Cotton producers who encounter glyphosate-resistant horseweed in their fields may be tempted to fall back on a solution that served their fathers and grandfathers well: cold steel.

Before you pull that disk out of the weeds on the back side of the equipment lot, however, think about this: Do you really want to spend all that extra money on diesel fuel and labor AND undo the benefits of conservation-tillage you've worked so hard on all these years?

And there's another consideration, according to Larry Steckel, Extension weed scientist with the University of Tennessee, and a speaker at Cotton Incorporated's recent Crop Management Seminar in Memphis.

“You have to be careful,” says Steckel, displaying a photo of a freshly disked field with green horseweed plumes sticking up in it. “If you don't do a thorough job of disking, you can wind up with a worse problem than when you started.”

There's no doubt glyphosate-resistant horseweed has set back conservation-tillage efforts in Tennessee, says Steckel, who spoke on “The Impact of Glyphosate-Resistant Horseweed and Pigweed on Cotton Weed Management and Costs.” (The University of Georgia's Stanley Culpepper and Arkansas' Ken Smith were co-authors.)

In a 2004 survey, county Extension agents said glyphosate-resistant horseweed had reduced conservation-tillage farming in Tennessee by 18 percent. Even more telling, the survey showed the percentage of farms using conservation-tillage in the largest cotton counties in Tennessee had dropped from 80 to 40 percent.

Glyphosate-resistant horseweed has spread much more quickly than anticipated when Bob Hayes, a weed scientist with the West Tennessee Experiment Station in Jackson, discovered it in west Tennessee's Lauderdale County.

“It's in all our cotton acres now,” Steckel told Crop Management Seminar participants. “Horseweed can grow in Tennessee 11 months out of the year. It has a very aggressive tap root, and it loves a no-till environment.”

Horseweed or marestail, as it is sometimes called, also competes well with cotton. Studies show horseweed can reduce cotton yields by 40 percent when left unchecked through the two-leaf stage. If not controlled between planting and first bloom, losses can reach 70 percent.

The staggering increase in glyphosate-resistant horseweed followed a spectacular rise in the amount of glyphosate products (Roundup, Touchdown and others) being applied in cotton and other glyphosate-tolerant crops.

“We saw a 752-percent increase in glyphosate applications between 1997 and 2003 at the expense of just about everything else with the exception of diuron (Karmex, Direx),” said Steckel. (Applications of diuron jumped 101.1 percent during the same period while those of other herbicides declined.)

As most farmers now know, weed scientists with the University of Georgia have documented cases of glyphosate-resistant Palmer pigweed in southwest Georgia. More recently, glyphosate-tolerant Palmer pigweed has been found in Crockett and Lauderdale counties in Tennessee and Mississippi County in Arkansas. Resistant waterhemp, a cousin of pigweed, has also been found in Missouri.

Culpepper, a weed scientist with the University of Georgia, also discussed Palmer pigweed resistance in Georgia at the Cotton Incorporated seminar.

“Scientists at Georgia, South Carolina and North Carolina have found an 8x to 12x level of resistance to glyphosate in Palmer pigweeds in their states,” said Steckel. “We've seen pigweed survive 2x to 4x rates of glyphosate in Arkansas and Tennessee.

Weed scientists say glyphosate-resistant horseweed and pigweed can be managed with a combination of herbicides, but it will cost growers more.

One approach has been to burn down with glyphosate or paraquat (Gramoxone Inteon) plus 8 to 12 ounces of dicamba (Clarity, Oracle) in early February and come back with Gramoxone at 48 ounces plus Ignite at 29 ounces plus Caparol at 32 ounces, Cotoran at 32 ounces or Direx at 16 ounces 21 days before planting.

Some growers have also been making a fall (November or December) herbicide application with Valor at 2 ounces plus Clarity or Oracle at 8 ounces. Others have applied Valor plus Caparol, Cotoran or Direx in February.

“A fall application of Valor has been getting a lot of attention from growers,” says Steckel. “You've got to get some residual control out there to keep the horseweed from emerging during the winter.”

Envoke has also received a label from EPA for fall and early winter application in cotton fields. Envoke will provide residual and knockdown control of glyphosate-resistant horseweed and other winter annuals. The use rate will be 0.10 ounce per acre.

“Trying to burn down large horseweed that got its start the summer of the previous year or early in the fall is going to be hard with anything,” said Steckel. “If a grower catches these populations early with a residual herbicide, he will be ahead of the game.”

Cotton farmers can spend an extra $20 per acre to control glyphosate-resistant horseweed by the time they add Valor, Clarity and Caparol to their program, according to Steckel.

For glyphosate-resistant horseweed and pigweed, the cost could rise $27 an acre if they have to apply a maximum rate of glyphosate; add Dual Magnum over-the-top with the first or second glyphosate spray, followed by a post-directed application of Caparol or Dual and Valor or Caparol in a hooded sprayer.

But that's not as expensive as what growers already face in southwest Georgia, says Steckel.

Control costs for glyphosate-resistant Palmer pigweed in Georgia can range from another $45 an acre to as high as $92 an acre in fields where farmers have had to resort to hand weeding to remove the problem weed.

“Glyphosate-resistant Palmer pigweed can be much more problematic than horseweed due to its more competitive nature,” says Steckel. “On average, GR Palmer could cost cotton producers an extra $40 per acre or more to manage.

“Because of that, we think glyphosate-resistant pigweed is a much bigger threat to cotton production, and every year we can delay its arrival in the Mid-South can mean big savings to our producers.”