May 13, 2022
Written by: Peter Newman
“If I were given one hour to save the planet, I would spend 59 minutes defining the problem and one minute resolving it,” Albert Einstein.
We spend a lot of time defining and understanding the problem in herbicide resistance research. Perhaps we overdo it sometimes, and according to Albert, we’re on the right track.
We don’t have cinmethylin (Luximax®) resistance in ryegrass yet, but when it does turn up, we already know the resistance mechanism.
We know the cause of the problem before we even have the problem. How good is that!
Two years ago, we told you that Luximax® gets its own box (in other words, there is no cross-resistance from other herbicides), and that wheat tolerates cinmethylin likely by P450 metabolism. This message hasn’t changed.
Now, some recent AHRI research by Dr. Danica Goggin and others has shown that it looks like the same metabolic pathway will be responsible for resistance in annual ryegrass when it arrives. How do we know? The researchers found three populations of ryegrass that had 3 to 8 fold higher tolerance to cinmethylin. However, it isn’t classified as resistant because the label rate of herbicide still kills it. The team studied these populations and found a lot of similarities between wheat and ryegrass tolerance.
We don’t know for sure that this tolerance will turn into resistance in the field, but based on past experience with other herbicides, it seems likely. We understand the problem, now can we use this knowledge to resolve it?
Let’s start with a quick recap of the “Luximax gets its own new box” edition of AHRI insight.
Dr. Roberto Busi exposed 130 field populations and a few other nasty populations of ryegrass to cinmethylin. He found resistance to other pre-em herbicides, but no resistance to cinmethylin. From this, we concluded that “Luximax gets its own box”. In other words, no cross-resistance was found. In this research, Roberto also concluded that wheat tolerated cinmethylin likely by P450 enzymes.
In this current study, Danica worked with three populations of ryegrass with reduced sensitivity to cinmethylin. The populations aren’t resistant, but they can tolerate more cinmethylin than totally susceptible ryegrass.
Two of these populations were direct from a field near Tammin, Western Australia. A third population from Wickepin, Western Australia was further selected with two low doses of cinmethylin. These populations are 3 to 8 fold more tolerant to cinmethylin than susceptible ryegrass.
Keep in mind that the label rate of cinmethylin in Australia is 375gai/ha. The label rate gives high levels of control of all of these populations, which is why we say they have reduced sensitivity rather than resistance.
Phorate turns off P450 enzymes
The researchers used the common technique of using an insecticide called phorate, a known P450 inhibitor. They applied cinmethylin with or without phorate in agar to look for the resistance mechanism. They found that phorate “turned off” cinmethylin tolerance, improving the control of wheat and the three tolerant ryegrass populations mentioned above. This gave them confidence that P450’s were involved in both wheat and ryegrass tolerance to cinmethylin.
When a herbicide is broken down in a plant by P450’s, the herbicide molecule is broken into pieces called metabolites. The researchers found that cinmethylin was broken down into the same metabolites in both wheat and ryegrass, and phorate inhibited the production of this metabolite.
What’s good for wheat is good for ryegrass
For wheat to tolerate a herbicide, there must be a mechanism. In some instances, weeds such as ryegrass can evolve exactly the same mechanism, and this appears to be what we are seeing here.
We are in a new era. Our understanding of herbicide resistance is so advanced that our researchers are sometimes able to determine the mechanisms or resistance before resistance even happens in the field. This is excellent information that can serve to help us maximise the life and effectiveness of new herbicides. We are on the way to understanding this future problem, now to resolve it!