Palmer amaranth has reared its ugly head in Ohio as reported by Mark Loux, OSU Extension weed specialist, in the Aug. 14 issue of the C.O.R.N. Newsletter. Loux says the state’s first confirmed infestation near Portsmouth is likely the result of contaminated CREP or wildlife seed. The population is both ALS and glyphosate resistant, according to his report.
“There are several other suspect sites in Ohio, which we are in the process of investigating,” Loux writes. “There are also new infestations of Palmer amaranth in Michigan and Indiana, and the source of these appears to be contaminated cottonseed shipped from the southern US for use as animal feed here in the Midwest.”
“That’s extremely bad news,” says Sarah Gehant, a field rep for Syngenta based in northern Kentucky. Gehant, too, has been dealing with Palmer this summer in her territory. “This is the first time it’s been in Kentucky,” she says. “But already we are seeing it in multiple counties.”
Gehant says most likely the weed arrived in Kentucky in chicken litter, soybean meal or cotton seed hulls brought in to feed dairy cows. She notes that Palmer has male and female plants. Only one of the parents needs to herbicide resistant to pass the trait along. With pollen that can blow a mile or more the males are especially suited to producing resistant offspring.
The weed grows to 7 or 8 feet tall, she says, and has a stem “the size of your forearm.” The seed head is 10 to 12 inches long and stands 3 or 4 feet above a soybean canopy. It can produce up to 600,000 seeds and germinates throughout the growing season. As if that’s not enough bad news it also can produce seeds when it is as small as 3 inches, she says. That’s especially true in dry conditions like we all had this summer.
Gehant has visited farms further south where Palmer has become established “wall-to-wall.” “It’s a (heck) of a wreck,” she says.
Controlling the weed with herbicides will require “layering” as many as 6 different modes of action, according to Gehant. For soybeans she is recommending an early burn down with a growth regulator, 2, 4 D or Clarity. She says add Sharpen if you have marestail too. At planting you will need a residual combination of Gramoxone and Boundary. “You want the field to be completely clean when the beans emerge,” she says.
Then 14 days later she recommends farmers come back with Flexstar GT or another chemical from the PPO inhibitor category. “Using a PPO is the pillar of Palmer control,” she says. “You really need another mode of action. And you will have to have overlapping residual herbicides both pre-emerge and post-emerge to keep the weeds from coming up.” she says. “Don’t count on any benefit from a canopy either. The Palmer will grow right through it.”
Finally, at harvest many farmers in the South use Gramoxone as a harvest aid to control any Palmer that has managed to escape the residual treatments. “I’ve never had as many calls asking about Gramoxone as I‘ve had this year,” Gehant says.
A corn rotation is also critical for suppressing the weed. “This is a multi-year pest,” Gehant says. “And a strong herbicide program in corn is one of the best ways to control it.”
All that said, she is convinced that palmer is manageable in the Midwest. “This is going to change the way we farm. It’s manageable, but we have to be proactive.”
It will be costly, but maybe not as much as the alternative. “There are places in the South where they have applied double rates of glyphosate and still had pay laborers to go in and chop the Palmer out by hand with machetes so they can harvest a crop,” Gehant reports.