Farmers still have a fighting chance to stop a tough, yield-robbing weed from taking root in the state, experts say.
Palmer Amaranth, a member of the pigweed family, was first officially identified in Iowa in September, 2013. As of spring 2014, there were documented cases in Harrison, Page, Muscatine, Fremont and Davis counties, though experts suspect it’s more widespread. Herbicide resistance, primarily to glyphosate, is an issue.
Iowa State University (ISU) Extension and Outreach weed specialists say it’s not too late to keep Palmer from spreading if farmers take action now. The Iowa Soybean Association (ISA) and ISU Extension are working together to provide information to keep the weed at bay.
“We’re at a point that we can really restrict how quickly it spreads. We can’t say that about most weeds,” said Mike Owen, ISU weed specialist. “If we ignore it, in the next 10 years it could be infesting half the (row crop) acres in Iowa.”
That will decimate yields and the bottom line, officials contend.
Native to the southwest United States, Palmer has slowly migrated north. It looks almost identical to waterhemp, the most prevalent herbicide-resistant weed in Iowa, but is more hardy and tougher to control.
Palmer produces twice the amount of biomass as waterhemp, which means it steals more water and nutrients from crops. It can grow tall, producing a large canopy, which interferes with photosynthesis of soybeans.
“Based on experience down South, it will eat up a soybean crop. There may not be an opportunity to harvest,” Owen said.
A mild to moderate infestation of Palmer can result in yield losses of up to 30 percent. That could mean a potential revenue hit of more than $200 per acre.
“A farmer can easily get their money back in control,” Owen added.
Early identification and action are critical to mitigate the spread of Palmer, experts agree.
Late May is the time to start scouting for the weed, experts say. However, vigilance all during the growing season is important since Palmer will germinate in the late summer and fall, which waterhemp usually doesn’t.
Palmer and waterhemp have morphological differences. Palmer generally has a longer and thicker flower stalk than waterhemp. The bracts (modified leaves at the base of flowers) of Palmer are 3 to 7 millimeters long, while waterhemp bracts are shorter.
“The difficulties differentiating between the two could be the downfall of containing it,” said Dr. Bob Hartzler, ISU weed expert. Owen and Hartzler will help farmers with identification.
Ed Anderson, ISA senior director of Supply and Production Systems, said not identifying specific weeds, as some farmers tend to do, could prove costly.
“There’s increasing importance to know what species you have in order to figure out better prescriptive control methods,” Anderson said.
Control strategies that work for waterhemp will work for Palmer. But the difference is waterhemp is a “relatively wimpy weed” and farmers can get by with weaker herbicide doses. That’s not the case with Palmer, which is why identification is so important.
Owen and Hartzler urge farmers to use soil-applied, residual herbicides and multiple effective modes of action to control Palmer. Group 3, 15 and some group 14 products work well. Post-emergent products are limited due to herbicide resistance, and should be used sparingly.
Good intentions, but only using herbicide will no longer cut it.
“All that does is continue to lead us down the path of resistance and increase the problem,” Owen said. “The buzz word, as it has been, is diversity. Anything that is simple and convenient isn’t sustainable.”
Cover crops and spot cultivation are also effective control measures.
Owen said there’s still time for farmers to act. Some of the first cases of Palmer were in sandy fields.
“That tells me at this point it’s probably not really well adapted to our highly productive soils. But it will adapt,” Hartzler said. “Managing fields differently and appropriately is the way to do it.
For more Palmer Amaranth management tips, go to www.takeactiononweeds.com/.