It's not too early to start thinking about herbicide programs for 2008 — because like the continued threat from Asian rust to soybeans, weed resistance to glyphosate herbicide is here to stay.

“That's especially true for cotton and soybean growers who have depended on glyphosate over the past decade,” says Stanley Culpepper, University of Georgia cotton specialist. “Of greatest concern in the South is glyphosate-resistant Palmer amaranth pigweed.

“If growers don't control glyphosate-resistant Palmer amaranth (pigweed) now, it will be potentially devastating,” says Culpepper.

Considering that more than 75% of cotton, 70% of soybeans and 35% of corn is planted in Roundup Ready varieties, serious problems may result.

Roundup Ready Flex is used by many growers because of its season-long weed control with over-the-top application. In previous Roundup Ready varieties, application could be made over the top through the fourth leaf stage.

But Roundup Ready Flex has created a Catch 22. Ease of weed control has increased. However, continued sole reliance of glyphosate will likely increase the selection pressure for more glyphosate resistance problems.

Weed resistance isn't new in cotton. Some weeds couldn't be controlled by yellow herbicides even in the 1970s. “So what's the big deal now?” asks Culpepper. “Well, the big deal now is that weeds that are extremely competitive are resistant to extremely important classes of herbicide chemistry such as glyphosate and ALS herbicides.

“Pigweed has resistance to ALS herbicides (such as Staple) in 19 states. And if you don't have glyphosate-resistant horseweed (marestail), it's just a matter of time,” Culpepper says.

He stresses that pigweed can outgrow virtually anything in the field, especially cotton plants. “It grows at least 1 in. a day and can have 400,000 seeds per female plant,” he says. “One pigweed plant per 20 ft. of row can reduce cotton yields by at least 7%. And the glyphosate resistance can be transferred by pollen.”

There are several hundred acres of cotton showing resistance to pigweed out of some 13 million planted. That's a small percentage, but one that can easily grow if it is not controlled.

“The convenience of glyphosate has been a real blessing to us up until this point,” says Ken Smith, Extension weed scientist, University of Arkansas, an area that saw some of the first pigweed resistance. “However, glyphosate-resistant pigweed will absolutely destroy our agriculture if we do not learn to manage it. Residual herbicides applied preplant, pre-emergence or before the weed germinates must be in the equation.”

Glyphosate is a tool but nota complete herbicide program,says Smith. “If we start using it just as a convenience and forget that we need residuals, there will be problems.”

Smith suggests using Flex cotton for increased flexibility, but feels farmers must continue to use two of the three options including:

  1. Dual Magnum over-the-top

  2. Post direct with residuals such as Caparol, Cotoran, or Direx

  3. Layby with a residual such as Valor or Direx.

Culpepper says managing resistance can be done through using different modes of action. For those who already have glyphosate-resistant pigweed, control “will be extremely challenging and expensive,” he says. But for growers who do not currently have resistance, he recommends management programs that include at least two alternative modes of herbicide chemistry in addition to glyphosate to delay impact of this resistant pest.

The cotton experts suggest that growers consult their local Extension specialists and/or consultants to determine the best modes of action for their region and varieties that may work best.

Monsanto, makers of Roundup herbicide, the most popular glyphosate used on over-the-top applications, is working closely with university scientists to develop recommendations to help slow/prevent and manage resistant weed problems.

“We understand that some growers already have resistant weeds and are spending more money to treat them,” says Paul Callaghan, Monsanto cotton traits marketing manager in St Louis, MO. “We are trying to promote recommendations for growers that are the most effective and economical while encouraging the use of multiple modes of action.”

Monsanto's Parrlay metolachlor herbicide will be priced to minimize the cost of applying a residual herbicide in the Roundup Ready cotton system. “It helps growers use a residual second mode of action to minimize the risk for resistance,” says Callaghan.

“We are encouraging growers, in every opportunity, to use a residual second mode of action in their cotton,” adds Callaghan, noting that Roundup Ready Flex should benefit growers in their residual program.

Syngenta and other companies are promoting residual herbicides to help combat glyphosate resistance. Syngenta's Sequence is applied when weeds are less than 3 in. tall. Envoke can be fall or winter applied for pre-emergence residual control of resistant horseweed and winter annuals, applied in season tankmixed with glyphosate or in a layby application. Reflex is another Syngenta product promoted for control of pigweed in resistant situations.

The LibertyLink herbicide from Bayer Crop Science is also seeing more use by growers eager to spread their weed control options. Valor, marketed by Valent U.S.A. Corporation, helps control glyphosate-resistant or glyphosate-tolerant weeds.

Culpepper adds that growers can take steps to delay resistance. “Rotating cotton with corn can help,” he says. “However, you can count on resistance coming back for Roundup Ready cotton. You need to detect resistance early then develop residual modes of action.

“It's challenging for growers,” he says, “and survival of growers who ignore this potentially devastating problem is questionable.”

Weed control recommendations

During the 2007 Beltwide Cotton Conference, weed resistance received major attention. Scientists like Stanley Culpepper, University of Georgia, stressed early weed control during the critical first 40 days of production.

Among the first 40 days recommendations were:

  • Stop sole reliance on glyphosate.
  • Where appropriate, rotate to multiple modes of action with the LibertyLink trait technology system or conventional herbicides.
  • Use appropriate rates as stated in the label guidelines; consider use of residual herbicides and weed populations when developing a program.
  • Historical problems by field and/or areas within fields should be the focus.
  • Types, weeds and timing of oversprays are important when selecting tankmixes.
  • Be aware of tankmix limitations when selecting herbicide combinations.

Culpepper says growers should treat weeds in a timely manner. “They should eliminate weed competition for six to nine weeks after planting,” he says. “Allowing weeds to grow after this period may not reduce yield, but then can adversely affect harvest and lint quality.”