Herbicide-resistant weeds threaten farms, food

Bill Knight column for Thurs., Fri., or Sat., March 20, 21 or 22

Weeds are getting more resistant to common herbicides, and Illinois farmers must anticipate and battle the botanical intruders as they spread. These “super-weeds” are resistant to glyphosate, the active ingredient in many herbicides, according to scientists, and the worst may be the variety of pigweed called Palmer amaranth, which can grow more than six feet tall and produce hundreds of thousands of seeds.

There’s little question that Palmer amaranth will adapt to Illinois acreage, scientists say.

“Perhaps a more important question now is to define the damage niche of Palmer amaranth populations in Illinois agronomic cropping systems,” said Associate Professor Aaron Hager, a weed science specialist at the University of Illinois at Urbana-Champaign.

According to Hager, who’s studied the phenomenon for more than 20 years, Palmer amaranth has been shown to result in soybean yield losses of almost 80 percent and corn yield losses of more than 90 percent. With a fast growth rate, it’s very competitive with crops common to Illinois agriculture.

Glyphosate-resistant weeds were practically unknown before the introduction of Roundup Ready (RR) crops in 1996. It was considered a 1 in 100-year breakthrough in weed control when it became more widely used. The increasing use of RR seeds – which permit farmers to use that herbicide without killing the crop – led to an overreliance, and repeated applications contributed to weeds that can no longer be killed by glyphosate. Now, Roundup is becoming inefficient, as resistant Palmer amaranth, horseweed, giant ragweed, waterhemp and other weeds infest millions of acres of farmland. Since weeds compete for water, light and nutrients, and glyphosate is only one of several herbicides that have become less effective against many weeds, food production could be at risk.

“Skyrocketing herbicide use is news to the public at large, which still harbors the illusion, fed by misleading industry claims and advertising, that biotechnology crops are reducing pesticide use,” says Dr. Margaret Mellon of the Union of Concerned Scientists. “Such a claim was valid for the first few years of commercial use of GE [genetically engineered] corn, soybeans and cotton. But it is no longer.”

It’s no longer financially beneficial either.

“Roundup Ready crops were a godsend; weed-control costs were approximately $20 per acre in the Roundup system. But now, we spend $50 to $60 per acre for residual herbicides in addition to our Roundup Ready program,” Jimmy Wilson, a North Carolina farmer, told Progressive Farmer magazine. “Resistant pigweed can take a farm in two years.”

GE seeds now dominate corn and soybean production, and herbicide use on GE acres has shot up.

Resistant weeds force producers to use more herbicides – and more toxic types, such as paraquat and 2,4-D.

“Growing reliance on older, higher-risk herbicides for management of resistant weeds on herbicide-tolerant crop acres is now inevitable and will deepen the environmental and public-health footprint of weed management,” said Dr. Charles Benbrook of the Organic Center. “This footprint will [also] grow more diverse, encompassing heightened risk of birth defects and other reproductive problems, more severe impacts on aquatic ecosystems, and much more frequent instances of herbicide-driven damage to nearby crops and plants.”

Not native to Illinois, Palmer amaranth evolved as a desert species in the U.S. southwest.

“It is likely that Palmer amaranth was introduced by seeds moved into Illinois from areas where Palmer amaranth has become the dominant pigweed species,” Hager said. “Palmer amaranth populations have been confirmed in 26 counties throughout the state, with some showing glyphosate-resistant populations.

“This certainly is a threat,” he added. “We’re really running out of options and can’t afford to be lulled into complacency.”

Successful, long-term management of Palmer amaranth here will likely require more than herbicides, he added.

“The greatest likelihood for successful management is with systems that employ multiple effective management tactics,” he said. “Palmer amaranth is perhaps the personification of a weed species that requires an integrated management approach.”

Hager suggests ways to fight the invasive weed:

* Fields with Palmer amaranth populations should be the last fields planted.

* Areas where Palmer amaranth has produced seed should be marked to use aggressive weed management plans to prevent future seed production.

* Fields in which Palmer amaranth seeds were produced should not be tilled. Leaving its seeds near the soil surface increases the chances that grain-eating species will consume them.

Other ideas include hand-weeding and cover crops, which shade the ground to prevent resistant weeds from emerging early.

“It’s not too early to begin planning an integrated Palmer amaranth management program,” Hager said. “An integrated herbicide program should include soil residual herbicides applied at full recommended use rates within two weeks of planting and followed by post-emergence herbicides applied before Palmer amaranth plants exceed three inches tall.”

[PICTURED: Hager, photo from University of Illinois.]