Monitoring by Dr Vijaya Bhaskar at Teagasc Oak Park in Ireland has found a complex resistance picture in mixed grass weed populations that helps inform herbicide choice.
Samples are sent to Dr Bhaskar from fields where herbicide resistance is suspected because the control is less than expected. The work is part of Teagasc’s Weed Watch initiative and has been running since 2019.
The research tested popular contact-acting herbicides based on ACCase (clethodim, cycloxydim, pinoxaden, propaquizafop) and ALS chemistry (mesosulfuron, iodosulfuron, pyroxsulam).
It initially focused on sterile brome and wild oats, but as other grass weeds spread on Irish farms, black grass and Italian ryegrass also became targets.
“For blackgrass and Italian ryegrass, we see new fields every year,” says Dr Bhaskar. “The situation is quite complex due to the spread of native weeds, and those we suspect have been introduced. Rats tail fescue is now also becoming an issue in a min-till situation.”
The results show a complex resistance pattern for each weed population, amplified at field level if there is a mixed grass weed population. Dr Bhaskar believes resistance has evolved in Irish strains, much of this due to the innate tendency for resistance to develop to herbicide actives.
“Herbicides like clethodim and cycloxydim rarely develop enhanced metabolism non-target site (NTS) resistance. The likelihood of resistance developing in a weed population is through target site mutations.
“However, clethodim is still less affected by target-site resistance as the degree of resistance depends on specific mutations and zygosity, as well as the ploidy level of a grass-weed,” explains Dr Bhaskar.
Dr Bhaskar’s analysis is mirrored in the results from 2023. Centurion Max (clethodim) was effective against all the blackgrass samples (picture 1), whereas 42% were cross-resistant to all the other herbicides tested.
The picture for Italian ryegrass is more complex and challenging (pictures 2 and 3). 15 of the 17 samples analysed had resistance to ALS or ACCase chemistry, the exception being Centurion Max, which remained effective against all but four of the samples.
Bromes were more positive, with all the herbicides effectively controlling the samples.
Antonia Walker, technical specialist for UPL, believes the difference in performance seen from Centurion Max when compared to the other ACCase herbicides is because clethodim is much less affected by the 1781 mutation, which makes grass weeds highly resistant to other herbicides in this group.
“The main mutation to impact clethodim’s performance is 2078,” says Antonia. “Studies have shown this to be much less common in UK black-grass populations. However, Italian ryegrass has it at a higher frequency, although our studies have shown that clethodim will still have some effect.”
UPL continues to watch the resistance levels in grass weeds and is monitoring samples gathered from fields, explains Antonia. She advises growers to participate in initiatives such as the Teagasc Weed Watch programme, taking grass weed seed samples for testing and keeping a record of the information for each field.
Advice to get the best grass weed control in oilseed rape this season
UPL’s Antonia Walker believes the opportunity to use alternative modes of action in oilseed rape could help farmers get back on track after a difficult season for grass weed control.
“UPL has investigated autumn black grass control programmes in oilseed rape and has discovered that the most effective control came from a mid-October application of Centurion Max followed by propyzamide applied in mid-December.
“If the propyzamide is delayed until mid-January, then this reduces black-grass control compared to a mid-December application by almost 10%.
“We believe this is because it allows some of the black grass that survived the Centurion Max to recover sufficiently to develop roots below the subsequent propyzamide layer in the soil,” advises Antonia.
For growers planning Centurion Max this autumn, Antonia says there are several steps farmers can take to maximise its effectiveness.
She cites research conducted by independent consultant Stephen Moss investigating the effect of water conditioners. Across six field trials, a water conditioner increased black-grass control by 11% on average, regardless of the water hardness. Significantly, where the populations were more resistant to clethodim, the benefit was even greater than the average, says Antonia.
“We recommend that Centurion Max is not applied after the 15th of October or when the crop goes beyond six true leaves. This is primarily a crop safety measure because the plant must be growing to metabolise the clethodim and avoid damage from the herbicide.
“The ideal timing to spray black grass with Centurion Max is when it is at three true leaves and has not tillered.
“There should be a ten-day interval between applying any pesticide and Centurion Max and a fourteen-day window after it has been used where no other pesticide should go on the crop,” advises Antonia.
