NUTRITION
PIH-86
pork industry handbook
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA
Management to Prevent Drug Residue Problems in Pork
Authors:
Gary L. Cromwell, University of Kentucky
LeRoy G. Biehl, University of Illinois
Robert A. Wilcox, Kansas State University
Reviewers:
Glenn Brown, Delphi, Indiana
Dixon Hubbard, USDA-SEA, Washington, DC
Dean Pretzer, Diller, Nebraska
Antibiotics and other feed additives are widely used in the swine industry for growth promotion and for the reduction of mortality and morbidity in pigs. Certain feed additives require a withdrawal period prior to slaughter in order to insure that residues do not occur in the carcass. The additives that require withdrawal and their withdrawal times are given in Table 1.

The feed additives causing the greatest residue problem in recent years have been the sulfonamides (or sulfa drugs). The sulfonamides are commonly used in combination with certain antibiotics in pig feeds. The feed additive combinations that include sulfa are Aureo SP-250 and Chlorachel-250 (chlortetracycline, penicillin and sulfamethazine), Tylan-Sulfa (tylosin and sulfamethazine) and CSP-250 (chlortetracycline, penicillin and sulfathiazole). The approved level of sulfamethazine and sulfathiazole is 100 grams per ton. In addition, the sulfas are sometimes used as water medications for controlling pneumonia, scours and other bacterial infections.

Sulfa drugs are primarily used with young pigs during the early growth stages. Nearly all starter feeds and approximately 75% of grower feeds are medicated. Approximately 60% of these medicated feeds contain sulfa. One reason for the popularity of the sulfa-antibiotic combinations is that they are very effective growth promoters, as shown in Table 2.

A summary of 378 experiments involving over 10,000 pigs indicates that pigs fed sulfa-antibiotic combinations from 19 to 57 lb gained 21.7% faster and required 8.2% less feed per pound of gain than control pigs that received no antibiotics. For 10 other antibiotics, the average improvements in daily gain and feed efficiency were less, 13.7% and 6.5%, respectively.

Similar trends were found in a summary of 280 experiments involving slightly heavier pigs, fed from 37 to 109 lb. The sulfa-containing feed additives also have been shown to be effective in maintaining performance in herds having
chronic or acute respiratory infections such as Bordetella atrophic rhinitis.
The regulatory tolerance level for sulfa in pork tissue (liver, kidney or muscle) is 0.1 ppm, as established by the Food and Drug Administration. Regulations require that sulfamethazine be withdrawn from the feed for 15 days and sulfathiazole for 7 days in order to insure that tissues do not exceed the tolerance level for sulfa.

During the early 1970s, a national monitoring program was initiated by the USDA. By the mid-1970s, it was discovered that about 15% of hog carcasses were in violation because of sulfa residues. In almost all cases, sulfamethazine was the sulfonamide found in the tissues. A major effort was initiated in 1977 by the USDA, the Federal Extension Service and the National Pork Producers Council to solve this problem by means of research and educational programs. Although the problem has not been completely solved, the industry has been successful in getting the violation rate down to below 5% (Table 3).
Causes of Sulfa Residues
What is the reason for the high incidence of sulfa residues, and why has it been so difficult to eliminate the problem? Initially, producers were blamed for not complying with the withdrawal period. However, it was later realized that many violations were from farms where producers were following proper withdrawal times. In some cases, violations were even being reported on farms in which pigs were not known to have had access to any sulfa medication.

Finally, results of research conducted at Iowa, Illinois and Kentucky shed new light on the problem. It was found that very small amounts of sulfamethazine in the feed would cause a residue problem in the tissue. An early study at Kentucky indicated that as little as 1 gram of sulfamethazine per ton of feed would cause a high incidence of residues in the liver. Table 4 illustrates data from a later
Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U. S. Department of Agriculture Cooperating. H. G. Diesslin, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to its programs and facilities without regard to race, color, sex, religion, national origin, age or handicap.