No one wants to be responsible for a drug residue in a pork carcass, but there are many ways it can occur – contaminated feeds, water, environment and equipment, not to mention misuse and many others.

Still, the main source is cross-contamination of non-medicated feed with medicated feed. This is an avenue that can be controlled with attention to detail. Here are some points to check and tips for residue prevention, provided by Wayne Du, Swine Advisory Team of the Ontario Ministry of Agriculture, Food and Rural Affairs.
1. Adopt a quality assurance program. Implementing an effective quality assurance program is the key to producing high-quality and safe pork.
2. Consider using approved drugs that require no withdrawal time.
3. Choose the right form of sulfa drugs. This is an old, but highly applicable recommendation. Studies show that sulfamethazine is more prone to drug residue than sulfathiazole.
4. Use the granular form of sulfa drugs. The powder form is extremely electrostatic and dusty, increasing the potential for lingering contamination .
5. Empty and clean feeders. When switching pigs from medicated feeds to non-medicated feeds the feeders must be emptied and cleaned if the same system will be used to finish out pigs.
6. Use a proper feed mixing sequence. If you mix your own feeds on the farm you should have a written procedure for the feed-mixing sequence.
7. Ensure correct dosage and uniform mixing. Do not use higher dosages than accepted levels, and make sure scales are accurate and calibrated regularly. Sufficient mixing is key to uniform drug dispersal.
8. Clean all equipment. A thorough cleaning of all feed-mixing and delivery equipment is essential to reducing the chance of drug carryover.
9. Flush the drinking-water system. Like a feed-delivery system, a drinking-water system can harbor drugs and cause drug residues in pigs.
10. Identify all individually treated animals. Treated animals should be marked and recorded or separated from the rest of the group if in the finishing phase.
11. Clean out manure containing drug residues. Studies repeatedly show that manure left in pens from pigs fed medicated feeds or drugs could cause residue-positive tests.
12. Avoid using recycled lagoon water. If
lagoon water is recycled, don't use it in the finishing area because it could contain residual drugs and cause a positive residue test.
13. Avoid extra-label drug use. Using any drug outside of its label approval is illegal without an appropriate veterinary prescription.
14. Follow drug withdrawal times at all times – period.
15. Ensure there are no drugs in meat and bone meals. Meat and bone meals could be a source of drug residues.
16. Prevent delivery error. Feed bins should be labeled clearly so drivers know exactly which bin receives which feed. Drug carryover in feed can occur if medicated and non-medicated feeds are hauled at the same time or if the conveying system is not cleaned out properly between deliveries.