Fact Sheet Peer reviewed
In-feed antibiotics
Jay Y. Jacela, DVM; Joel M. DeRouchey, PhD; Mike D. Tokach, PhD; Robert D. Goodband, PhD; Jim L. Nelssen, PhD; David G. Renter, DVM, PhD; Steve S. Dritz, DVM, PhD
JYJ, DGR, SSD: Department of Diagnostic Medicine/Pathobiology, Kansas State University, Manhattan, Kansas. JMD, MDT, RDG, JLN: Department of Animal Science and Industry, Kansas State University, Manhattan, Kansas. Corresponding author: Dr Jay Y. Jacela, I-102 Mosier Hall, 1800 Denison Ave, Manhattan, KS 66506; Tel: 785-532-4845; E-mail: jjacela@vet.ksu.edu.

RIS citationCite as: Jacela JY, DeRouchey JM, Tokach MD, et al. Feed additives for swine: Fact sheets – acidifiers and antibiotics. J Swine Health Prod. 2009;17(5):270–275.
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Fast facts

Use of in-feed antibiotics in pigs is regulated by the Food and Drug Administration and they must be used only as approved.

No extra-label usage is allowed for in-feed antibiotics.

The best responses in growth performance are seen in nursery pigs.

Magnitude of responses may differ depending on herd-health status and sanitation.

Concerns are increasing about the negative consequences of antibiotic use in food animals.

Antimicrobial agents, such as antibiotics, have been used in pig production for over 50 years. Early studies indicated significant improvements in pig growth performance when antibiotics were fed. With the improvements in production practices and health status of pig herds, positive responses to in-feed antibiotics may not be as large in today’s modern facilities. Additionally, the magnitude of response differs with the stage of pig growth. Use of antibiotics as feed additives is subject to regulatory policies to prevent residues and enhance public health. It is therefore important to be aware of the current information available concerning the effects of commonly used in-feed antibiotics in pig production.

How do antibiotics enhance growth?

Antibiotics are non-nutritive feed additives, which means that they do not provide further nourishment to the pig, and their absence in a well-balanced diet will not result in nutritional deficiency. Antibiotics are included in swine feed for their therapeutic potential as well as their ability to promote growth. Some of the proposed possible mechanisms by which antibiotics improve growth include inhibition of subclinical pathogenic bacterial infections; reduction of microbial metabolism products that may negatively affect pig growth; inhibition of microbial growth, thereby increasing nutrients available to the pig; and an increase in uptake and utilization of nutrients through the intestinal wall.1

Efficacy of in-feed antibiotics

Studies2 on the effects of antibiotic feed additives have indicated significant improvements in growth rate and feed efficiency (Table 1). These studies, however, were conducted more than two decades ago, when disease pressures in pig farms were relatively greater than in today’s facilities. With numerous improvements, such as multi-site pig production, nutrition, biosecurity, and overall pig husbandry practices in the last two decades, responses may not be as great. A more recent study3 on the use of in-feed antibiotics in modern production systems showed that such additives are still effective in improving growth in nursery pigs, although the magnitude of the response is less (Table 2). However, in finishing pigs, no improvement is noted. Many factors can affect the efficacy of antibiotic feed additives, including nutrition, management practices, and health status. When these factors are optimal, less or almost no response to antibiotics can be expected, especially with excellent sanitation practices and lack of bacterial disease pressure. The data on feeding antimicrobials in sow diets, however, is much more limited than that in growing pigs. Antibiotics in sow diets may improve reproductive performance in herds with a high incidence of reproductive problems due to greater disease challenge.4,5 Thus, herds experiencing problems with conception rates and litter size associated with bacterial infections may benefit from the addition of antibiotics to sow diets. Chlortetracycline and oxytetracycline, the two in-feed antibiotics approved for use in sow diets, are indicated to reduce the incidence of abortion due to Leptospira interrogans serovars and reduce shedding of these organisms. However, routine feeding of antibiotics to the breeding herd is discouraged.

Choosing the proper antibiotic

When the antibiotic appropriate for a specific herd is selected, a number of important things must be considered, for example, the disease organisms present in the herd. Certain antibiotics may be more efficacious in treating respiratory problems, while others may be more effective against enteric pathogens. Stage of production and withdrawal period also will determine the specific antibiotic of choice. While in-feed antibiotic use is most prevalent in nursery diets, it is sometimes necessary to use antibiotics in grow-finish diets, eg, during outbreaks of bacterial disease. Observing the proper withdrawal time for an in-feed antibiotic is important to avoid residues in the meat. Improper consideration of withdrawal time may result in delays in marketing pigs. The product also must be approved for use in swine, as no extra-label usage is allowed for in-feed antimicrobials. Ultimately, choosing the proper in-feed antibiotic depends on the benefit in production efficiency compared to cost and risk of residue.

