The metabolic effect of an antibiotic is defined as a response that alters the rate or pattern of some biochemical process at the tissue (muscle) level. Most growth-promoting antibiotics are not present in high enough concentrations at the tissue level to have a bacteriostatic or bactericidal effect on bacteria.

Choosing a feed additive

Several growth-promoting feed additives are available to swine producers. Table 1 lists several commonly used feed additives, their approved levels, and their withdrawal times.

Consider the required withdrawal time before slaughter when choosing a feed additive. Some feed additives remain in tissues longer than others. The level fed and the duration of feeding also influence tissue retention of additives. Abide by the required withdrawal times and use only the approved concentrations.

Also consider your farm's environment, management conditions, and the stage of production cycle when choosing a feed additive. These things will cause the specific feed additive and the level needed for the best response to vary.

The degree of response to feed additives will vary with the control of disease organisms in the pig's environment. Response will also vary by stage of production; in certain stages, response to feed additives is clearly seen; in others, a response is less evident.

The first few weeks of the pig's life are by far the most critical in terms of nutritional needs or health protection. Born with no protection against disease organisms, the pig enters an environment saturated with a variety of bacterial organisms.

The first milk, colostrum, of the sow will supply the pig with antibodies against the disease organisms in the surroundings, if the sow has been previously exposed to the organisms for enough time to synthesize the antibodies and concentrate them in the colostrum. However, by three weeks of age, this acquired immunity begins to disappear. The pig does not begin producing antibodies until five or six weeks of age. Thus, between three and five weeks, the young pig is most vulnerable to disease, especially to any new, infective organism that enters the pig's environment.

Also, during the early weeks of a pig's life, it is exposed to several stress conditions that render it more susceptible to diseases: castration, weaning, treating for anemia, ear notching, vaccination, climatic stresses, and exposure to internal and external parasites. Research has shown marked responses to antibiotics during these early production stages. By the time the pig reaches 40 to 50 pounds body weight, its own disease protective system — antibody formation — is functioning well, and it has adapted to environmental stresses. This is why a practical feed additive program calls for a reduction in levels as the pig develops or progresses to market weight. Fast-growing, healthy pigs may not benefit from antibiotics or chemotherapeutics during the finishing phase. However, on high health farms, improved average daily gan can be observed when low concentrations of antibiotics are fed to alter the enteric flora and prevent subclinical levels of enteric diseases that reduce nutrient absorption, such as the endemic presence of Lawsonia (ileitis) or Salmonella species.

Table 1. Feed-grade antibiotics for targeted diseases and response.

