The 2011 growing season served a grim reminder that the fight against resistant weeds in the U.S. isn’t going well.

During field trips in September, No-Till Farmer executive editor/publisher Darrell Bruggink saw numerous Iowa corn fields infested with waterhemp, a member of the pigweed (or Amaranth) family that has developed resistance to glyphosate in many states — including severe levels in the South.

University of Nebraska greenhouse studies also confirmed glyphosate resistance in multiple giant ragweed populations.

The problem isn’t just with glyphosate. A waterhemp population in southeast Nebraska was confirmed to be resistant to 2,4-D, the university says. It’s the sixth herbicide mode of action to which waterhemp has developed resistance in the U.S.

Dean Riechers, associate professor of weed physiology at the University of Illinois, says in a recent university news release that 29 auxin-resistant weed species have been discovered worldwide since the 1950s. In comparison, 21 glyphosate-resistant weed species have been discovered since just 1996, when Roundup Ready soybeans were commercialized.

Two problematic weeds in Roundup Ready soybeans and cotton — common waterhemp and Palmer amaranth — aren’t on the list of auxin-resistant weeds.

Riechers says tankmixing auxinic herbicides, such as 2,4-D, with glyphosate may be the best short-term option for broad-spectrum, post-emergence weed control.

“Farmers can’t imagine going back to 2,4-D,” he says. “But herbicide resistance is bad enough that companies are willing to bring it back. That illustrates how severe this problem is.”

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