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Incorporating sub-therapeutic levels of antibacterial feed additives in swine and poultry feeds has been a relatively common practice since the early 1950s. Sub-therapeutic inclusion of antibacterial compounds in feeds can be defined as inclusion of these compounds in complete feeds at doses below what would be intended to treat clinical disease but at a level that will promote more rapid growth and improved feed efficiency or maintenance of growth in the presence of sub-clinical disease. The actual inclusion rate for sub-therapeutic growth promoters ranges from 10 to 200 grams per ton of complete feed and is dependant on the specific product being used and classification of animal being fed. For swine, there are currently 17 antibacterial and chemotherapeutic growth promoters approved by the Food and Drug Administration for use in the United States (Feed Additive Compendium, 2000).
Based on data from the Centers for Disease Control and the Animal Health Institute, about 23 million kilograms of antibiotics are produced annually in the U.S., of which about one third are used for animal purposes. Use of growth promoting antibacterial agents in swine production is relatively common. A recent survey indicated that over 80 % of U.S. swine farm sites with nursery pigs use antibacterial growth promoters in starter feeds (NAHMS Survey, 2001). Furthermore over 60 % of sites with finishing pigs used growth promoters in growing-finishing swine feeds.
Antibacterial feed additives continue to be used by swine producers for a simple reason. They are effective. In starter pigs, improvements in growth rate can be as high as 16% and feed efficiency (less feed per lb. of gain) can be improved as much as 7%. In growing-finishing pigs growth rate can be improved as much as 11% and feed efficiency as much as 5%. Return on investment for using antibacterial feed additives in swine will vary depending on individual situations, but estimates of $3.00 per pig marketed have been indicated (Cromwell, 2000). This is substantial when one considers that the profit margin per market hog sold is typically only a few dollars. Improvements in performance and efficiency with growth promoters are ultimately beneficial to consumers in the form of more competitively priced pork products in food stores.
However, at various times since the established use of antibacterial growth promoters, concerns have been raised about potential negative aspects. These concerns have centered on the potential for drug residues in meat products and the possibility of development of drug resistant bacterial that could compromise disease treatment in humans or animals. The drug residue issue has largely subsided with more frequent residue testing of meat products in processing facilities and the careful use of feed additives and specified withdrawal times by livestock producers. But recently, concerns about anti-microbial resistance have increased.
In the European Union countries, these concerns have resulted in some significant changes. In July of 1999, the use of the feed additive antibiotics virginiamycin (Stafac), spiramycin, tylosin phosphate (Tylan), and zinc bacitracin (Alban and Baciferm) were banned and additional products are reportedly being targeted by the European Union for addition to the ban in the near term future (Muirhead, 2001). Concerns are also being expressed in this country. In an editorial in a recent issue of the New England Journal of Medicine, Dr. Sherwood L. Gorbach of Tufts University Medical School called for the elimination of use of growth promoting antibacterial agents in livestock and poultry production (Schuff, 2001). To those who follow the debate, there seems to be two distinct points of view. Some argue with good basis that antibacterial growth promoters have been used in livestock and poultry production for over 50 years and continue to be highly effective with little or no direct evidence that the effectiveness of human or animal disease treatment is jeopardized (Cromwell, 2000). Others would argue that enough evidence exists to call for a ban in growth promoter use in order to avoid the potential risk of antibacterial resistance that may impact disease treatment.
In the midst of this controversy, what are swine and other food animal producers to do? First and foremost, they should continue to focus attention on the wise and prudent use of antibacterial feed additives. Care should always be taken to use feed additive products in complete accordance with the FDA approved use label. This includes growth promoters in the form of medicated complete feeds delivered to the farm or those added to feeds manufactured on the farm. Under all circumstances, use of antibacterial growth promoters should be viewed as a production tool to be used in concert with good sanitation and herd health practices and not as something to compensate for poor herd management, poor housing or inadequate sanitation.
Another important factor includes keeping accurate records of feed additive use for each group or classification of animals being produced. Some antibacterial feed additives have pre-slaughter withdrawal periods and proper feeding records insure that these withdrawal periods are met. Of the 17 FDA approved growth promoters available for use in swine feeding, 9 have pre-slaughter withdrawal periods during which the drug must be removed from the feed before the pigs are sent to market. In addition, several swine de-wormer products designed to be included in feeds also have a required pre-slaughter withdrawal period. Antibacterial growth promoter and de-wormer products for swine that do have a required pre-slaughter withdrawal period are summarized in Table 1.
Table 1. Summary of swine antibacterial feed additives and feed delivered de-wormers that require pre-slaughter withdrawal periods (source: National Pork Board Pork Quality Assurance Level III manual, 2001 edition).
Example Trade Name
Pre-slaughter withdrawal, days
Ivomec swine premix
Tramisol Hog De-wormer
(10 to 50 grams/ton)
(10 mg/lb. body wt.)
Oxytetracyclin Plus Neomycin
(neomycin < 140 gm/ton)
(neomycin = 140 gm/ton)
Banminth premix- 48
Aureo mix 500,
Aureo SP 250,
Tylan Sulfa G