Proper use of in-feed antibiotics

While most in-feed antibiotics are available without veterinary supervision, they should not be used indiscriminately. They should be used only for purposes specified on the labels. A good reference for the list of drugs that can be used as feed additives is the Feed Additive Compendium,6 which is updated regularly to provide up-to-date information and provides guidelines on the proper use of antibiotics in feed. Each country has its own regulatory policies regarding use of feed additives in pigs. Thus, the recommendations in this fact sheet may not apply outside of the United States. It is, therefore, important for US producers to be aware of which antibiotics are forbidden in countries that import pork from the United States.

Which antibiotics are approved for use as feed additives in pig diets in the United States?

Antibiotics and combinations approved for use in swine diets, including withdrawal times, are listed in Table 3. Florfenicol and tilmicosin, which are classified as Veterinary Feed Directive (VFD) drugs, are also included in the list. Veterinary Feed Directive drugs can be used only under the order and professional supervision of an appropriately licensed veterinarian.7 Before a VFD drug can be used, the producer must first contact the veterinarian to diagnose and treat the existing health problem. A VFD order can be written only by a veterinarian for drugs that are approved for that swine category and under a valid client-patient relationship.7 This is accomplished by filling out a form in a format approved by the Food and Drug Administration Center for Veterinary Medicine. All pertinent information must be provided by the veterinarian. The veterinarian, producer, and feed miller must all follow the responsibilities outlined by the Food and Drug Administration Center for Veterinary Medicine when using VFD drugs. Issued VFDs for florfenicol and tilmicosin have an expiration period of 90 days.


Increased productivity, efficiency, and profitability are the goals of every swine-production business. Antibiotics have been used in swine diets for several decades to improve growth performance, as well as to control and treat diseases. Because of the improvements made in housing, nutrition, production, and health-management practices over the years, the impact of antibiotics on growth performance may not be as large or as consistent in response as those observed during the early years of antibiotic use. In-feed antibiotics remain an effective tool in improving production efficiency, but are not a substitute for good production management. These products must be used properly and responsibly.


1. Gaskins HR, Collier CT, Anderson DB. Antibiotics as growth promotants: mode of action. Anim Biotechnol. 2002;13:29–42.

2. Cromwell GL. Antimicrobial and promicrobial agents. In: Lewis AJ, Southern LL, eds. Swine Nutrition. 2nd ed. Boca Raton, Florida: CRC Press LLC; 2001:401–426.

3. Dritz SS, Tokach MD, Goodband RD, Nelssen JL. Effects of administration of antimicrobials in feed on growth rate and feed efficiency of pigs in multisite production systems. JAVMA. 2002;220:1690–1695.

4. Cromwell GL. Why and how antibiotics are used in swine production. Anim Biotechnol. 2002;13:7–27.

5. Alexopoulos C, Fthenakis GC, Burriel A, Bourtzi-Hatzopoulou E, Kritas SK, Sbiraki A, Kyriakis SC. The effects of the periodical use of in-feed chlortetracycline on the reproductive performance of gilts and sows of a commercial pig farm with a history of clinical and subclinical viral and bacterial infections. Reprod Domest Anim. 2003;38:187–192.

6. Feedstuffs. Feed Additive Compendium. Minnetonka, Minnesota: Miller Publishing Co; 2008.

7. Animal Drugs @ FDA. US Food and Drug Administration Center for Veterinary Medicine Web site. Available at: http://www.accessdata.fda.gov/scripts/animaldrugsatfda/. Accessed 10 July 2009.



Table 1: Effectiveness of in-feed antibiotics on production responses in pigs*

Parameter Control Antibiotic Difference (%)
Starter phase (15 to 55 lb)
ADG (lb) 0.86 0.99 16.4
F:G 2.28 2.13 6.9
Grower phase (37 to 108 lb)
ADG (lb) 1.30 1.45 10.6
F:G 2.91 2.78 4.5
Grow-finish phase (53 to 196 lb)
ADG (lb) 1.52 1.59 4.2
F:G 3.30 3.23 2.2

*  Adapted from Cromwell (2001)2 as adapted from Hays VW (Effectiveness of Feed Additive Usage of Antibacterial Agents in Swine and Poultry Production. Washington, DC: Office of Technology Assessment, US Congress; 1977) and Zimmerman DR [Role of subtherapeutic antimicrobials in animal production. J Anim Sci. 1986;62(Suppl3):6]. Data from 453, 298, and 443 experiments, involving 13,632, 5783, and 13, 140 pigs for the three phases, respectively.

