Herbicide resistant weeds force greater herbicide use

Herbicide tolerant crop varieties have led to an increase in overall herbicide use in U.S. corn, cotton and soybeans, according to a recent study.


Professor Charles Benbrook of Washington State University found that the emergence of weeds resistant to glyphosate has led to 239 million more kilograms of herbicide used in the United States since 1996 than what would likely have been used in the absence of herbicide resistant crops.


Benbrook’s peer reviewed study, which was published in Environmental Science Europe, analyzed data from the U.S. Department of Agriculture to track pesticide use trends since 1996, when farmers began growing HT crops.


In an interview, Benbrook said herbicide resistant weeds pose a significant threat to agriculture, and that the onus is on the chemical and biotechnology companies to solve the problem. 


“The weed science community from all over the world was warning from the day this technology was commercialized in 1996 that if farmers become too dependent on it and plant Roundup Ready crops after Roundup Ready crops, that it would be a perfect storm for triggering the emergence of glyphosate resistant weeds,” Benbrook said. 


Eighty-five percent of corn and 95 percent of cotton and soybean acres in the U.S. are planted to varieties genetically modified to be herbicide tolerant.


Weeds resistant to glyphosate began to emerge in 2000 and by 2005 had led to a substantial increase in the amount of herbicide used in Roundup Ready cotton and soybeans. 


He said an extra pound per acre of glyphosate is applied today compared to when the technology was first brought onto the market.


The study predicts that the incremental increase of herbicide use will continue as herbicide resistant weeds spread. As well, farmers will be forced to return to older chemicals that have greater health and environmental effects, such as 2,4-D.


“(2,4-D) is one of the higher risk old chemicals that American and Canadian farmers have moved away from because of the public health and environmental risk,” Benbrook said. 


“It would be a real shame if it came back because there would most certainly be a price to pay in terms of reproductive problems.” 


Stephen Yarrow, vice-president of plant biotechnology at CropLife Canada, said weeds have adapted to weed management systems since the advent of agriculture.


When asked if herbicide tolerant crop systems have caused an increase in the use of herbicides, Yarrow said: “In sheer volume, then the answer is yes, particularly when we’re talking about glyphosate and glufosinate type herbicides. But on the other hand, the number of different herbicides that farmers have to use had decreased. They can use just one herbicide rather then multiple types of herbicide to control their weeds. It’s a much more efficient means of weed control, and it’s now allowing the use of more environmentally benign herbicides.”


Even though resistant weeds are emerging in herbicide tolerant crops in Canada, Yarrow said the situation is far more serious in the United States. 


“What we’re hearing is it’s all about rotation,” he said. “Farmers in the U.S., for various reasons where these issues have arisen, are not rotating their crops and different types of herbicide controls as frequently as they are in Canada. The situation in Canada is going to be less serious because of that.”


Benbrook also said alternating weed control methods is crucial in slowing the development of resistant weeds.


“By far the most important recommendation for farmers all over the world is when they manage their weed control system, is don’t rely too heavily on any one tactic or tool. In the world of herbicides, don’t rely on only one herbicide with only one active ingredient.”


The study also charts a decrease in the application of insecticides since the implementation of b.t. crops that manufacture one or more forms of the natural bio-insecticide bacillus thuringiensis. 


However, the amount of the insecticide displaced is less than the amount of bioinsecticide produced by the crops. 


“It is certainly true that b.t. generally poses a less worrisome set of risks on the environment and human health than the typical corn insecticide,” Benbrook said. 


“I think given the volume that are now being produced across large areas in the American Midwest because of the use of b.t. corn, it really warrants a closer look at what is going on in terms of impact on soil microbial communities, plant health and aquatic ecosystems.” 


Yarrow said HT crops have other benefits that fall outside the study’s scope, such as those associated with zero till.


Benbrook’s study, Impacts of genetically engineered crops on pesticide use in the U.S. — the first 16 years, is available online at bit.ly/esebenbrook2012.