Every cliché and adjective in the book has been used to characterize it – “the 600-pound gorilla,” “devastating,” catastrophic,” “annihilating,” “calamitous,” “desolating,” “destructive,” “disastrous,” “overwhelming”, etc.
But if you’ve ever come face to face with glyphosate-resistant pigweed in your own fields, then you know there are no words to adequately describe it.
Resistant Palmer amaranth pigweed is Georgia’s most problematic weed in cotton and it continues to spread rapidly, according to Stanley Culpepper, University of Georgia Extension weed scientist.
The pest’s growth and damage potential alone can be alarming — two Palmer amaranth per 20 row feet of cotton can reduce yield by at least 23 percent, and in dryland production in Georgia, a single female plant can produce 450,000 seeds when competing with DP 555 BRR cotton for an entire season.
“Spread of this resistant pest is rapid through traditional means such as custom harvesting, failure to clean equipment, and the spreading of infested materials,” says Culpepper. “But the resistant trait also is moving rapidly via pollen.”
And resistant pigweed is an equal opportunity pest, threatening both conservation-tillage and conventional cotton production, he says. “Growers who have resistant Palmer amaranth pigweed must adopt aggressive management programs. More importantly, growers who do not have resistance must delay its arrival, as there are no economical programs for managing this pest in cotton,” says Culpepper.
Research conducted during 2008 showed that resistant pigweed control was positively impacted in dryland cotton production when land was deep turned, a yellow herbicide was incorporated preplant, or Ignite-based programs were started. For full 2009 University of Georgia recommendations for controlling glyphosate-resistant pigweed, go to http://mulch.cropsoil.uga.edu/weedsci/HomepageFiles/Palmer2009.pdf.
Glyphosate-resistant pigweed has spread to an estimated 500,000 to 1 million acres in Georgia and South Carolina. Alabama Cooperative Extension System specialists and USDA-ARS weed scientists confirmed the presence of glyphosate-resistant palmer pigweed in Barbour County in the eastern portion of Alabama during 2008.
“We anticipate it will spread across south Alabama fields in the coming years,” says Auburn University Extension Weed Scientist Mike Patterson. “Managing this weed in cotton will require the use of soil-residual herbicides and timely postemergence herbicides other than glyphosate.”
Soil-applied (residual) herbicides available for weed control in all varieties of cotton in Alabama include the following, says Patterson: metolachlor (Dual, etc), pendimethalin (Prowl), trifluralin (Treflan, etc.), fluometuron (Cotoran, etc.), prometryn (Caparol, etc.), diuron (Karmex, etc.), norflurazon (Solicam), pyrithiobac (Staple), and flumioxazin (Valor).
“Trifluralin must be incorporated within eight hours of application to be effective. Metolachlor must be applied early postemergence after cotton emerges in order to prevent crop injury on most Alabama soils. Valor must be used either early preplant or post-directed to prevent crop injury. Metolachlor, trifluralin and pendimethalin primarily control small-seeded broadleaf weeds (pigweed, Florida pusley, etc.) and annual grasses (crabgrass, goosegrass, etc.).
“Norflurazon and metolachlor have some nutsedge activity. Fluometuron, prometryn, diuron, norflurazon, pyrithiobac and flumioxazin also have significant activity on large-seeded broadleaf weeds like morningglory. All of these soil-residual products have significant activity on pigweed (including glyphosate-resistant biotypes) and are foundations for resistant pigweed management in crop rotations,” he says.
There are limited options for postemergence over-the-top weed management in cotton, he adds. Pyrithiobac (Staple), trifloxysulfuron (Envoke) and MSMA (at low rates) are available for broadleaf weed control in all cotton varieties after emergence, he says.
“They have activity on pigweed, but the weeds must be less than 2 inches tall in most cases. Envoke application must be delayed until cotton has at least five true leaves. If soil-applied herbicides fail due to lack of activation, then it will almost be impossible to control glyphosate-resistant pigweed with these herbicides due to the weed’s rapid growth rate,” says Patterson.
Liberty-Link cotton varieties are available that are genetically modified to tolerate gluphosinate (Ignite) herbicide, he says.
“Ignite has good postemergence activity on small pigweed (3 inches or less). Post-directed herbicides including prometyrn, diuron, linuron (Lorox, etc.), lactofen (Cobra), oxyflurofen (Goal), and flumioxazin all have good activity on emerged pigweed, but they must be directed to the base of the cotton stalk and some require cotton a minimum of 12 inches tall to prevent crop injury. They should also be mixed with MSMA for better foliar activity,” he says.
Paraquat (Gramoxone, etc.) has activity on pigweed but must be applied with a hooded sprayer for crop safety, according to Patterson. “Most of these herbicides, in addition to fluometuronm can also be used as layby treatments in cotton. Layby treatments with residual activity will be extremely important in controlling glyphosate-resistant pigweed.”
Finally, in order to maintain season-long control of weeds, especially glyphosate-resistant pigweed in cotton, a systems approach should be used, he says. “Start with a good soil-applied residual herbicide or herbicides, follow with the appropriate postemergence over-the-top foliar herbicides in a timely manner — when weeds are small — and follow lastly with residual layby herbicide applications.”
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