June 4, 2025
Written by: Peter Newman
At first this research just looks like another resistance mechanism described, but wait, there’s more. This story has a few twists and turns.
Some exciting new AHRI research by Dr Danica Goggin and others discovered a resistance mechanism for a herbicide, how our crops tolerate the herbicide, and in the process, may have even discovered a new herbicide.
The research has identified a resistance mechanism in annual ryegrass to the herbicide bixlozone which is sold in Australia under the trade name Overwatch®. From here on I’m going to call it Overwatch to keep it simple.
The Overwatch resistant ryegrass was discovered by Dr Roberto Busi in 2019 from samples sent to a resistance testing service. The crazy thing is, this population was discovered before Overwatch was released to market.
Then Dr Danica Goggin researched the mechanism to find how wheat, barley and resistant ryegrass tolerate Overwatch and how this differs to how wild radish and canola tolerate it. That’s where it gets weird!
Figure 1: Overwatch (bixlozone) susceptible ryegrass on the left v resistant on the right
How Overwatch works
Overwatch is a pro herbicide. If you’re a nerdy, regular reader of AHRI insight, you’ll remember that a pro herbicide is one which must be converted into its toxic form once inside the plant.
In this case, Overwatch is taken up by the plant and a couple of P450 enzymes turn it into 5-ketobixlozone.
To keep it simple, let’s call this KetoOverwatch from now on.
It’s this KetoOverwatch that kills the ryegrass.
Figure 2: 5-ketobixlozone (KetoOverwatch)
How does wheat, barley and resistant ryegrass tolerate Overwatch?
It’s pretty simple.
For Wheat, Barley and resistant ryegrass, instead of converting Overwatch into KetoOverwatch, they convert it something else.
They have other P450 enzymes that turn Overwatch® into, let’s call it, NonToxicOverwatch. For the nerds, these are non-herbicidal metabolites.
If you want to picture what non-toxic-Overwatch looks like, it’s commonly Overwatch with an OH (hydroxyl) added to it by a P450 enzyme, and later a glucose molecule is added as well. Doesn’t look much like Overwatch anymore does it!
Fortunately, Overwatch resistance is extremely rare at this stage so we have many years of weed control with this herbicide ahead.
Cross resistance pattern
I spoke to Dr Roberto Busi about whether there was any obvious pattern of cross resistance to Overwatch . That is, does resistance to another herbicide automatically confer resistance to Overwatch? The answer is no. There have been several populations of Overwatch resistant ryegrass discovered in the field, and at this stage, there is no pattern.
How do wild radish and canola tolerate Overwatch?
Also pretty simple.
Canola and wild radish are tolerant because they hardly metabolise Overwatch® at all, meaning that they don’t convert it to the toxic KetoOverwatch.
It seems that they don’t have the P450 enzymes necessary for this step.
A new herbicide?
If you make KetoOverwatch in the lab, and apply it to wild radish or canola, it dies!
And here’s the kicker.
If you apply KetoOverwatch to grasses, including wheat and barley, it doesn’t die!
It seems that grasses and cereals don’t absorb KetoOverwatch very well.
Could this be a new herbicide?
It’s a long shot but it’s possible. We assume that the company may have already looked into this, but you never know.
Fun fact, if you have a new product made in the lab it costs a fortune. Spraying the first sample of “KetoOverwatch” in the field would have cost $280,000 per hectare!
Summary
Fortunately, Overwatch® (bixlozone) resistance is very rare, so most grain growers still have many years of this herbicide to look forward to. And it seems that there is no obvious cross resistance pattern emerging, so it will be difficult to predict where resistance to this herbicide pops up. But, we now know the mechanism and who knows, we may have even found the next herbicide.
Papers
Check out these papers here:
Posted in: AHRI Insight, Herbicide evolution and technology, Herbicide resistance mechanisms