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From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. It's one of the most controversial practices in agriculture, feeding small amounts of antibiotics to animals to make them grow faster. Critics say it raises the chances that bacteria will become resistant to those drugs and that it could increase the risk of drug resistant infections in humans. NPR's Dan Charles reports there's now evidence that when it comes to fattening pigs, much of this drug use doesn't even work.

DAN CHARLES, BYLINE: When bacteria-killing drugs, antibiotics, came on the scene about 60 years ago, they were a medical miracle. And as Steve Dritz from Kansas State University tells the story, they quickly moved on to the farm.

STEVE DRITZ: People started treating animals and feeding them and finding that they had increased growth rates and increased feed efficiencies.

CHARLES: Young pigs, for instance, could grow 12 to 15 percent faster if they got some bacteria-killing drugs in their feed. The animals also needed less feed to reach full weight. Other studies showed similar results in chickens and cattle. This was great for farmers. They could make more money.

Feeding low doses of antibiotics to farm animals became standard practice. Fast-forward to the 1990s. Steve Dritz was starting his career as a scientist in pig nutrition, and pork production was changing dramatically.

DRITZ: We instituted this thing called multi-site production.

CHARLES: It used to be pigs were born and raised all in one barn, but that meant infections kept getting passed on from one generation to the next, sort of the way that kids pick up germs on the playground and bring them home. Under the new system, after piglets are weaned, they move to a whole different place and the new space is totally clean, free of diseases.

Like this pig nursery that Craig Rowles runs outside Carroll, Iowa.

CRAIG ROWLES: This room just got washed, completely washed and disinfected, and now it's going to sit here and dry for a while.

CHARLES: A whole group of pigs will come in here together, and later they will move out together to yet another site.

ROWLES: And that group of pigs will stay together until they go all the way to market.

CHARLES: The groups are kept strictly separated from each other. If workers move between the groups, they'll first have to change their boots. When farmers adopted multi-site production, it cut down on disease and pigs actually grew faster. Steve Dritz wondered is all this better hygiene delivering the same benefits that farmers used to get through antibiotics. Did they still need to use the drugs?

He ran some studies on pigs in these multi-site systems, comparing animals that got antibiotics with others that didn't.

DRITZ: Lo and behold, in these studies, we found there wasn't nearly the magnitude of the growth response.

CHARLES: There was no growth boost in older pigs. Younger pigs grew a little faster with antibiotics, but not as much as people had expected. Steve Dritz put out the word to pork producers, you can stop buying most of those drugs that you're using for growth promotion, he said. Many farmers took his advice. But Dritz was surprised and a little discouraged to see how many did not. They weren't convinced by the evidence.

DRITZ: It's very hard to change people's perceptions.

CHARLES: Even more surprising, Dritz found that many veterinarians also didn't change their tune. They kept advising farmers to use the drugs. Dritz thinks the reason is partly rooted in the culture and psychology of veterinarians.

DRITZ: They are action-oriented people, most of them. You know, they want to do something. And so not doing something just doesn't seem right, as a professional, you know?

CHARLES: Now, there's a growing number of critics outside the meat industry who say using antibiotics for growth promotion isn't just unnecessary, it's irresponsible. The Food and Drug Administration, in fact, now says medically important antibiotics should not be used for to make animals grow faster. Within a few years, such uses of antibiotics may no longer be legal.

Among farmers, there's some grumbling about this. But at least for pork producers, dropping the drugs may actually save them money. Dan Charles, NPR News.

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