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Ontario Grain Farmer Magazine is the flagship publication of Grain Farmers of Ontario and a source of information for our province’s grain farmers. 

Weed resistance


THE DEVELOPMENT OF glyphosate resistance has created a challenging obstacle for farmers to tackle at all stages of growth across all crops. Each stage of a crop’s development can be affected by weed pressure, which is why control is required in a timely fashion.



Dr. Peter Sikkema, field crop weed management associate professor at the University of Guelph Ridgetown Campus, believes that by using a variety of agronomic programs and modes of action farmers can ensure the usefulness of glyphosate for many years in the future and reduce the potential for additional glyphosate-resistant weeds. He conveyed this message, along with a lesson on the history of glyphosate, at the 2014 ‘DuPont Touch Tomorrow: Innovating for Success’ conference in London, Ontario attended by retailers and growers.

1950:    Glyphosate was invented by Swiss chemist Dr. Henri Martin who worked for a small pharmaceutical company named Cilag

1959:    Cilag was purchased by Johnson and Johnson, which sold its research samples including glyphosate to Aldrich Chemical now known as Sigma-Aldrich Chemicals out of the U.S.

1960s:    Aldrich sold small amounts to several companies but no biological activity was ever reported Monsanto was developing potential water softening agents. These compounds were tested as herbicides by Dr. Phil Hamm and Dr. John Franz

1974:    Roundup® first registered as a herbicide

1978:    Roundup is listed in Publication 75 – Guide to Weed Control

1997:    Roundup Ready® soybeans

2001:    Roundup Ready corn

2008:    Glyphosate resistant giant ragweed

2010:    Glyphosate resistant Canada fleabane

2011:    Glyphosate resistant common ragweed

There are currently 24 species in the world that are glyphosate resistant. Of those 24 species, 14 can be found in the U.S. and four can be found in Canada. Three species are in Ontario, Giant ragweed (2008), Canada fleabane (2010), and Common ragweed (2011).

Weeds can become resistance to herbicides in three ways: target site resistance, non-target site resistance, and gene amplification. Target site resistance occurs when the herbicide reaches the target site at a lethal dose but there are changes that limit herbicide impact. Non-target site resistance involves different mechanisms that minimize the amount of active herbicide that reaches the target site, such as reduced absorption. Gene amplification occurs naturally when a selective gene increases more than others within the same stand of DNA, similar to the duplication of cancer cells. 

Glyphosate stewardship is an integral part of reducing reliance on this one herbicide. Sikkema recommends that growers implement a diverse crop rotation that includes non- Roundup Ready® crops, such as Winter wheat or spring cereals, Identity Preserved (IP) soybeans, dry beans, and forages. He also suggests including crops with alternative herbicide resistant traits, such as tolerance to dicamba or 2,4-D. Executing well planned pre and post emergent application programs and using more than one herbicide mode- of-action on every acre, every year, will help combat resistance. •


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