CHAMPAIGN, Ill. (AP) — 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 24 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.
"It's getting to be a big deal," said Mike Plumer, a 61-year-old farmer and University of Illinois agronomist who grows soybeans and cotton near the southern Illinois community of Creal Springs. "If you've got it, it's a real big deal."
When Monsanto introduced Roundup in 1976, "it was like the best thing since sliced bread," said Garry Niemeyer, who grows corn and soybeans near Auburn in central Illinois.
The weed killer, known generically as glyphosate, is absorbed through plants' leaves and kills them by blocking the production of proteins they need to grow. At the same time, the U.S. Environmental Protection Agency considers it to have little toxicity to people and animals, and aside from the plants it's sprayed on, it's less of a threat to the environment because it quickly binds to soil and becomes inactive.
Monsanto's introduction of seeds designed to survive Roundup made things even better for farmers because they could spray it on emerging crops to wipe out the weeds growing alongside them. Seeds containing Monsanto's Roundup Ready traits are now used to grow about 90 percent of the nation's soybeans and 70 percent of its corn and cotton.
With increased reliance on Roundup, herbicide use on corn decreased from 2.76 pounds an acre in 1994 to 2.06 in 2005, the most recent year for which the U.S. Department of Agriculture has data. Spread that out over the 81.8 million acres planted in 2005, and it's a decrease of more than 57 million pounds of herbicides annually.
Farmers also found they could cut back or in some cases eliminate tilling, reducing erosion and fuel use.
But with any herbicide, the more it's used, the more likely it'll run into individual plants within a species that have just enough genetic variation to survive what kills most of their relatives. With each generation, the survivors represent a larger percentage of the species.
St. Louis-based Monsanto maintains the resistance is often overstated, noting that most weeds show no sign of immunity.
"We believe that glyphosate will remain an important tool in the farmers' arsenal," Monsanto spokesman John Combest said.
