Identifying Different Types of Weed Resistance

When weed resistance strikes a field, the most immediate concerns are how to control those weeds and manage their spread. However, when the alarm subsides, it can be important for growers to identify what type of resistance they’re dealing with. And not just what sites of action or herbicide groups the weeds are resistant to, but also the method of resistance that’s underway. Dean Grossnickle, Syngenta agronomy service representative based in central Iowa, explains the two main types of resistance….

“The two types are target site resistance and metabolic resistance. Target site resistance is the most common type of resistance, and it relates to the site of reaction that is used. On the converse, metabolic resistance happens when the weed breaks down the herbicides, or the herbicide has a chance to effectively kill the weed.”

Grossnickle says target-site resistance is more common to encounter…

“The herbicide reaches its site of activity. Well, basically, it’s unable to act. The same sites are being hit with the herbicides time and time again, and then as the time progresses, the weeds basically builds up a resistance due to that repetition. There are 18 known sites of action. The herbicide targets that specific site of a weed and binds to it in order to kill that weed. A good way to think about this is to imagine that weeds have multiple kill switches, each protected by its own lock door. In an effective herbicide, it’s like a key that is specifically designed to unlock that door.”

He explains the risks of metabolic resistance…

“Metabolic resistance occurs when that weed metabolizes or breaks down that herbicide. It renders it to non-toxic before it ever has a chance to reach the target site that is designed to affect. A weed with a metabolic resistance mechanism may not have the ability to metabolize 100% of that herbicide, but it can metabolize enough of that herbicide, really doesn’t have an opportunity to kill that plant. An example, the plant may appear injured from that herbicide, but it will fully recover. In kind of that same lock-and-key analysis, some of that key actually reach in the site of action, that plant can just dissolve that key, and there’s no way that that target site will ever be affected by that herbicide.”

However, Grossnickle says that regardless of the type of resistance a grower is facing on his or her farm, the weed resistance management strategies are the same…

“We actually need to attack that population with multiple sites of action. Another thing that I like to do is use overlapping residuals. Additionally, using full effective rates. And last but not least, we can’t throw out the mechanical aspect of this and use any different cultural practices like cultivation. There’s not a weed out there that’s resistant to steel, and then using that row spacing and planting cover crops could all be effective cultural tools that don’t come with a lot of costs but yet can help keep our herbicides for the future for us to use.”

Syngenta works with growers through its Resistance Fighter® program to teach weed identification, coach in resistance management and provide localized solutions.

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