Published Oct 10, 2011

Growers grapple with resistant superweeds & PFI’s Annual Conference

By Sarah Carlson

FYI—PFI member Craig Fleishman and Dr. Mike Owen, ISU Extension Weed Specialist will speak about this topic at PFI’s Annual Conference on January 14, 2012 in Ames, IA. Plan to attend!

Growers grapple with resistant superweeds

( – Farming costs, food prices and agricultural pollution may rise as a result of nature’s strike back against a biotechnology that has revolutionized modern farming.

“Superweeds” resistant to the herbicide glyphosate, also known by the trade name Roundup, have infested millions of hectares of cropland through much of the U.S. and areas of southwestern Ontario.

That means farmers may no longer be able to reap the benefits of Roundup Ready crops, which are genetically modified to be resistant to glyphosate, allowing farmers to control weeds with the herbicide without harming the crops themselves.

Bill Johnson, a weed scientist at Purdue University in West Lafayette, Ind., told CBC’s The Current that the development of Roundup crops was among agriculture’s top one or two most important in the past 60 or 70 years because it allowed farmers to control weeds that had become resistant to a variety of other herbicides. That resistance had been forcing farmers to use complicated mixtures of chemicals to control weeds.

It also meant farmers no longer had to till their fields to control weeds, Johnson said.

“It greatly reduced soil erosion. It allowed farm sizes to expand,” he said, noting that tillage is time-consuming and expensive because it uses lots of fuel. Forgoing tillage has also reduced the amount of polluting agricultural run-off into waterways, he said.

Johnsons said glyphosate-resistant weeds began popping up in fields around a decade ago, and by 2002 or 2003, there was a large area in southeast Indiana where over 80 per cent of soybean fields had a glyphosate-resistant strain of mare’s tale, a weed also known as Canada fleabane.

Such weeds can double or triple the costs of weed control, he said, and lead to more tillage, more erosion, more water pollution from run-off, increased costs, yield losses and higher food prices.

Philip Shaw, a farmer and agricultural economist near Dresden, Ont., said glyphosate-resistant giant ragweed, a “very aggressive” weed that can grow up to three-metres tall, first appeared on his farm about 10 years ago. The plant can destroy 93 per cent of the yield of soybeans in the surrounding area, he added.

Shaw said he wonders if Monsanto, the U.S. biotechnology company that makes both Roundup and Roundup-Ready crops, has some responsibility to deal with this problem: “Because some of these weeds are getting away from what it says on the label will be killed.”

Trish Jordan, director of public and industry affairs for Monsanto Canada, said her company is committed to working with farmers and academics to make sure glyphosate continues to be effective weed control.

Jordan downplayed the impact of Roundup-resistant crops in Canada, where she says they are a relatively new phenomenon and confined to parts of Ontario. She credited good crop rotation practices and lower adoption of Roundup Ready crops compared to the U.S.

The company has been recommending practices such as crop rotation, tilling their fields from time to time if appropriate, and using other herbicides to help control weeds.

It is also working on genetically modified crops that are resistant to other herbicides, such as dicamba-resistant soybeans.

“In the U.S., we think that will be a potential option to help grower who have been relying on roundup ready soybeans to introduce a new technology into their fields that has a different mode of action.”

However Chris Willenborg, a weed scientist at the University of Saskatchewan, cautioned, “The solution is not always more and different pesticides.”

He suggested using additional methods such as crop rotation and high seeding rates to keep weed populations low and minimize the chance that they become resistant to Roundup.

Johnson noted that the process of natural selection inevitably leads to the appearance of weeds resistant to any widely used herbicide. The genetic variation within a population eventually produces an individual that can survive the pesticide, and over time, that strain will come to dominate, since all the other strains will have been killed off.

He said companies need to stay ahead of the resistance curve by developing new herbicides and investigating other means of controlling weeds.

He added, “We’ve simply gotten too accustomed to relying too heavily on a very good technology.”