ADG = average daily gain; F:G = feed-to-gain ratio.



Table 2: Effectiveness of in-feed antibiotics in nursery and grow-finish pigs reared in modern production systems*

Parameter Control Antibiotic†
Nursery phase
ADG (lb) 0.96† 1.01†
F:G 1.44 1.42
Grow-finish phase
ADG (lb) 1.72 1.72
F:G 2.90 2.90

*   Adapted from Dritz et al, 2002.3 Data from five and four experiments, involving 3648 and 2660 pigs, for the nursery and grow-finish phases, respectively.

†   ADG was greater (5.0% difference) in nursery pigs treated with antibiotics than in controls (ANOVA; P < .05)

ADG = average daily gain; F:G = feed-to-gain ratio.


Table 3: Withdrawal periods for FDA-approved in-feed antibiotics and combinations*

Antibiotic Indication Inclusion rate
Withdrawal period (days)
disalicylate (BMD)
Increased ADG and feed efficiency 10-30 0
Grow-finish: control of swine dysentery 250 0
Sows: control of clostridial enteritis in suckling piglets 250 0
chlortetracycline (CTC)
Increased ADG and feed efficiency BMD: 10-30;
CTC: 400†
Treatment of bacterial enteritis and
bacterial pneumonia
Bacitracin zinc Increased ADG and feed efficiency 10-50 0
Bambermycin Increased ADG and feed efficiency 2-4 0
Carbadox Increased ADG and feed efficiency 10-25 42
Control of swine dysentery and salmonellosis 50 42
CTC Increased ADG and feed efficiency 10-50 VW‡
Reduction of jowl abscesses 50-100 VW‡
Control of leptospirosis in sows 400† VW‡
Control of proliferative enteropathies (ileitis) BW dosage:
10 mg/lb/d†
CTC + sulfathiazole + penicillin Reduction of abscesses; treatment of bacterial
enteritis; maintenance of weight gain in the
presence of rhinitis
100 CTC;
100 sulfa;
50 penicillin
Florfenicol¶ Control of bacterial respiratory disease 182 13
Lincomycin Increased ADG and feed efficiency 20 0
Control of swine dysentery and ileitis 40-100 0
Reduce severity of mycoplasmal pneumonia 200 0
Neomycin Treatment and control of bacterial enteritis BW dosage: 10 mg/lb/d for 24-48 h beyond remission of symptoms, ≤ 14
consecutive days
Neomycin/oxytetracyline Increased ADG and feed efficiency 10-50 5
Treatment of bacterial enteritis and
bacterial pneumonia
BW dosage:
10 mg/lb/d, 7-14 d
Control and treatment of leptospirosis in breeders BW dosage:
10 mg/lb/d, 7-14 d
Oxytetracycline Increased ADG and feed efficiency 10-50 0
Treatment of bacterial enteritis and
bacterial pneumonia
BW dosage:
10 mg/lb/d, 7-14 d
Control of leptospirosis in sows BW dosage:
10 mg/lb/d, 7-14 d
+ carbadox
Treatment of bacterial enteritis and
bacterial pneumonia
10-25 carbadox; oxytetracycline BW dosage: 10 mg/lb/d 42
+ neomycin
Prevention or treatment of bacterial enteritis and
dysentery; maintenance of weight gain in the
presence of atrophic rhinitis
50-150 oxytetracycline; neomycin BW dosage: 35-140 mg/lb/d 10
Tiamulin Control of dysentery and ileitis 35 2
Treatment of swine dysentery 200 7
Tiamulin + CTC Control of dysentery; treatment of bacterial enteritis and bacterial pneumonia 35 tiamulin + 400 CTC (BW dosage: 10 mg/lb/d) 2
Tilmicosin¶ Control of bacterial respiratory disease 181-363 7
Tylosin Increased ADG and feed efficiency in finishers 10-20 0
Increased ADG and feed efficiency in growers 20-40 0
Increased ADG and feed efficiency in nursery pigs 20-100 0
Control of swine dysentery 40-100 0
Control of dysentery and ileitis 100 0
Virginiamycin Increased ADG and feed efficiency 5-10 0
Control of swine dysentery 25 0
Treatment of swine dysentery 100 0

*   Sources: 2008 Feed Additive Compendium6 and Food and Drug Administration Center for Veterinary Medicine7.

†   Limitations: feed continuously for ≤ 14 days at approximately 400 g/ton of feed, varying with body weight (BW) and feed consumption to provide 10 mg/lb BW/d.

‡   Voluntary withdrawal to meet residue limits of certain export markets.

¶   Veterinary Feed Directive drug.