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Antimicrobial Feed Additives and the Swine Industry

Livestock Update, June 1999

Allen Harper, Extension Animal Scientist - Swine, Tidewater AREC

Subtherapeutic levels of antimicrobial feed additives have been used in swine diets since the 1950s for improved growth rate and feed efficiency and to maintain pig performance in the presence of sub-clinical disease. The most prominent and efficacious use is in the diets of weanling and young growing pigs but responses are also obtained in finisher pigs and breeding swine. Zimmerman (1986) summarized the data from 239 separate experiments and reported that average improvement response to antimicrobial feed additives in starter pigs was 15% for growth rate and 6% for feed efficiency. In older growing-finishing pigs the improvement was 4% for growth rate and 2% for feed efficiency. It is believed that weanling and starter pigs are more susceptible to stress and subclinical disease and consequently show a greater response to growth promoting antimicrobial products. Studies have also indicated that both starter and finisher pigs have a greater response to antimicrobials under farm conditions than at swine research facilities, possibly because the disease, sanitation and housing stresses are typically greater at commercial farms than in research facilities.

The consistent effectiveness of antimicrobial feed additives has led to extensive use in the swine feeding industry. Cromwell (1991) estimated that at least 85% of all starter pig feeds, 75% of all grower pig feeds, 55% of all finisher pig feeds and 20% of all sow feeds are fortified with antimicrobial feed additives. There are 17 antimicrobial feed additive products currently approved by the U.S. Food and Drug Administration for use in swine diets. General reviews on the use and effects of antimicrobial feed additives for swine are available in the antimicrobial agents chapter of the Swine Nutrition text edited by Miller, Ullrey and Lewis (Cromwell, 1991) and in Pork Industry Handbook fact sheet 31, Feed Additives for Swine (Parker and co-workers, 1994). For product labels and detailed use and pre-slaughter withdrawal information, the Feed Additive Compendium published annually by Miller Publishing and the Animal Health Institute should be consulted.

Despite their effectiveness, use of antimicrobial feed additives faces an uncertain future. There is increasing pressure from regulatory agencies and from consumer groups to curb the use of antimicrobial additives in livestock and poultry feeds. Of particular concern is that regular use of antimicrobial feed additives may lead to the development of resistant microbes that may compromise the effectiveness of antibiotics in treating both animal and human disease. However, there is still considerable debate as to whether the use of antimicrobial feed additives will lead to a significant risk of inability to treat disease. For example, long-term studies have shown that using feed grade antimicrobials on swine farms for extended periods of time does lead to the development of resistant organisms but that the growth promoting effects of the products continue to be realized. Furthermore, within a herd in which antimicrobial use was suspended for over 13 years, significant levels of resistant microbe strains were still identified (Langlois and co-workers, 1986).

Last year European authorities issued a ban effective July 1, 1999 on four prominent feed grade antimicrobials used within the European Union member countries. These additives included virginiamycin, spiramycin, tylosin and zinc bacitracin. Three of these products are currently approved and used in U.S. swine feeds. It remains an open question if these countries will apply the same ban to pork products imported from outside of Europe. In the U.S., the Food and Drug Administration (FDA) has proposed new regulations that will restrict approval of new antimicrobial additives while the resistant bacteria issue is studied further.

In the future, regulatory action and consumer preferences may force swine producers to alter antimicrobial feed additive use. For now there are several key points to consider as this issue continues to develop.

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