Problem grass weeds such as wild oats, sterile brome, Italian ryegrass and blackgrass are increasing in tillage fields. They are very competitive, can cause yield losses of over 80% and can explode in numbers if left uncontrolled.
Weed control generally involves a combination of herbicide and cultural tools.
The ACCase (Group A) and ALS (Group B) herbicides are most commonly used for selective grass weed control (Table 1).
While there are a number of chemicals for use on wheat, there are no herbicides to control sterile brome or blackgrass in barley, and virtually no options at all for oats.
Our limited herbicides belonging to the ACCase and ALS groups carry a very high risk of resistance development.
Monitoring and mapping of herbicide-resistant grass weeds began recently following reports of poor control. This is being done through the Enable Conservation Tillage (ECT) programme.
As a part of the ECT programme, grass weed samples were collected from a number of problem fields across the country between 2019 and 2020 for glasshouse resistance testing.
Spring-germinating wild oats
Field populations of six wild oats (R1 to R6) were collected in Wexford, Kilkenny, and Cork prior to the 2019-harvest. Their response to ACCase herbicides was tested, in comparison with two susceptible populations (S1 and S2).
Plants were sprayed at the three- to four-leaf stage, with rates ranging from 0.25 to 8.0 times the recommended label rates of Axial, Falcon and Stratos Ultra. Herbicide effectiveness was measured by estimating the effective dose rates (ED90) required to kill 90% of the treated plants.
For Axial, the recommended label rate of 30g/ha did not kill 90% of the plants in populations R1 to R5. Instead, 1.5 to 3.0 times the recommended rate was required to achieve that level of kill. This confirmed that these five populations are pinoxaden-resistant (Figure 1A).
Similarly, for Falcon, the effective dose rates required to kill 90% of the plants in all six populations ranged between 2.5 and > 8.0 times the recommended rate of 100g/ha (Figure 1B).
For Stratos Ultra, the effective dose rates causing 90% mortality was about 1.5 times the recommended rate of 150g/ha in populations R2 to R4 (Figure 1C), while population R5 was extremely cycloxydim-resistant.
Overall, the resistance pattern profile indicates R2 to R5 were resistant to all three ACCase actives, R1 was cross-resistant to Axial and Falcon, and R6 was resistant to Falcon only. Different cross-resistance within a single field (for populations R1 to R5) limits the options of changing or alternating actives belonging to the ACCase group.
For R6, adequate control may initially be achieved by using ACCase-Axial or Stratos Ultra. Nevertheless, ALS-Pacifica Plus at recommended rate was found to be highly effective on all six ACCase-resistant populations, when used at the correct plant growth stage.
Furthermore, of 78 field populations of wild oats screened to date, ACCase resistance was identified in 10 populations, while there was no evidence of ALS resistance (Pacifica Plus or Broadway Star). However, growers must note that ALS resistance has already been reported in spring-germinating wild oats in other countries.
Autumn-germinating sterile brome
Seeds from 18 field populations of sterile brome were collected in 2020. Plants were sprayed (two- to three-leaf stage) with half or full recommended label rates of ALS/ACCase herbicides.
Glasshouse screens indicated that 89% of the populations had a wide range of tolerance or resistance, with survival varying from 8% to 68% when sprayed with half rate of ALS-Pacifica Plus. An additional 56% of the populations had some degree of tolerance to full label rate, with survival varying from 2% to 24%.
With the ACCase-Stratos Ultra, some populations (28%) recorded a lower level of tolerance, with survival from the half-rate dose varying from 4% to 26%.
These results confirm early stages of resistance evolution within and among sterile brome populations, which may be hastened by the repeated use of reduced rates.
However, ALS-Broadway Star and ACCase-Falcon (both half and full rates), and Stratos Ultra (only full rates) were found to be highly effective on all ALS/ACCase-tolerant populations when used at the correct growth stage.
