Managing weeds effectively in tillage crops is becoming increasingly difficult due to evolving herbicide resistance (where the herbicide no longer works effectively).
Often, resistance develops in weed populations not because of one factor but due to a combination of factors, including:
Limited crop rotations.Earlier sowing of autumn crops and non-inversion tillage adoption.Allowing weeds with a high risk of developing resistance (eg blackgrass, Italian ryegrass or common poppy) to grow and replenish seedbanks with viable seeds. Using fewer cultural/non-chemical weed control methods.Herbicide-only weed control based on high-risk ACCase/ALS herbicide groups.Using rates lower than the recommended rate or spraying on older or larger plants.Lack of on-farm biosecurity measures.While the development of herbicide-resistant grass weeds has received most attention recently, resistance in broad-leaved weeds cannot be overlooked. ALS chemistries (eg Ally Max, Cameo Max and Pacifica) form the backbone of most broad-leaved weed control programmes.
In this article, I will present the results of our glasshouse resistance testing of suspected populations of grass (blackgrass and Italian ryegrass) and broad-leaved weeds (common poppy) from the 2022 harvest that were submitted by growers/advisers or collected following growers’ reports.
The results suggest that most, if not all, cereal growers with these critical species have multiple resistance issues on their farms.
These tests took place in the glasshouse which is a controlled environment. Testing in the glasshouse can help to detect resistance early, so that growers can work to protect chemistry in the field.
It should be noted that controlled conditions in the glasshouse mean that spray is applied in optimum conditions.
Once we get to the field, weather immediately plays a part in how effectively a product works as well as the operation of the machine. So chemicals may not work as effectively. This should be kept in mind when reading this article.
If blackgrass is found on the farm it should be destroyed immediately. If seeds have set these plants will need to be picked and placed in a plastic bag so the seeds do not spread.
Resistance in blackgrass
Ten blackgrass populations were sprayed at the two- to three-leaf stage with field label rates of ACCase (Falcon at 1L/ha, Stratos Ultra at 2L/ha, Centurion Max at 1L/ha) and ALS (Pacifica Plus at 0.5 kg/ha) herbicides (Figures 1a and 1b).
Figure 1a: Symptoms of sensitive and resistant populations of blackgrass following application of ACCase Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus. Where plants are absent from a square full control was achieved.
Figure 1b: symptoms of sensitive and resistant populations of blackgrass following application of ACCase Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus. Where plants are absent from a square, full control was achieved.
Eight out of 10 populations tested were resistant, while samples Dublin-2 and Dublin-3 were fully controlled by all herbicides.Except for the Cork population, which developed resistance to all herbicides from the ACCase group, the remaining seven populations exhibited multiple resistance to both ACCase and ALS groups.Although Centurion Max offers limited control of ACCase cross-resistant blackgrass, in field situations its efficacy may not be sufficient to prevent seedbanks from increasing. It is noteworthy that the sensitivity results for Waterford and Cork populations were similar to 2020 resistance testing suggesting that once resistance has developed via target-site mutations, it does not go away even if herbicide selection pressure is removed.Resistance in Italian ryegrass
Thirteen Italian ryegrass samples (some farms had two or more field samples denoted by alphabets) were sprayed at the two- to three-leaf stage with field label rates of ACCase (Axial at 0.55L/ha, Falcon at 1.5L/ha, Stratos Ultra at 1.5L/ha, Centurion Max at 1L/ha) and ALS (Pacifica Plus at 0.5kg/ha and Broadway Star at 265g/ha) herbicides (Figures 2a and 2b).
Figure 2a: Symptoms of sensitive and resistant populations of Italian ryegrass following application of ACCase Axial, Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus and Broadway Star. Where plants are absent from a square full control was achieved.
Figure 2b: symptoms of sensitive and resistant populations of Italian ryegrass following application of ACCase Axial, Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus and Broadway Star. Where plants are absent from a square full control was achieved.
