Teagasc has confirmed that the threat of herbicide resistance in blackgrass, Italian ryegrass and spring wild oats is now very real.
Priority action on the part of tillage farmers is now required, where this matter is concerned. But, what does this mean in practical terms?
Farms where Italian ryegrass and blackgrass are present should be treated as resistance-suspect, and samples sent for analysis prior to harvest.
A zero tolerance approach should be taken to avoid weed seed being returned.
On farms where Acetyl-CoA carboxylase (ACCase)-resistant spring wild oats are present, there is only one control option available – acetolactate synthase (ALS) herbicides: Pacifica or Broadway.
These products should be carefully used in conjunction with hand rogueing to reduce the seed bank number.
Herbicide resistance
The continued use of the one herbicide chemistry will lead to resistance. No full herbicide-resistant brome strains have as yet been detected on Irish farms.
Both ALS (Pacifica, Broadway) and ACCase (Falcon, Stratos) have been found to be effective when applied at full label rates on small and actively growing plants.
But, the loss of sensitivity with lower application rates of these herbicides is concerning.
Cultural control methods – stale seedbeds, rotation, later sowing etc., – should be practiced where brome is present.
In addition, the limited opportunities for autumn pre-emergence use, and the on-going wet weather challenges will affect the timely spring weed control programme in 2024.
Products used now should be reviewed based on weed spectrum, crop type and growth stages.
Increased vigilance on the part of tillage farmers in dealing with the threat posed by herbicide-resistant grass weeds will be required.
This approach will include resistance testing and integrated approaches, such as the use of cultural control systems and judicious use of herbicides into the future.
Significantly, Teagasc’s 2023 testing of resistance-suspect grass weeds revealed a worrying situation: 60% of samples submitted were found to be resistant to a wide range of herbicides.
Take Italian ryegrass as a case in point – a total of 17 different populations were grown alongside a herbicide-sensitive control.
All were sprayed at the two to three-leaf stage with label rates of ACCase (Axial, Falcon, Stratos Ultra, Centurion Max) and ALS (Pacifica Plus or Monolith, Broadway Star) herbicides
The results, now available, confirm that 15 out of the 17 populations tested were resistant.
All 15 populations were ALS-resistant and 12 of those were also ACCase-resistant.
Moreover, populations that developed resistance to Stratos Ultra also had an impact on the efficacy of Centurion Max, resulting in no chemical options within crops for four populations.
Target-site resistance was the main mechanism for populations resistant to ALS herbicides.
However for ACCase, target-site (for populations cross-resistant to all ACCase herbicides) and metabolic resistance (for populations resistant to Axial and/or Falcon) were both involved.