Glyphosate grabs the headlines when it comes to resistant weeds, but weed scientists say it is not the only culprit as they prepare to observe a dubious milestone next year, which marks the 60th anniversary of the first reports of herbicide-resistant weeds. In contrast, glyphosate is only 42 years old, and glyphosate-resistant crop technology is only 20 years old.
The first report of herbicide resistance was thought to have happened in 1957 in two separate incidences in Hawaii and Canada. That summer, a biotype of spreading dayflower gathered from a Hawaiian sugarcane field survived a 5x dose of synthetic auxin herbicide. And a strain of wild carrots growing along the roadside in Ontario, Canada, was found to have similar synthetic auxin resistance.
Fast forward 59 years, and resistant weeds are a true worldwide phenomenon. Weed scientists have documented 250 resistant species in 55 countries, which disrupt the production of 86 different crops.
“Given all the media attention paid to glyphosate, you would think it would have the greatest number of resistant weed species,” David Shaw, Mississippi State University weed scientist, points out. “Though there are currently 35 weed species resistant to the amino acid synthesis inhibitor glyphosate, there are four times as many weed species resistant to ALS inhibitors and three times as many resistant to PS II inhibitors."
The glyphosate resistance buzz has an upside, however, Shaw adds.
“[It has] made glyphosate the public face for the pervasive problem of resistance,” he says.
Even so, Shaw adds that resistance issues are “far broader than a single herbicide” and growers have had the best success adopting a broad range of control methods.
In fact, farmers need to look beyond herbicides when assembling a control strategy for resistant weeds, according to Stanley Culpepper, weed scientist with University of Georgia.
“It would be naïve to think we are going to spray our way out of resistance problems,” he says. “Although herbicides are a critical component for large-scale weed management, it is paramount that we surround these herbicides with diverse weed-control methods in order to preserve their usefulness – not sit back and wait for something better to come along.”