Herbicide resistance a truly global problem

People may think that herbicide-resistant weeds are a new phenomenon linked to the overuse of glyphosate in genetically-engineered crops, but nothing could be further from the truth.

The Weed Science Society of America reports next year will mark the 60th anniversary of the first reports of herbicide-resistant weeds, while this year marks only the 20th anniversary of glyphosate-resistant crops.
The first known report of herbicide resistance came in 1957 when a spreading dayflower (Commelina diffusa) growing in a Hawaiian sugarcane field was found to be resistant to a synthetic auxin herbicide. One biotype of spreading dayflower was able to withstand five times the normal treatment dosage. That same year, wild carrot growing on roadsides in Ontario, Canada, was found to be resistant to some of the same synthetic auxin herbicides.

Since then, 250 species of weeds have evolved resistance to 160 different herbicides that span 23 of the 26 known herbicide mechanisms of action. They are found in 86 crops in 66 countries, making herbicide resistance a truly global problem.

Scientists say what is unique about glyphosate resistance is the severity of selection pressure for resistance development. More than 90 percent of soybean, corn, cotton and sugar beet acres in the U.S. are glyphosate tolerant and get glyphosate treatments -- often multiple times per year.

"The sheer size of the crop acreage impacted by glyphosate-resistant weeds has made glyphosate the public face for the pervasive problem of resistance," said David Shaw, a Mississippi State University weed scientist. "But resistance issues are far broader than a single herbicide and were around long before glyphosate-resistant, genetically engineered crops were even introduced."

Research shows that resistant weeds can evolve whenever a single approach to weed management is used repeatedly to the exclusion of other chemical and cultural controls -- making a diverse, integrated approach to weed management the first line of defense.