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.

Summary

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.

References

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.