The closed, isolated herd at the university’s research facilities at Princeton was involved in a long-term study to answer those very questions.

The herd was fed continuous levels of antibiotics until 1972, when antibiotics were permanently removed, meaning it has been drug-free for three decades.

During the first few years after antibiotic withdrawal, antibiotic resistance (as measured by the percentage of fecal coliforms that were resistant to tetracycline) dropped from almost 90% to about 50%.

"Since that time, 30% to 70% of the fecal coliforms continue to be resistant to tetracycline, even though the pigs have received no antibiotics in their feed for treatment purposes or as injectables since 1972," stresses Cromwell.

In another University of Kentucky herd at Lexington, antibiotics were fed continuously during this same time period. Microbes in the gut of these pigs were solidly resistant to tetracycline.

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Source: National Hog Farmer - 15th November 2002