One thing that makes Palmer amaranth so much tougher than other weeds is that one plant can produce 500,000 to 1 million seeds. A combine can scatter seeds from a couple plants across an entire field, Johnson said, and the untrained eye can't tell the difference between Palmer amaranth and more common but less aggressive Corn Belt weeds, such as waterhemp and other kinds of pigweed.
Palmer amaranth probably took root in Kendell Culp's fields near Rensselaer in northwestern Indiana last year, but he wasn't aware of it until a seed salesman spotted it this summer. Culp pulled it up by hand — filling a pickup truck bed from one spot and a half load from another.
"Unfortunately I think it's going to be a pretty difficult weed to control for us," Culp said. He's working with a consultant on strategies for deploying herbicides on his 1,750 acres of corn, soybean and wheat.
Palmer amaranth often hitches a ride on dirt stuck to farm machinery. It may also hide in grass seeds planted as cover for conservation programs, experts say. But they disagree on whether the seeds spread through animal feed containing cottonseeds or hulls, which are commonly added to dairy cattle rations.
Johnson said the weed is often seen near dairy farms, and the presumption is that when manure from those cattle is spread on fields, the seeds can spread with it. But Culpepper said the research he's seen doesn't back up that theory, adding that spreading the idea without proof could hurt demand for cottonseeds — and the entire cotton industry.
The infestation found this August in two western Iowa soybean fields probably got there by truck, Iowa State University weed scientist Bob Hartzler said.
Despite those fields being adjacent to a stretch of flood plain with poor soil where sludge from a Nebraska company has been spread as fertilizer, he said there's no reason to think the sludge contained Palmer amaranth seeds. His suspicion is that the seeds were stuck in mud on trucks that hauled the sludge.