Most of the major poultry producers in the U-S have stopped using antibiotics in chicken feed. That's in part because giving fewer antibiotics is thought to be a way to protect humans from drug-resistant diseases.

Maryn McKenna is the author of the new book Big Chicken: The Incredible Story of How Antibiotics Created Modern Agriculture and Changed the Way the World Eats. McKenna says we're in a moment of change.

Harvest Public Media brings us an excerpt of an interview between McKenna and KCUR "Up To Date" host Steve Kraske about the history of antibiotics in the meat industry.

Steve Kraske: Maryn, when did this practice of dishing up antibiotics to chickens begin back in the day, when did all this start?

Maryn McKenna: The fascinating thing, and I didn't know this until I started researching this, is that all of this begins right as antibiotics come on the scene. So, a little bit of history, probably in high school biology you learned about Alexander Fleming, who in 1928, left a window open in his laboratory in London and something blew in the window and killed the plates of bacteria that he was working on. And that … compound became the first antibiotic, penicillin. But not until the 1940s. It look Fleming and his collaborators a while actually to develop it into a drug.

So, penicillin comes out in the middle of World War II and it really makes a difference in to the war in about 1943 and the other drugs that start the antibiotic era all by 1948. All just being used in humans. And then in 1948, one of those manufacturers of those drugs gets the idea that maybe they could also find a market for their drugs in animals.

And simultaneously, the food system is kind of suffering because of the end of the war because during the war there'd been a huge guaranteed market because of all the soldiers and sailors deployed across the world. When that market goes away, the meat production system needs to cut costs to keep from having its economy crash entirely. So they start feeding animals cheaper feed and they need supplements to make the feed more nutritious.

And one of the scientists at that company that makes one of the first antibiotics gets the idea to try the leftovers from their manufacturing of the antibiotic in the animal's feed. And when he weighs the chicks that are his experimental animals - chickens are the first ones to get these growth-promoter antibiotics - he discovers the ones that got the tiny antibiotic doses gained more than twice as much weight as birds that had not gotten a supplement at all. And out of that, this whole worldwide industry of giving antibiotics routinely to meat animals was born … within five years.

Steve Kraske: I was going to say, these antibiotics have this upside, which is what you're leading to I think, is it makes chicken awfully awfully cheap.

Maryn McKenna: That's right. It made animals, chickens first, and then these antibiotics were given to pigs, as well, and then it took a little while longer before they were trialed in cattle. They made it possible to grow an animal more quickly. So you could either give an animal more feed and get the animal through lesser amount of feed in the same time or the same amount of feed and get the animal out much faster. One way or another, it started people thinking about birds first and then other animals as something you could move through a production system, almost like an assembly line for cars.

And from that, we get the concentration of farming, we get larger barns and much larger feedlots, we get all the things that together have contributed to our protein being extremely, maybe artificially, inexpensive.

Steve Kraske: And yet, scientists were warning even right from the start that all these antibiotics might not be a good thing. What were some of them saying?

Maryn McKenna: You know, this is kind of the heartbreaking thing about this story. There were warning signs at the beginning. In fact, even before that first experiment in 1948, Alexander Fleming, the discoverer of penicillin had said in 1945, when he accepted the Nobel prize: that if we underdosed people with antibiotics, then resistant bacteria would result and would undermine the action of these precious drugs. Now, Fleming didn't know it, but what he was describing was exactly what agriculture was going to start doing … underdosing animals for some purpose other than curing an infection.

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Note: You can listen to this interview excerpt on our podcast.