CHAMPAIGN, Ill. — When the weed killer Roundup was introduced in the 1970s, it proved it could kill nearly any plant while still being safer than many other herbicides, and it allowed farmers to give up harsher chemicals and reduce tilling that can contribute to erosion.
But 34 years later, a few sturdy species of weed resistant to Roundup have evolved, forcing farmers to return to some of the less environmentally safe practices they abandoned decades ago.
The situation is the worst in the South, where some farmers now walk fields with hoes, killing weeds in a way their great-grandfathers were happy to leave behind. And the problem is spreading quickly across the Corn Belt and beyond, with Roundup now proving unreliable in killing at least 10 weed species in at least 22 states. Some species, like Palmer amaranth in Arkansas and water hemp and marestail in Illinois, grow fast and big, producing tens of thousands of seeds.