Resistant giant ragweed doesn’t get the media attention of other resistant weeds like Palmer pigweed and horseweed. But it is becoming more and more of a problem in west Tennessee, the Missouri Bootheel and northeast Arkansas, according to University of Tennessee weed scientist Larry Steckel.

Fortunately, giant ragweed is not a fast-spreading weed like Palmer pigweed and horseweed, resistant biotypes which have spread over hundreds of thousands of acres seemingly overnight. “It’s more of an evolution process,” said Steckel, a Delta Farm Press contributor.

“Giant ragweed has historically been confined to ditches and fence rows around crop fields. Invariably, growers will clean out ditches with their backhoes, they dump it in the field and we start seeing giant ragweed in the fields. This wasn’t an issue when glyphosate did good job on them, but we’re starting to develop a biotype that is starting to walk through Roundup. There is also a lot of genetic diversity in the populations.”

Steckel says giant ragweed typically germinates early “in February and March in our part of the world. The new resistant biotype is not only showing glyphosate resistance, but it’s showing the ability to emerge over a long period of time.”

One factor that could help west Tennessee cotton producers with control of glyphosate-resistant giant ragweed is the state’s widespread adoption of WideStrike cotton, particularly, PhytoGen 375 WRF. Just over 40 percent of the state’s 300,000 cotton acres were planted to the variety in 2009.

Steckel noted that cotton varieties containing WideStrike have very good Ignite tolerance, although the tolerance is not as good as it is in LibertyLink cotton varieties. Growers have not readily adopted LibertyLink cotton in west Tennessee as of 2010.

Steckel estimates that 60,000 acres of WideStrike cotton was sprayed with Ignite in west Tennessee in 2009.

Research in the Southeast cotton-growing region indicates that some injury will occur when Ignite is sprayed over the top of varieties containing WideStrike. The visual injury ranges “anywhere from 15 percent to 20 percent. We did not see any loss in yield at the end of the year from Ignite applications,” Steckel said. He said further research is needed on the potential for injury causing a delay in maturity, “which is some concern because Tennessee lies in the northern end of the Cotton Belt.”

Steckel also reported lint yields improved by a factor of 3.3X “with an Ignite-based system versus a Roundup-based system in the presence of weeds that are highly glyphosate-resistant.”

A 2009 study on a west Tennessee field heavily infested with glyphosate-resistant giant ragweed confirmed the effectiveness of Ignite-based systems on the weed, Steckel noted. Applications, made with a backpack sprayer, applied over the WideStrike cotton variety PhytoGen 375 WRF were: a single application of Ignite at 29 ounces; sequential applications of 29 ounces of Ignite at 2-leaf and 5-leaf cotton; one shot of Roundup; a tank mix of Roundup and Ignite, an Ignite plus Staple tank mix; a Roundup and Staple tank mix; and Roundup plus Envoke.

The cotton grew out of any Ignite injury within 30 days after the application, Steckel said.

As far as weed control was concerned, “At 10 days after application, we had very good control with one shot of Ignite,” Steckel said. “The giant ragweed was a little bigger than we would have liked — it ranged from 4 inches to 12 inches tall. We basically had no control with glyphosate. Ignite and Roundup was good early, and Ignite and Staple looked good. Our two standard treatments of Roundup and Staple and Roundup and Envoke didn’t work very well.”

At 30 days after application, “we didn’t see much grow back,” Steckel said. “We did see control with the single Ignite application drop to 80 percent but some of that was subsequent emergence. Ignite followed by Ignite was very good. We had 35 percent control with Roundup at 30 days. It was interesting that the Roundup and Ignite tank mix showed some antagonism. Ignite and Staple looked pretty good, with the Staple giving us some residual control of giant ragweed.”

The plots with a single shot of Ignite and two shots of Ignite both yielded nearly 1,200 pounds, while there was “not much cotton to pick” in the Roundup program. With Ignite and Roundup, we had some escapes due to antagonism. The Ignite plus Staple plots looked very good.”

e-mail: erobinson@farmpress.com