March 22, 2023
Written by: Peter Newman
Elon Musk’s co-worker, Franz von Holzhausen, broke the window of the new Tesla Cybertruck that wasn’t supposed to break. How embarrassing!
AHRI researcher Roberto Busi teamed up with Masters student Geide Figueiredo (also known as JD) to try and break the herbicide Luximax (cinmethylin), and failed. How embarrassing!
I’m sure it was pretty disappointing for the researchers because when they’ve tried to break herbicides in the past, and evolve resistance in the lab, they’ve been pretty successful.
They gave it a good crack though, starting with 492 populations of ryegrass that growers had sent to Dr Roberto Busi for resistance testing in 2020. These were populations that were suspected to be resistant to a range of herbicides, and plenty of them tested as resistant. They sprayed them with cinmethylin and 457 of the populations died.
They then took 28 populations that showed some level of increased tolerance and did their darndest to find some resistance, resorting to attempting to “breed” resistance by exposing them to low doses.
And still nothing!
So disappointing for weeds researchers!
But great news for Australian farmers and agronomists. No signs of cinmethylin resistance just yet, but perhaps a little increased tolerance. We never say never, but at least we put this one to a good solid test, and it won.
It’s pretty tricky for a company to release a new herbicide these days, because it needs to control weeds that have already evolved resistance to numerous herbicides, and some of these resistance mechanisms can cause cross-resistance to unrelated products.
Cinmethylin is an old product that was given new life a few years back when it was released as the product Luximax by BASF. Back in 2020, AHRI researcher Roberto Busi, tested Luximax against 130 ryegrass populations from his resistance testing service, and found no resistance.
This led us to declare that Luximax gets its own box!
Roberto Busi then managed to convince Geide Figueiredo (JD) to have another go at finding resistance, this time over nearly 500 ryegrass populations.
Step 1 – test the lot
Test Luximax (cinmethylin) across 492 populations of gnarly ryegrass at 0, 375, 500 and 750 g active/hectare. The label rate is 375g/ha.
There were zero survivors to the label rate in 457 of these populations.
Step 2 – have a closer look at 28 populations
JD took the few survivors from 28 populations, allowed them to set seed, then conducted a dose-response test across these populations. He found these populations had a resistance index (RI) ranging from 1.1 to 6.4. In other words, the LD50 (lethal dose to kill 50% of the population) was up to 6.4 times the susceptible.
This sounds like a lot, but to classify a population as moderate to highly resistant, we need to see an RI of at least 10.
But let’s put this into context. The LD50 of the susceptible population was 10.2 g/ha, and the LD50 of the most tolerant population was 65g/ha, against the label rate of 375g/ha.
In all, 16 populations had an RI of 4 or greater, and these are considered to have reduced sensitivity to Luximax, but we have a long way to go before they will be declared resistant.
Step 3 – breeding resistant weeds
The final step was to take five populations, expose them to a couple of doses of Luximax (250 and 375 g/ha), collect seed from the survivors and see if they have increased tolerance. This is called recurrent selection.
The chart below shows the parent progeny of population 22 in black, and the dose-response of its progeny after exposure to a low dose of Luximax in red. The researchers saw a shift in the dose-response, but not enough to get too excited. However, this is after just one selection. Researchers typically undertake three or four recurrent selections in these studies, so watch this space.
Survival response (% of untreated controls) of P22 progeny and parental Lolium rigidum populations, a susceptible (S) control L. rigidum population, and wheat (‘Mace’) to increasing doses of cinmethylin (g ai ha−1).
Summary
When we saw significant evidence of cross-resistance between a number of our well-known pre-emergent herbicides, we were concerned that this would continue to happen with new pre-emergent herbicides as they come to market. Fortunately, this is not the case with Luximax (cinmethylin). Yet! We did witness some increased tolerance, but no evidence of resistance at this stage. Let’s keep it that way for as long as we can.
Paper
Posted in: AHRI Insight, Herbicide evolution and technology