On July 6, 2011, Joe Steinkamp changed his entire weed-management program.

Steinkamp raises seed soybeans and white corn on Ohio River bottomland near Evansville, Ind. On that July day two years ago, his neighbor brought over some weeds that had survived Roundup. “He said, ‘Joe, do you know that these are?’ I said, ‘I sure do!’ ”

Just the day before, Steinkamp was in Tennessee with the Indiana Soybean Alliance to see the ravages of an aggressive pigweed that is rapidly moving north into Corn Belt states. Palmer amaranth is a faster-growing and more competitive cousin of common waterhemp, and it’s frequently resistant to glyphosate and other herbicides.

After seeing his neighbor’s soybean field thick with uncontrolled Palmer amaranth, Steinkamp went looking on his own farm. He discovered foot-high infestations in both soybeans and corn. The seeds probably arrived with Ohio River floodwaters, he says. He also spotted the weeds in drainage ditches.

“Palmer pigweeds can produce up to half a million seeds per plant. And they are easily carried by water, wind, tillage tools, combines; even on your clothes,” Steinkamp says. In Michigan, where the weed was first detected in 2010, the tiny seeds were probably transported in cottonseed fed to dairy cattle and spread to fields in the manure.

Early weed detection and aggressive management are essential, says Christy Sprague, Michigan State University weed scientist. “It’s difficult to control, especially in soybeans.” In fields where it’s not correctly identified or managed, a few plants “can rapidly multiply to several hundred plants in just a year’s time.”