Would a total ban on use of antibiotics in food animals have an impact on resistance and make pork safer for consumers? Gary Cromwell, professor of swine nutrition at the University of Kentucky, doesn’t think so.

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.

Yet when antibiotic efficacy experiments were conducted, the pigs with antibiotic resistance still responded to the antibiotic in terms of enhanced growth rate and improved feed efficiency, he adds.

"In the Princeton herd, we found that age of the pigs, type of housing and moving stress had as much or more of an effect on shedding of antibiotic-resistant fecal bacteria than did the presence of antibiotics in the diet," declares Cromwell. "So it appears that even if antibiotics were banned, it would not completely eliminate the problem of antibiotic resistance."

The Princeton herd, without antibiotics, also experienced poorer reproductive performance. About 10 years of production data before and after the elimination of antibiotics was compared.

Conception rates averaged almost 9% lower, litter size dropped half a pig, number of pigs weaned at 21 days declined by over a pig, survival of live born pigs went down almost 9% and incidence of mastitis, metritis and agalactia (MMA) went from under 10% to a striking 66% of the sow herd.