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24 POTATO GROWER | APRIL 2017 You may think weeds resistant to herbicides are a new phenomenon linked to the overuse of glyphosate in genetically engineered crops, but nothing could be farther from the truth. This year marks the 60th anniversary of the first reports of herbicide resistant weeds, while 2016 was 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 (Daucus carota) 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 modes of action. They are found in 86 crops in 66 countries, making herbicide resistance a truly global problem. "Given all the media attention paid to glyphosate, you would think it would have the greatest number of resistant weed species," says David Shaw, a Mississippi State University weed scientist. "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." 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 sugarbeet acres in the U.S. are glyphosate tolerant and receive 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," says Shaw. "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. Many growers have had great success fighting resistance by adopting a broader range of controls. One example is found in the experiences of cotton growers in the southern U.S. After years of relying Herbicide resistance predates GMOs by decades 24 POTATO GROWER | APRIL 2017