Life was much simpler for no-till or reduced-till farmers before horseweed became resistant to glyphosate herbicides. Growers sprayed Roundup in the spring and then came in with the planter and were on their way.

But Nature has a way of complicating things, and many producers are trying different herbicide strategies to cope with the problem of hard-to-kill horseweed. Others appear to be giving up on herbicides for now.

“This was the typical horseweed burndown in 2004 and 2005,” says Larry Steckel, referring to a PowerPoint photo of a tractor pulling a disk through a good stand of horseweed in a field that had been sprayed with a burndown herbicide.

“A lot of our growers had that feeling of shock and awe when they sprayed a glyphosate herbicide that first year and found they hadn’t made a dent in the horseweed,” said Steckel, a speaker at the Regional Cotton Pest Management Workshop in Memphis, Tenn.

Steckel, weed specialist with the West Tennessee Experiment Station in Jackson, says glyphosate-resistant horseweed can now be found in almost all of the cotton acres and about 80 percent of the soybeans in west Tennessee.

“We also saw what appeared to be glyphosate-resistant horseweed in some corn fields in the Dyersburg area in 2005,” he said. “That was the first time I’ve seen that situation in corn.”

Since the University of Tennessee’s Bob Hayes first documented glyphosate resistance in horseweed in 2000, a tank-mix of glyphosate and dicamba (Clarity) has become the standard burndown recommendation for the pest. But even that combination has its drawbacks.

“This tank-mix has done a good job for us,” Steckel notes. “Unfortunately, it doesn’t fit all of our producers’ needs. One problem is timing — you can get a rain and get delayed and get closer to planting cotton or you have breakdowns that keep you out of the field.

“The other issue we have with Roundup-Clarity is that after we get that initial burndown, Clarity doesn’t give us a world of residual control on horseweed, and we get new emergence.”

So other options are needed. Some farmers have turned to conventional-tillage as indicated in Steckel’s PowerPoint photo. “Bob Hayes and a lot of these guys worked for years to promote no-till, and it seemed like we started going backwards in my first year,” he said. “We did a survey, and it turned out a lot of that was due to glyphosate-resistant horseweed.

“But what I have noticed with tillage is that you need to be very aggressive with it if you’re going to kill horseweed. Going after it with light tillage doesn’t seem to have much impact.”

Steckel has been looking at other herbicides and tank-mixes to try to help farmers continue to reap the advantages of no-till and reduced-tillage. Among those are glufosinate or Ignite, the old/new herbicide that can be applied over-the-top of Liberty Link cotton, Gramoxone Max, an old standby, and combinations of residual herbicides.

“One of the things we’ve noticed — and I think others have also — is that Ignite is very, very temperature sensitive,” he said. “In general, the earlier we applied Ignite the less effective the control of horseweed.

Over two years, Steckel applied Ignite at 32 ounces per acre on March 30, April 6 and April 14. The March 30 treatment provided 30 to 40 percent control 40 days after treatment; the April 6, about 50 percent; and the April 14, nearly 90 percent. (The five-day average for the March 30: 46 degrees; April 6: 60 degrees; and April 14: 63 degrees.)

“I don’t want to get too hung up on temperatures, but it appears that Ignite performance was reduced when average daytime temperatures were less than 60 degrees,” he said. “Applying Ignite two weeks later gave us 90 to 100 percent control of horseweed. The best fit for Ignite alone in a burndown program is close to planting, provided daytime temperatures are adequate.”

(Bayer CropScience representatives say their research also shows that Ignite is not as effective under cool, low humidity conditions. For burndown, the company is recommending that growers apply 29 ounces of the new formulation of Ignite 14 days or less before planting. Before 14 days, they recommend adding Clarity or 2,4-D for horseweed control.)

Steckel says his research indicates the addition of Clarity at 8 ounces per acre to Ignite can provide 90 to 100 percent control of horseweed when applied around March 30.

Tank-mixing the standard rate of Ignite with Caparol at 32 ounces, Cotoran at 32 ounces or Direx at 16 ounces also improves the horseweed control on that April 14th application, but it also provides residual control of later flushes of horseweed.

Applying 32 ounces Gramoxone Max per acre in late March also provided excellent control of horseweed 40 days after treatment, says Steckel. But the same rate of Gramoxone Max applied April 14 gave the researchers only 70 percent control.

“Tank-mixing 32 ounces of Gramoxone Max with 32 ounces of Caparol or Cotoran or 16 ounces of Direx gave us much better horseweed control on that April 14 application date than Gramoxone Max alone,” he notes. “And there’s the residual aspect, as well.

“Ignite provides good horseweed control even under cold conditions when tank-mixed with eight ounces of Clarity or a photosystem II inhibitor (Caparol, Cotoran or Direx),” said Steckel, summing up the results of the early timing study with tank-mixes. “Gramoxone Max is not as temperature sensitive and will control small rosette horseweed. Larger horseweed can be controlled with Gramoxone plus the PS inhibitor.”

The University of Tennessee scientist says he believes growers registered a large increase in soil residual herbicides such as Caparol, Cotoran, Direx, Dual Magnum, Prowl and Valor in 2005, primarily for help in controlling resistant horseweed and Palmer amaranth.

“The soil residuals bring a lot to the table, including greatly increasing the burndown of GR horseweed when tank-mixed with Gramoxone or Ignite,” he said. “Applied behind the planter they also provide good control until growers can make a hooded application.”

But they also have drawbacks: They typically require rainfall or irrigation for activation and they can cause crop injury under certain conditions.

For that reason, Steckel and other scientists have begun recommending lower application rates; that is 1 quart or 32 ounces for Caparol vs. 1.5 to 2 quarts in the past; 1 quart for Cotoran vs. 1.5 to 1.9 quarts; 1 pint of Direx 4L vs. 1 pound of Direx 80 WP; and 1.2 pints of Prowl vs. 1.5 to 2 pints previously.

If growers are unable to control horseweed before their cotton emerges, they have options, he says. Dual Magnum can provide control later in the season. Envoke at 0.15 ounce can stunt horseweed until growers can post-direct or apply other herbicides under a hooded sprayer. And Ignite can kill good-size horseweed in Liberty Link cotton. Farmers can also apply Suprend, Ignite or Direx plus MSMA with a post-directed rig or at layby.

“The real keys to managing glyphosate-resistant horseweed are to start clean, stay clean (using residuals) and making a good layby application,” notes Steckel. “If you allow horseweed to continue growing in the field after the cotton emerges, you will be fighting an uphill battle for the remainder of the season.”

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