Early in 2007, glyphosate-resistant Palmer amaranth was confirmed in Tennessee. Now, a year later, giant ragweed has been added to the official glyphosate-resistant list.

In a number of Tennessee counties, giant ragweed “just isn't being controlled with Roundup in cotton and soybeans,” says Larry Steckel, row crop weed specialist at the University of Tennessee.

“Lake and Lauderdale counties are, by far, the two counties with the most affected acreage. I'm sure there is some resistance in Dyer County, too. I just haven't had the chance to take a boom out and do the work yet. That will come.

“Really, all five of the counties (adjacent to the Mississippi River) — Shelby, Tipton, Lauderdale, Dyer and Lake — have resistant giant ragweed. One of the worst cropping field areas for it is around Ripley. There's also a fair amount of it around Ridgely, in Lake County.”

For several years before confirmation, Palmer amaranth, a pigweed, was suspected of resistance. The screening process to prove resistance, or tolerance, can take much greenhouse work.

“To prove a weed resistant means spraying it with Roundup and having the plants live and produce seed. Then, that seed is grown out and must live through Roundup applications. We finally got that process done for Palmer pigweed in late 2006.”

In late January, Steckel and colleagues finished the same work on giant ragweed. Proper testing completed, both resistant weeds are now listed with the International Survey of Resistant Weeds (www.weedscience.org) led by Ian Heap (see http://deltafarmpress.com/news/070117-weed-resistance/index.html).

How did Steckel find the resistant giant ragweed?

“When Ripley-area farmers were unable to control it with Roundup, I traveled there. I sprayed 40 ounces of Roundup on it at about 3 inches tall and it did nothing. I sprayed another 88 ounces of Roundup WeatherMax at 6 inches tall. Those lived. I then sprayed 88 ounces again at 10 inches tall. That's a lot of Roundup and the ragweed came through fine.”

The weeds did so well, “they reached head-high and went to seed, which I collected. Bob Scott and Jason Norsworthy (both weed scientists/researchers with the University of Arkansas) have been screening ragweed in Arkansas — they have a good check system, good greenhouses, and are already set up for the research. So I sent some of this Tennessee seed to them.”

As of early February, Norsworthy had put 44 ounces of Roundup WeatherMax — a 2X rate — on some plants grown from seed collected in Tennessee's Lauderdale County. After 43 days, all the susceptible check plants were dead. Meanwhile, all the Tennessee plants were alive.

“That helps confirm what we'd already seen here.”

How about control options?

“I've been trying to look at a bunch of (preplant herbicides),” says Steckel. “But it was so dry last year, (they) wouldn't work. So I have some suggestions on post applications, not preplants. Hopefully, we'll get some rain this year that will activate the preplants and we can check them out.”

Right now — based largely on research done on soybeans in Lake County — “the best game in town for resistant giant ragweed in soybeans is 0.6 ounce of FirstRate and 44 ounces of WeatherMax. That's the only thing I saw working as a post application once the weed got 4 to 6 inches tall.”

Unfortunately last year the treatment cost about $35 per acre. Even more unfortunate: this year, with the higher price of Roundup, it'll cost well over $40 per acre.

For cotton, the best control Steckel found “was .15 ounce of Envoke. That killed some of them and stunted the others enough to where I could see some height separation between the giant ragweed and cotton. Then we ran over the crop with a hood and applied 29 ounces of Ignite. That was the only way I found to manage the weeds.”

How well did it work? “In the cotton, I could control everything between the rows. But where I couldn't get the hood over them, some of the weeds lived. We didn't get 100 percent control.”

Some of the giant ragweed in soybeans also lived after the FirstRate application. “We killed most of them, but others struggled on. They didn't appear to be as competitive.”

How resistant is the giant ragweed Steckel has dealt with? “I put a quart of Roundup on my check and it looked like I'd applied water — the giant ragweed was head-high and you couldn't even see the soybeans. Where I put out the 44 ounces of Roundup and 0.6 ounce of FirstRate, you could see the soybeans — the giant ragweed was stunted under them, or dead.”

Steckel has faced a frequent barrage of control queries during winter meetings. “Farmers are worried, especially with the giant ragweed. Everyone has picked up on Palmer pigweed being an issue and the need for preplant herbicides, particularly in soybeans. Then going over-the-top with good pigweed residual — like Dual or Reflex.”

One thing that surprises the researcher is how many people claim Palmer pigweed is only a problem in western Tennessee. “I spoke with growers having control problems with giant ragweed from Knoxville to Ripley. They all have it and it's creeping into fields.”

Giant ragweed is described by Steckel as a “ditch-weed,” much as marestail once was. Resistant marestail has since moved quickly through much of the Mid-South through ample, wind-blown seed.

With giant ragweed, “one of my top recommendations so far is to understand it's largely confined to field edges and let's keep it there. Many farmers clean ditches and move soil into their fields. That's how a problem can really get going. After moving the dirt, the farmer farms the field as they normally have with Roundup. When Roundup isn't controlling the giant ragweed, pretty soon the field will be covered with it.

“So I'm stressing that if resistant giant ragweed isn't across the whole field, keep it in the ditches. Prevention at that stage is well worth the effort.

“Once resistant giant ragweed gets into a soybean field, I don't know of a cheap way to control it. And in cotton, there's not a great way to control it either.”

Steckel, who is constantly hearing about and looking into suspiciously resilient weed species, says the confirmed incidents of resistance could be just the start. “I think we'll see more — this is just the tip of the iceberg. We've got to begin managing it better using (preplant herbicides) and residuals over-the-top. That's the only way I know we can do it.”

And since glyphosate prices have jumped so much in the new year, “the (preplant herbicides) — particularly in soybeans — make economic sense. They basically replace a Roundup application and may mean cutting an over-the-top Roundup application.”