WHILE cinmethylin (Luximax) resistance does not yet exist in ryegrass, when it does turn up, researchers have already discovered the resistance mechanism and therefore the cause of the problem, before the problem even exists.
Two years ago, Luximax was put in its own box as there was no cross-resistance from other herbicides and wheat tolerated cinmethylin likely by P450 metabolism.
That has not changed, however Australian Herbicide Resistance Initiative (AHRI) research has shown the same metabolic pathway would be responsible for resistance in annual ryegrass when it arrives.
The researchers, led by Danica Goggin, found three populations of ryegrass that had three to eight-fold higher tolerance to cinmethylin.
However, it wasn'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 and while they don't know for sure that this tolerance will turn into resistance in the field, based on past experience with other herbicides, it seems likely.
In this current study, Dr Goggin worked with three populations of ryegrass with reduced sensitivity to cinmethylin.
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"The populations aren't resistant, but they can tolerate more cinmethylin than totally susceptible ryegrass," Dr Goggin said.
The researchers used the common technique of using an insecticide called phorate, a known P450 inhibitor.
"We applied cinmethylin with or without phorate in agar to look for the resistance mechanism," Dr Goggin said.
"We found that phorate 'turned off' cinmethylin tolerance, improving the control of wheat and the three tolerant ryegrass populations mentioned above.
"This gave us confidence that P450s were involved in both wheat and ryegrass tolerance to cinmethylin."
When a herbicide is broken down in a plant by P450s, the herbicide molecule is broken into pieces called metabolites.
The researchers found cinmethylin was broken down into the same metabolites in both wheat and ryegrass and phorate inhibited the production of this metabolite.
"For wheat to tolerate a herbicide, there must be a mechanism," Dr Goggin said.
"In some instances, weeds such as ryegrass can evolve exactly the same mechanism, and this appears to be what we are seeing here."
The finding from AHRI shows researchers are in a new era of discovery as their understanding of herbicide resistance is so advanced they are sometimes able to determine the mechanisms of resistance before resistance happens in the field.
It's a positive step for the future of herbicide resistance and the information will help researchers to maximise the life and effectiveness of new herbicides.
