Many west Tennessee cotton producers have introduced residual herbicides into their weed control programs to combat glyphosate-resistant horseweed, which now comprises between 80 percent of 90 percent of the horseweed infesting the region.

While the additional cost of a residual can be a concern, the use of the chemicals could actually help producers on two fronts — fighting the development of glyphosate resistance in other weeds, such as pigweed, and in some cases eliminating a second glyphosate application on Roundup Ready cotton.

Glyphosate-resistant horseweed was first documented in the Mid-South in 2002 by UT weed scientist Bob Hayes, after it appeared in one field in west Tennessee in 2001. It has also been confirmed in Arkansas and Mississippi.

The University of Tennessee is conducting a study, now in its second year, to determine options for burndown and in-season control of the weed. UT Extension specialist Larry Steckel discussed the results of the study during the University of Tennessee Weed Tour, held at the West Tennessee Experiment Station in Jackson and the Milan Experiment Station in mid-June.

The standard recommendation in west Tennessee for resistant horseweed is Clarity and glyphosate. “It’s worked very well,” Steckel said. “You have to spray it 21 days ahead of planting, and there are some logistics issues from time to time. You can’t always get out there and physically get it done.”

Other options include Ignite herbicide, “which has worked well if temperatures are fairly warm (above 60 degrees) and weeds are actively growing. We have had some cool springs, so we have had some inconsistent performance. We have found that if we add herbicides like Direx, Cotoran or Caporal, we get much better control, even at cold temperatures.

“We’ve also been looking at Gramoxone as a burndown and it’s performed fairly well, particularly this year. But we like it mixed with something, Clarity at 21 days or one of the photosystem II inhibitors, like Ignite, with Direx, Caporal or Cotoran.”

Use of residual herbicides at burndown and planting has increased in importance because horseweed can emerge 10 months out of the year in Tennessee, according to Steckel. “We can get a good kill early and five days before planting, we get another flush. So any kind of residual like Valor or the photosystem II inhibitors work very well.”

“If you can get a residual on at planting, it can carry you well into the season when you start to get some shade. We did a quick survey of retailers in the area and found that about 90 percent of our cotton acres got a pre-emergence treatment. Most of it was Cotoran.”

The pre-emerge treatment has a couple of other benefits, according to Steckel. “It often will eliminate one glyphosate shot prior to the fifth true-leaf. As large as a lot of farms are getting these days, producers can’t physically get around to all their cotton anyway.

“Plus we’re getting another mode of action out there. We could be helping manage resistance in some other weeds like pigweed.”

There are not many options for in-season control of glyphosate-resistant horseweed, according to Steckel. “Envoke won’t kill resistant-horseweed, but it does stunt it and make it fairly uncompetitive. It can get you to the hoods where you can hit it with Direx/MSMA or Suprend.”

Resistant horseweed has had a negative impact on no-till and reduced-till practices in west Tennessee, Steckel said. About 80 percent of west Tennessee cropland is under a reduced or no-till program. According to a UT survey, “last year, we lost close to half those acres to cultivation. Some was to take out ruts. Some was due to producers having a good cotton year and expanding equipment, and they had to re-hip a lot of it. But some of it was due to resistant horseweed.”

If you see horseweed growing in west Tennessee, chances are good that it is resistant. Steckel said glyphosate-resistant horseweed makes up about 80 to 90 percent of the horseweed species in west Tennessee. “There is a range within that. Some horseweed may be two to four times resistant, others up to 10X. But it’s all very tolerant.”

UT researchers also studied the emergence of horseweed in various types of residue, including cotton, corn, soybean and fallow. “We had much more emergence in cotton residue, very similar to what we would with fallow,” Steckel said.

e-mail: erobinson@primediabusiness.com