Disease: Bacterial diarrheaApproved treatment levels
E. coliLawsoniaSalmonellaDysenteryCompoundProphylactic
(grams per ton)
Withdrawal timeTheraputic
(grams per ton)
Withdrawal timeFeeding duration
x   Apramycin15028 days  14 days
   x1Arsanilic acid (AA)45-905 days905 days 
   x1AA and bacitracin MD (BMD)45-90 and 10-305 days   
   x1AA and bacitracin zinc (BZ)45-90 and 10-505 days   
   x1AA and C or O  45 to 90 and >1005 days 
   x1AA and P  45 to 90 and 1005 days 
   xBacitracin MD (BMD)  250 (for G/F)  
    BMD for gestating sows2  250  
   xBMD and R  250 and 22.7 to 345 days 
  xxCarbadox (CX)  50 (>15 percentCP; <75 pounds body weight)70 days 
  xxCX and pyrantel tartrate  50 and 96  
x x Chlorotetracycline (C)  10 milligrams per pound body weightSee label14 days
x x C and BMD  10 milligrams per kilogram body weight and 10 to 30See label14 days
    C and hygromycin B  10 milligrams per kilogram body weight and 1215 days14 days
   xC and R  10 to 50 and 181.55 days6 days
x x C and R  10 milligrams per pound body weight and 22.7 to 345 days14 days
x xxC and R  10 milligrams per pound body weight and 181.55 days6 days
  xxC + SM + P  100 + 100 + 5015 days 
  xxC + ST + P  100 + 100 + 507 days 
x xxC and tiamulin H fumarate (THF)  10 milligrams per pound body weight and 352 days 
   xLincomycin40 100 21 days
    Neomycin (N)(used in combinations)
x x Oxytetracycline (O)  10 milligrams per pound body weight5 days7 to 14 days
  xxO and R  >100 and 22.7 to 345 days 
  xxO and N3  50 to 100 and 35 to 1405 days 
   xRoxarsone (R)  181.55 days6days
    R and P  22.7 to 34 and 1005 days 
    Sulfamethazine (SM)(used in combinations)
    Sulfathiazole (ST)(used in combinations)
   xTiamulin H fumarate (THF)352 days200 (for 14 days)7 days 
   xTylosin (T)  40 to 100 21 days
 x  T  100 21 days
   xT and pyrantel tartrate  40 to 100 and 96  
   xT + SM  10015 days 
   xVirginiamycin425 50 to 100 14 days at 100 grams per ton
Disease: Bacterial respiratory diseaseApproved treatment levels
PasturellaMycoplasmaCompoundTheraputic (grams per ton)Withdrawal timeFeeding duration
x Chlorotetracycline10 mg per pound body weightSee label14 days
x C and BMD400 and 10 to 30See label14 days
x C and R10 milligrams per kilogram body weight and 22.7 to 345 days14 days
 xLincomycin200 21 days
xxOxytetracycline10 milligrams per pound body weight 7 to 14 days
x Tilmicosin181 to 3637 days 
x T + SM10015 days 
Disease: Bacterial reproductive disease5Approved treatment levels
LeptospirosisCervical abscesses (Septicemia)CompoundTheraputic (grams per ton)Withdrawal timeFeeding duration
x Chlorotetracycline400 14 days
x Oxytetracycline10 milligrams per pound body weight 7 to 14 days
 xC + SM + P100 + 100 + 5015 days 
 xC + ST + P100 + 100 + 507 days 
Source: 1999 Feed Additive Compendium, Minneapolis, Minn.: Miller Publishing Co.

Proper use of feed additives

Consumers are increasingly concerned about bacterial resistance and drug residues in animal tissues. Producers should use extreme caution and follow feeding directions on the label exactly. The Food and Drug Administration regulates the use of feed additives. The Animal-Plant Health Inspection Service of the U.S. Department of Agriculture and the state feed regulatory agencies are responsible for compliance with FDA regulations and are actively initiating more rigid controls for monitoring feed and pork at packing houses for residues of feed additives. Producers should abide by the FDA regulations on removal of certain additives before selling hogs for slaughter. Disregarding these regulations could cause costly losses to producers because of tissue residues and the loss of certain effective compounds for use as feed additives.

In addition to the formation required for nonmedicated feeds, the FDA requires that all medicated feeds carry the following information on the tag:

  • Purpose of the medication. The FDA evaluates the drugs for effectiveness in growth promotion or disease prevention and treatment. Those purposes that have been adequately tested will appear on the label.
  • Directions for use
    Mix at proper levels and do not use unapproved combinations.
  • Names and amounts of all active drug ingredients
  • The withdrawal period
    A warning or caution for withdrawal is on the label when required for the particular drug contained in the feed.
  • Warning against misuse
    This will explain the adverse effects of using too high levels or feeding at the wrong stage of production.

If you use medicated feeds:

  • Read the feed tag
    Be sure you're using the medicated feed for the right stage of production and for tested and approved reasons.
  • Meet the withdrawal times to avoid residues and to ensure safe and wholesome pork.
  • Do not assume withdrawal will just happen
    The time after last feeding required for a drug to clear the system varies with the type of and level of drug. All approved drugs have been tested for clearance time, and the withdrawal period is based on research.
  • Use medicated feeds only for the purpose and species indicated
    Drugs that are effective and approved for use in other species may not be effective in pigs, or the clearance time may differ.
  • Do not give additional drugs to animals on medicated feed without professional approval
    One drug may interfere with the effectiveness or clearance rate of another drug.
  • Do not permit other drugs to contaminate medicated or nonmedicated feed through mixer contamination or by other means

Although the responses to additives are more variable in gestating-lactating sows, you can expect a response during breeding and just before and after farrowing. Research has shown that feed additives in the breeding ration will increase conception rate and litter size, and additives in the farrowing ration increase pig survival and performance.

