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Glyphosate: What’s fact, fiction?

Labeled an “explosive topic,” glyphosate disease susceptibility and weed resistance was high on the agenda for the Pacific Northwest Direct Seed Association’s annual meeting in Spokane, Wash., earlier this year.

Trying to separate fact from fiction in the debate, USDA Agricultural Research Service agronomist Frank Young told the conference that claims that take-all, rhizoctonia and bare-patch disorders are associated with the herbicide have not been verified. “It seems to me that most of this is associated with the green bridge,” he said.

Glyphosate interactions with micronutrients and plant diseases has become a hot topic of late, amid charges that the world’s most widely used weed killer alters manganese and other minor nutrients, leading to root disorders.

But what seems obvious to Young and others who addressed the PNDSA meeting is these alleged negative effects are not surfacing in field studies. “Regarding the claim that roots exude glyphosate, I was able to find only one case of this in the literature I reviewed,” he said.

The allegations that glyphosate can damage crops by decreasing nutrient uptake and contribute to 40 or more plant diseases, causing dramatic yield decreases in wheat, alarmed the industry. These allegations were advanced by retired Purdue University plant pathologist Don Huber. In a 2009 European Journal of Agronomy article, he argued that widespread use of glyphosate can “significantly increase the severity of various plant diseases, impair plant defense to pathogens and diseases, and immobilize soil and plant nutrients rendering them unavailable for plant use.”

While noting that “all herbicides can influence growth and diseases severity of some pathogens,” Young said research in Roundup Ready wheat and soybeans shows they are no more susceptible to soilborne fungi disease than varieties that are not Roundup Ready selections.

Unfounded claims

“Claims that glyphosate has led to skyrocketing diseases are unfounded,” he said. “I do not think that you need to be concerned with micronutrient uptake and diseases” when using glyphosate, he added.

The importance of glyphosate to direct-seed growers is hard to overstate. The No. 1 concern of growers in the Pacific Northwest is losing this herbicide from the weed killer toolbox, Young said. Direct-seeders are totally reliant on glyphosate.

But while the product continues to lead the way in weed control, concern over weed resistances continues to trouble producers. The good news, said Oregon State University weed scientist Dan Ball, is of the 13 weed species resistant in Western states, most are not a concern in the PNW. Italian ryegrass is an exception.

While resistance is a growing problem, “it is not at this time the end of the world,” said Ball. Nevertheless, he characterized the glyphosate weed resistance as a “rapidly expanding problem. Rotation is very important,” he added. “We need to keep this important herbicide.”

Weed resistance to herbicides is not new, said Wayne Jelinek, Monsanto Roundup Ready manager. “Herbicides that you have been using all of your lives have had resistance problems,” he said. The company is probing the allegations and resistance situation of glyphosate, he added. “This product is our family jewel, and we are studying the resistance problems regularly.

“The sky is not falling [on glyphosate], but we must be proactive,” he said. “We take these claims [as from Huber] very seriously and have spent an inordinate amount of time addressing these issues.”

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GLYPHOSATE defenders: Researchers Dan Ball (left) of Oregon State University, Frank Young with USDA and Wayne Jelinek with Monsanto made up a panel discussing glyphosate at the PNDSA meeting.

This article published in the April, 2012 edition of WESTERN FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2012.