Weed resistance is a growing – and concerning – problem across Canada. But to limit the spread of resistant weeds and preserve the herbicide tools we have, it’s important to understand how the main types of herbicide resistance work. 

In this article, we’ll focus on target site resistance, while part two will take a closer look at non-target site resistance. 

So, what is a target site, anyway? 

Herbicides work like a “key and lock” system. Herbicides – as the “key” – work by binding to the target sites – or “lock” – of specific enzymes in weeds and de-activating them. This shuts down the production of amino acids that weeds need for growth, and eventually brings all plant function to a halt. 

Target site resistance happens when there is an alteration (or mutation) within the enzyme’s target site that prevents it from absorbing the herbicide. 

“Basically, what happens is that the herbicide ‘key’ no longer fits the target site ‘lock’,” explains Marijke Vanderlaan, Agronomic Service Representative with Syngenta Canada. “So, the enzymes are able to keep producing amino acids, and the weed itself lives to fight another day.” 

Another type of target site resistance that can occur is gene overexpression or over-amplification. A prime example of this phenomenon can be found in glyphosate-resistant waterhemp. 

In a normal, susceptible waterhemp plant, glyphosate binds to the target site of an enzyme and shuts down energy production.

However, a glyphosate-resistant waterhemp biotype over-expresses its amino acid-producing enzyme and makes numerous copies – too many for the glyphosate to bind to all of them. 

“The problem here is that there are more enzymes created than what the herbicide can handle,” says Vanderlaan. “Even if your herbicide successfully shuts down one enzyme target site, the additional enzyme copies ensure that the resistant waterhemp will continue to produce energy – and ultimately survive.”
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For more information including articles, resources and videos on resistance management, check out Manage Resistance Now. This is a collaborative effort of industry, academia and government experts, brought together by CropLife Canada, to raise awareness and promote the adoption of strategies to manage weed, insect and disease resistance.