When it comes to the growing problem of battling herbicide-resistant weeds in the South, you might say cotton farmers are just lucky like that.
Although resistant weeds have proven troublesome in corn, soybeans and rice, cotton growers have had a particularly difficult time with resistant species such as horseweed and Palmer amaranth or pigweed.
“The problem of weed control in cotton  is compounded by the fact that corn  or soybeans  are selected for the seed whereas in cotton, although we sell the seed, we have historically selected cotton for the lint it will produce,” says Cotton Incorporated’s Robert L. Nichols.
“So we have a perennial plant that’s been selected for lint and not seed size that is in fact the slowest to emerge of the three major row crops.”
Couple the slower emerging crop with the fact that fewer herbicides have been registered in cotton because of tolerance issues and you can see why producers have felt more of an impact from herbicide resistance.
“When we have resistant events and lose a herbicide we’re in double trouble because we have fewer herbicides to lose,” said Nichols, senior director of research at CI and a speaker at the Pan American Weed Resistance Conference. The conference in Miami attracted weed scientists from throughout the Western Hemisphere.
“So the current system we’re facing with herbicide resistance in the U.S. has really put cotton almost at the point of the spear. We are the people who are at this point most impacted by herbicide resistance because we have those two difficulties — a slowly emerging crop and fewer herbicides to rely on to do our weed control.”
Cotton producers, like those of other crops, select a combination of management techniques that minimize crop yield losses and the costs of transgenic-trait technologies, herbicides and field operations to fit their operations.
“Herbicide resistance events destabilize these programs,” Nichols said. “A resistance event deletes the option to utilize a herbicide mode of action across all crops within an affected area. The early effects of the event are usually the most severe, because the occurrence of a resistant species is often unanticipated and yield losses may be substantial.”
Losses are proportional to the numbers of the resistant weeds, the species’ competitiveness, its rate of spread, and the effectiveness and cost of the remaining management options.
“There are also chronic costs of herbicide resistance, because the next most efficient alternative for control is less effective and/or costs more than the one lost,” Nichols told participants in the Weed Resistance Conference, which was sponsored by Bayer CropSciences.
“Thus, economic losses due to weed resistance are not alleviated until a new technology is introduced.”
Resistance to the acetolactate synthase herbicides (sulfonylurea and imidazolinone) is the most commonly occurring one in the South and is particularly prevalent in the Mississippi Valley. Resistance to acetyl-carboxylase inhibiting herbicides occurs in several areas.
“Both of these resistances are largely managed in corn by the triazine herbicides and were managed in soybeans and cotton by the use of the broad-spectrum herbicide glyphosate, in conjunction with the glyphosate-resistance trait,” says Nichols.
“The use of glyphosate for preplant vegetation control, and in conjunction with the resistance trait for in-season control, greatly accelerated the adoption of reduced-tillage systems. Unfortunately in the northern tier of the southern states, horseweed became resistant to glyphosate.”
The supplemental measures required to manage it preplant and at-plant have raised costs of conservation-tillage about $22.67 per acre, according to research by university agricultural economists.
Glyphosate-resistant Palmer amaranth poses an even greater threat to conservation-tillage. It has spread rapidly and threatens effective broad-leaf weed control in cotton and soybeans east of Texas. Additional herbicides required to achieve control of glyphosate-resistant Palmer amaranth in cotton now cost approximately $52.63 an acre.
To see a video of Nichols comments, go to http://southeastfarmpress.com/video/forefront-of-resistance-0414/ 
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