For certain diseases and parasites, feed additives can control a specific problem through short-term use of higher levels than those allowed for growth promotion. Correctly diagnosing the trouble and matching the additives to the problem are important. Table 1 contains a list of specific diseases and the feed-grade antibiotics that have proved effective in controlling or treating the problem.

Producers should have their own feed additive programs as prescribed/defined by a veterinarian. Some may need to feed a certain antibiotic or chemotherapeutic compound during all stages of growth and development. Others may need to feed a certain additive or combinations for a certain period, then discontinue or change to another additive for additional growth periods only (pulsating). Each of these programs can be effective as long as you maintain a standard feeding program, using certain antibiotics or chemotherapeutics for growth promotion while reserving others for disease outbreaks. Table 2 presents some recommended levels and compounds of antibiotics for improvement in growth performance and feed efficiency.

This does not mean that you should stay with one antibiotic or chemotherapeutic year after year, but you should avoid continual switching every two or three weeks or using a number of different antibiotics within a short period. By following a standard antibiotic or chemotherapeutic program, you and your veterinarian can plan and adapt a treatment or a preventive medicine program accordingly.

Always follow good feeding, sanitation and disease control programs, and don't expect to buy management in a bag of medicated feed.

Table 2. Feed-grade antibiotics for improvement in growth performance.

Disease/CompoundApproved treatment levels
Prophylactic (grams per ton)Withdrawal timeFeeding duration (days)
Growth promotion — Feed efficiency (Presence of atrophic rhinitis)
Arsanilic acid (AA)45 to 905 days 
AA and bacitracin MD (BMD)45 to 90 and 10 to 305 days 
AA and bacitracin zinc (BZ)45 to 90 and 10 to 50 or 10 to 305 days 
Bacitracin MD (BMD)10 to 30 <250 pounds body weight
BMD and R10 to 30 and 22.7 to 345 days 
Bacitracin zinc (BZ)10 to 50  
Bambermycin2 to 4  
Carbodox10 to 2570 days>15 percent CP; <75 pounds body weight
Chlorotetracycline10 to 50  
C + SM + P100 + 100 + 5015 days<75 pounds body weight
C + SM + P6100 + 100 + 507 days10 pounds body weight up to 6 wk post-wean
Oxytetracycline10 to 50  
O and N50 to 150 and 70 to 1405 days 
Penicillin (P)10 to 50  
Roxarsone (R)22.7 to 345 days 
R and P22.7 to 34 and 1005 days 
Tiamulin H fumarate (THF)10  
Tylosin (See production phase.)
  Starter20 to 100  
  Grower20 to 40  
  Finisher10 to 20  
T and hygromycin B10 to 100 and 1215 days 
T + SM10015 days 
Virginiamycin5 to 10  
Source: 1999 Feed Additive Compendium, Minneapolis, Minn.: Miller Publishing Co.


All pork processing facilities will be expected to have a Hazard Analysis Critical Control Point (HACCP) plan in place that will ensure that pork processed in the United States is free of salmonella contamination. There will also be a greater emphasis placed on surveillance of pork carcasses for antibiotic residues when the HACCP plans are initiated. Therefore, as producers or advisers it is imperative that you have an understanding of the reasons that a feed additive will be formulated into the ration. The table included in this guide serves as a reminder of the legally approved use of the described feed-grade antibiotics and the mandatory withdrawal periods required for each compound. It is critical that the proper feed-grade antibiotic is selected according to the history of the farm or based on recommendations of a veterinarian for the specific pathogen or condition to be treated. It is also important that the proper withdrawal time for the feed-grade antibiotic or feed additive is strictly adhered to.

Publication No. G2353