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You can trace the following pattern in every kind of conflict: Somebody makes a radical improvement in offensive weaponry and then someone else improves the defense. It's true in war. It's true in football. And it's definitely true in the epic battle between farmers and weeds.

For the last decade or so, the weeds have been losing badly. Farmers have been winning with genetically engineered crops and the weed killer Roundup. Now some weeds are becoming Roundup resistant and there's a big argument about what farmers should try next. Here's NPR's Dan Charles.

DAN CHARLES, BYLINE: The combination of Roundup and so-called Roundup-ready crops was so amazing when it came on the market 15 years ago, it made grown men giddy. Roundup killed everything except for your cotton, soybeans, or corn. Those crops were genetically engineered to withstand the herbicide. It changed American agriculture. It made controlling weeds so simple, farmers went out and rented more land. Farms got bigger.

And then the magic started to wear off. At different places in different parts of the country, farmers realized that some weeds were not dying. In Georgia, it was a plant called Palmer amaranth, or pigweed.

RANDY BRYAN: It started just north of us here, three or four counties up.

CHARLES: Randy Bryan grows cotton in Irwin County, Georgia. At first he thought it was just somebody else's problem.

BRYAN: And then all of the sudden it was all over South Georgia. We had it everywhere.

CHARLES: This strain of Palmer Amaranth has a genetic mutation that makes it resistant to glyphosate, the weed-killing chemical in Roundup. Pigweed can grow three inches a day. A single plant can release close to a million seeds. This is a bully. Let it sprout beside cotton seedlings and the poor cotton won't stand a chance.

Farmers in Georgia like Vann Grantham are hiring people to go into cotton fields and pull the pigweed by hand.

VANN GRANTHAM: I've got a brother-in-law that told this year he spend $120 an acre on hand labor.

CHARLES: That's about four times what weed control used to cost. Farmers are looking for solutions, and the man they turn to is Stanley Culpepper. He's a weed scientist at the University of Georgia and the state's expert on cotton weeds. This time of year he spends his days driving from county to county delivering talks to cotton farmers.

STANLEY CULPEPPER: We all agree that there can be no glyphosate-resistant palmer amaranth up a planting, right? We've crossed that bridge. We know it has to be no palmer at planting, or you probably won't pick your crop. If you do pick your crop, you won't be economically sustainable.

CHARLES: Culpepper grew up in North Carolina and comes from a long line of farmers. He talks to these cotton growers like a football coach giving his players some tough love. The days of easy weed killing are over, he tells them. You can't be lazy anymore.

CULPEPPER: You go out and look at the fields, you say, oh, I've got me a few more days. And what happens when you say I'm going to wait a few days, those pigweeds come up and they're four or five inches when you get there and you can't kill them.

CHARLES: You'll have to spray a whole bunch of different chemicals to overwhelm the enemy, Culpepper says. Some will kill your cotton if you aren't careful.

Now, here, he says, here's something completely different. He puts up a new slide, a picture of a field that's covered with a layer of rye lying flat on the ground. This residue works as well as any weed killer, he says. Pigweed just hates it.

So we can grow this cover crop of rye and we leave some narrow gaps in it. That's where we plant our rows of cotton. I'm still working out a few kinks in this technique, he says. But in just a few years, I think this is something you should try.

CULPEPPER: If we can figure this out, this program is the most sustainable program we as cotton people could do, no question, bar none, for resistance management and for Palmer control.

CHARLES: So here's Stanley Culpepper's recipe for surviving in a world of Roundup-resistant weeds: Do lots of different things. Some of them involve chemicals, some don't. Some will mean more work.

But there are other recipes out there. And the one that may be really tempting for farmers is more genetically engineered crops. At this same meeting there are representatives from three big cotton seed companies: Dow, Monsanto and Bayer.

Those companies are selling, or plan to sell very soon, crops that have been engineered to tolerate other herbicides that will kill pigweed. So farmers may be able to spray those herbicides, old ones called 2-4-D and Dicamba, right over their crops.

Some environmentalists are angry about these new products. The Natural Resources Defense Council, for instance, says 2-4-D is dangerous and ought to be banned. And David Mortenson, a weed ecologist at Penn State, predicts that weeds will evolve resistance to these herbicides too. It's a kind of treadmill, he says, where farmers constantly need new weed killers.

DAVID MORTENSON: When one herbicide fails, then we add a second herbicide, and you turn it for a while, and then add a third herbicide to that package. And I am convinced that that is not a sustainable path forward.

CHARLES: Stanley Culpepper, meanwhile, stands somewhere in the middle of this argument.

CULPEPPER: Let's be clear. I want all the new technology that's economically and environmentally friendly for our growers that we can get.

CHARLES: But we can't misuse the technologies, he says. We can't overuse just one or two of them. He thinks his farmers have learned that lesson and what happened with Roundup will not happen again.

Dan Charles, NPR News.

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