There are two basic mechanisms that allow a weed to resist a herbicide’s ability to kill the plant. These include target site resistance and non-target site resistance. 

Marijke Vanderlaan, Agronomic Service Representative with Syngenta, explains that target site resistance can be described as a mutation within the plant that prevents the weed from absorbing the herbicide. For example, in the case of Group 2 herbicide resistance, the mutation prevents the herbicide from binding to a targeted enzyme. Without this bond, the herbicide is unable to stop this enzyme from producing critical amino acids that feed the weed and fuel its survival.

Non-target site resistance takes a different approach to avoiding the effects of a herbicide. In this instance, Vanderlaan notes that nothing actually happens at the site of the targeted enzyme because changes within the resistant weed prevent the herbicide from reaching the site, binding to the enzyme and controlling the weed. 

Glyphosate-resistant Canada fleabane

“For example, when it comes to glyphosate-resistant Canada fleabane, the herbicide is basically blocked by a process called vacuolar sequestration,” says Vanderlaan. “So, instead of the herbicide reaching the target site, the plant is able to sequester the herbicide and dump it into the vacuole, which is essentially the garbage can for the weed.”

Vanderlaan compares this process to how the liver functions in the human body. “The plant essentially identifies the herbicide as toxic material and dumps it into that vacuole. It never actually reaches the target site and the plant survives.”

Sequestration is one of several types of non-target site resistance. Enhanced metabolism is another form of non-target site resistance.

​​​​​​​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.