A train wreck has occurred in Mid-South agriculture with glyphosate-resistant weeds.
“A train wreck is coming to East Texas,” says University of Arkansas weed specialist Jason Norsworthy. “Weed resistance is definitely a threat to East Texas,” Norsworthy told participants in the Ag Technology Conference late last year in Commerce, Texas.
A sound rotation program already in place may help delay and control the problem, he said.
Herbicide-resistant weeds have caused serious trouble for Mid-South and Southeast farmers for several years.
“In 1992, U.S. farmers used 4 million pounds of active ingredient glyphosate,” Norsworthy said. “That was before Roundup Ready crops. In 2002, soybean farmers used more than 70 million pounds of Roundup and created tremendous selection pressure for weed resistance.”
By 2003, Northeast Arkansas fields had horseweed plant escapes resistant to glyphosate. “By 2004, most Northeast Arkansas fields were covered in mares tail (horseweed) that could not be controlled with Roundup. Farmers can’t plant into that.”
Norsworthy said wind disperses the seed from one farm to another. By 2006, the entire Mississippi Delta had serious resistance problems. “Now, anyone who farms in the Mississippi Delta has resistant mares tail.”
Control recommendations have changed. Norsworthy recommends a spring burndown treatment with Dicamba or 2, 4-D plus Valor for residual activity. “We don’t recommend fall residual applications. By March 15, residuals have broken down and farmers still have to go back in with glyphosate.”
He said common ragweed also shows glyphosate resistance “even with four times the normal use rates. Resistant common ragweed has shown up only in isolated fields so far.”
Giant ragweed is also becoming a resistance problem in Arkansas and Tennessee. Norsworthy said applications of up to 60 ounces of Roundup Weathermax have been ineffective.
“Last year we found Johnsongrass non-responsive to Weathermax applications. He said some farmers have abandoned fields because of Palmer Amaranth resistance. “These weeds can grow 3 inches a day in warm conditions. They also see this weed in Texas.”
He said resistance moves downstream along the Mississippi River with flooding. “Some resistant biotypes are 115 times more resistant than susceptible plants. As much as 2 percent of pigweed in all fields was non-responsive to glyphosate in a survey. That grew to 50 percent (in subsequent years).”
Norsworthy said the problem gets worse every year. One Roundup-resistant plant produces seed and the next growing season farmers see more infestations. Heavy rates of glyphosate do not take them out.
He said in some cases farmers disk to prevent weed survival.
Resistance has also spread into Georgia and other Southern States. “Georgia has serious Palmer Amaranth resistance issues,” Norsworthy said. “Farmers had to abandon a lot of fields.” Cotton and soybean fields in North Carolina have been choked out by Palmer Amaranth.
Some 2 million acres in the Mid-South and Southeast have resistant Palmer Amaranth infestations, Norsworthy said. “That’s a significant negative impact (across the region) and a major wreck in Arkansas.”
He said problems include the increasing complexity of weed control systems and the cost of treatments, up to an additional $23 an acre. “Multiply that by 2 million acres.”
Resistance also reduces harvest efficiency and results in crop loss, including abandoned fields.
Farmers with resistant weed problems are less likely to try or stay with conservation tillage systems. And resistance to one herbicide “perpetuates resistance to other broadleaf weed herbicides,” Norsworthy said.
He said widespread use of glyphosate may have limited ALS herbicide resistance problems and offered a godsend to labor-strapped farmers. The Roundup Ready system requires less time, less labor, the ability to cover more acres, and allows for more reduced tillage.
“But farmers abandoned cultivators and are not using pre-plant incorporated herbicides. They began to substitute convenience for accuracy and control to the point where the system has actually become less convenient. We may need to change and we are looking to other states (for new options).”
He said rotation, for both crops and herbicide mode of action, will be crucial. He said farmers should use a “lethal rate of herbicide. Avoid escapes with season-long control.”
New chemistry likely will not provide immediate relief, he said. “From 1986 through 1996, 96 new herbicides were labeled for use. We’ve had very few since.”
But growers do have options, such as:
• Use more modes of action.
• Integrate cover crops into cropping systems.
• Rotate with other crops.
• Use precision cultivation or bury weeds with deep tillage.
• Use other herbicide resistant crop traits such as Liberty Link.
He said farmers should “start clean with either tillage or a burndown herbicide — Gramoxone, Valor or a tank mix. Dual or Valor applied pre-plant and FlexStar early post emergence also may help.
“Liberty Link soybeans may be available in limited supplies in 2009. But remember, Ignite is not Roundup and timing is essential. Apply before weeds exceed 6 inches.”
Norsworthy said scientists also are looking at older herbicides, such as Atrazine and Banvel, that have been around since the 1950s and 1960s.
He recommends that farmers be aware of how resistance moves. “It’s most likely transferred from pollen movement, seed movement, farm equipment, flood events, gin trash and animal manure.”
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