All 13 samples tested were ALS-resistant; and 12 of these also carried multiple resistance to at least one ACCase herbicide.Among ACCase herbicides, both Stratos Ultra and Centurion Max were effective, but with some exceptions. The sensitivity of Italian ryegrass to ACCase herbicides varied among fields on the same farm. In the Cork farm, Italian ryegrass from field-1A exhibited resistance to Axial, Falcon and Stratos Ultra, while field-1B was resistant or less sensitive to Axial and Falcon only. Similarly, in Wexford farms, fields-1A and 1B samples were Axial- and Falcon-resistant, while field-1C was Axial-resistant only. Although Italian ryegrass is a genetically diverse grass, information on previous herbicide use, crop rotation, machinery hygiene, field proximity and land ownership (eg rented) is necessary to fully understand the resistance spectrum of an individual field.
No sensitivity differences were recorded in Meath-1 (A and B) and Dublin-1 (A and B) farms.
Two poppy populations which appeared to exhibit resistance were subjected to detailed dose-response analysis. Plants were sprayed at the four- to six-leaf stage, with rates ranging from 0.25 to eight times the field label rates of ALS (Ally SX at 30g/ha, Cameo Max at 60g/ha, Pacifica Plus at 0.5kg/ha and Broadway Star at 265g/ha), hormone-2,4-D (D50 at 2L/ha) and hormone + ALS (Zypar @ 0.75L/ ha) herbicides to determine the severity of resistance (Figure 3).
Figure 3: symptoms of resistant populations of common poppy following the application of Ally herbicide at dose rates from 0.25 to eight times the recommended label rates (highlighted in red). The herbicide was basically ineffective.
Ally, Cameo Max and Pacifica Plus, as well as 2,4-D, were virtually ineffective Another ALS chemistry, Broadway Star, was effective, but 1.5 times (this is above the legal limit for farmer use) the field rate was required to kill 99% of the treated plants, indicating developing sensitivity loss.Zypar, which has Arylex in combination with ALS-florsulam (the active in Broadway Star) was found highly effective.Multiple herbicide-resistant grass and broad-leaved weeds are emerging problems that will test growers’ management skills.Herbicide resistance can vary from field to field within a single farm, which is why resistance testing is critical to optimise weed control measures. Managing resistance relies on developing a total approach that combines long-term perspectives and proactive tactics.
Managing weeds effectively in tillage crops is becoming increasingly difficult due to evolving herbicide resistance (where the herbicide no longer works effectively).
Often, resistance develops in weed populations not because of one factor but due to a combination of factors, including:
Limited crop rotations.Earlier sowing of autumn crops and non-inversion tillage adoption.Allowing weeds with a high risk of developing resistance (eg blackgrass, Italian ryegrass or common poppy) to grow and replenish seedbanks with viable seeds. Using fewer cultural/non-chemical weed control methods.Herbicide-only weed control based on high-risk ACCase/ALS herbicide groups.Using rates lower than the recommended rate or spraying on older or larger plants.Lack of on-farm biosecurity measures.While the development of herbicide-resistant grass weeds has received most attention recently, resistance in broad-leaved weeds cannot be overlooked. ALS chemistries (eg Ally Max, Cameo Max and Pacifica) form the backbone of most broad-leaved weed control programmes.
In this article, I will present the results of our glasshouse resistance testing of suspected populations of grass (blackgrass and Italian ryegrass) and broad-leaved weeds (common poppy) from the 2022 harvest that were submitted by growers/advisers or collected following growers’ reports.
The results suggest that most, if not all, cereal growers with these critical species have multiple resistance issues on their farms.
These tests took place in the glasshouse which is a controlled environment. Testing in the glasshouse can help to detect resistance early, so that growers can work to protect chemistry in the field.
It should be noted that controlled conditions in the glasshouse mean that spray is applied in optimum conditions.
Once we get to the field, weather immediately plays a part in how effectively a product works as well as the operation of the machine. So chemicals may not work as effectively. This should be kept in mind when reading this article.
If blackgrass is found on the farm it should be destroyed immediately. If seeds have set these plants will need to be picked and placed in a plastic bag so the seeds do not spread.
Resistance in blackgrass
Ten blackgrass populations were sprayed at the two- to three-leaf stage with field label rates of ACCase (Falcon at 1L/ha, Stratos Ultra at 2L/ha, Centurion Max at 1L/ha) and ALS (Pacifica Plus at 0.5 kg/ha) herbicides (Figures 1a and 1b).
Figure 1a: Symptoms of sensitive and resistant populations of blackgrass following application of ACCase Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus. Where plants are absent from a square full control was achieved.
Figure 1b: symptoms of sensitive and resistant populations of blackgrass following application of ACCase Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus. Where plants are absent from a square, full control was achieved.
Eight out of 10 populations tested were resistant, while samples Dublin-2 and Dublin-3 were fully controlled by all herbicides.Except for the Cork population, which developed resistance to all herbicides from the ACCase group, the remaining seven populations exhibited multiple resistance to both ACCase and ALS groups.Although Centurion Max offers limited control of ACCase cross-resistant blackgrass, in field situations its efficacy may not be sufficient to prevent seedbanks from increasing. It is noteworthy that the sensitivity results for Waterford and Cork populations were similar to 2020 resistance testing suggesting that once resistance has developed via target-site mutations, it does not go away even if herbicide selection pressure is removed.Resistance in Italian ryegrass
Thirteen Italian ryegrass samples (some farms had two or more field samples denoted by alphabets) were sprayed at the two- to three-leaf stage with field label rates of ACCase (Axial at 0.55L/ha, Falcon at 1.5L/ha, Stratos Ultra at 1.5L/ha, Centurion Max at 1L/ha) and ALS (Pacifica Plus at 0.5kg/ha and Broadway Star at 265g/ha) herbicides (Figures 2a and 2b).
Figure 2a: Symptoms of sensitive and resistant populations of Italian ryegrass following application of ACCase Axial, Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus and Broadway Star. Where plants are absent from a square full control was achieved.
Figure 2b: symptoms of sensitive and resistant populations of Italian ryegrass following application of ACCase Axial, Falcon, Stratos Ultra and Centurion Max and ALS Pacifica Plus and Broadway Star. Where plants are absent from a square full control was achieved.
All 13 samples tested were ALS-resistant; and 12 of these also carried multiple resistance to at least one ACCase herbicide.Among ACCase herbicides, both Stratos Ultra and Centurion Max were effective, but with some exceptions. The sensitivity of Italian ryegrass to ACCase herbicides varied among fields on the same farm. In the Cork farm, Italian ryegrass from field-1A exhibited resistance to Axial, Falcon and Stratos Ultra, while field-1B was resistant or less sensitive to Axial and Falcon only. Similarly, in Wexford farms, fields-1A and 1B samples were Axial- and Falcon-resistant, while field-1C was Axial-resistant only. Although Italian ryegrass is a genetically diverse grass, information on previous herbicide use, crop rotation, machinery hygiene, field proximity and land ownership (eg rented) is necessary to fully understand the resistance spectrum of an individual field.
No sensitivity differences were recorded in Meath-1 (A and B) and Dublin-1 (A and B) farms.
Two poppy populations which appeared to exhibit resistance were subjected to detailed dose-response analysis. Plants were sprayed at the four- to six-leaf stage, with rates ranging from 0.25 to eight times the field label rates of ALS (Ally SX at 30g/ha, Cameo Max at 60g/ha, Pacifica Plus at 0.5kg/ha and Broadway Star at 265g/ha), hormone-2,4-D (D50 at 2L/ha) and hormone + ALS (Zypar @ 0.75L/ ha) herbicides to determine the severity of resistance (Figure 3).
Figure 3: symptoms of resistant populations of common poppy following the application of Ally herbicide at dose rates from 0.25 to eight times the recommended label rates (highlighted in red). The herbicide was basically ineffective.
Ally, Cameo Max and Pacifica Plus, as well as 2,4-D, were virtually ineffective Another ALS chemistry, Broadway Star, was effective, but 1.5 times (this is above the legal limit for farmer use) the field rate was required to kill 99% of the treated plants, indicating developing sensitivity loss.Zypar, which has Arylex in combination with ALS-florsulam (the active in Broadway Star) was found highly effective.Multiple herbicide-resistant grass and broad-leaved weeds are emerging problems that will test growers’ management skills.Herbicide resistance can vary from field to field within a single farm, which is why resistance testing is critical to optimise weed control measures. Managing resistance relies on developing a total approach that combines long-term perspectives and proactive tactics. 
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