From a total of 35 populations screened to date, no full herbicide-resistant sterile brome has been confirmed. However, there is a high likelihood that tolerance to ALS-Pacifica Plus may be widespread, which might reduce product efficacy and seems likely to get worse in time.
The introduction of herbicide-resistant blackgrass in recent years is likely to pose a significant challenge.
But some native populations may also carry a degree of resistance if they were regularly treated with herbicides that are active against blackgrass.
Samples from 12 blackgrass field populations were collected in 2020. Plants were sprayed (two- to three-leaf stage) with ALS-Pacifica Plus and Monolith, as well as ACCase-Falcon and Stratos Ultra at recommended label rates. Only some of these populations showed resistance.
One population (R1) from Cork survived the ACCase treatments only, while three populations (R2 to R4) from Dublin, Meath, and Waterford survived all ALS/ACCase treatments (Table 2).
For population R1, adequate control may initially be achieved using ALS-Pacifica or Monolith.
However, for multiple-resistant populations R2, R3 and R4, no chemical control options are available in any tillage crop and the clear recommendation would be to put it down to grass for a minimum of five years to help eliminate the seedbank.
Dose-response analysis is currently under way to quantify the levels of resistance in these four populations.
Symptoms of resistant and susceptible populations of blackgrass, with dose rates from 0.25 to 8 times the recommended rate of ACCase-Falcon, are shown in Figure 2. Falcon was virtually ineffective in all populations, especially population R1.
In addition to dose-response studies, genetic relatedness of English and Irish blackgrass populations is under way to identify whether Irish populations of blackgrass have independently evolved resistance or whether resistant blackgrass populations have been introduced.
Autumn-germinating Italian ryegrass
Italian ryegrass is not a major weed in winter cereals but infestations in cereals are an issue in places and some growers are experiencing control difficulties.
Field populations of four Italian ryegrass were collected in 2020. Plants were sprayed (two- to three-leaf stage) with ACCase-Stratos Ultra, Falcon and Axial Pro and ALS-Pacifica Plus and Broadway Star at recommended rates. Two populations (R1 and R4) from Tipperary and Meath were resistant to ALS treatments only, while populations R2 and R3 from Cork and Meath were resistant to multiple ACCase/ALS herbicides.
Nevertheless, all four resistant populations were totally controlled by ACCase-Stratos Ultra. Symptoms from the different treatments on the resistant populations are shown in Figure 3.
Pushing towards solutions
Herbicides from the same group can behave differently because actives are affected differently, there are different types of resistance and the proportion of resistant plants can be different. Most cases of weed resistance arose in situations where herbicides with the same mode of action have been used repeatedly in the same field. Earlier sowing of winter cereals, non-inversion tillage and low rates have also contributed.
However, growers have scope to practice aspects of integrated weed management to help manage resistant weeds and minimise further resistance development as follows:
It may even be justifiable to take out patches of the crops with glyphosate in severely infested fields. If weeds are surviving treatment, carry out resistance testing to establish which herbicides will effective.
As a general guide, delaying post-harvest cultivations for as long as possible encourages losses of all seed types. Shallow cultivation immediately after harvest encourages germination of freshly shed sterile brome seeds. For blackgrass, a similar tactic should be employed in moist soil conditions post harvest, but in dry soil conditions, leaving the soil uncultivated after harvest allows natural predation and germination of freshly shed seeds.
Shallow cultivation to establish a stale seedbed can be very useful in direct-drill or strip-till situations. While ploughing can bury weed seeds, it can also bring previously buried seeds back up in subsequent years. Alternating the plough with a few years of minimum cultivation can help further.
Delaying autumn drilling until the second half of October will help to bypass the main autumn flush of weed germination.
Moving fields into spring cropping can substantially reduce sterile brome, blackgrass, or Italian ryegrass, as these prefer to germinate and grow in the autumn. Growing non-cereal crops enables the use of other herbicide types, providing you do not have specific resistance to them.
The use of pre-emergence herbicides (Avadex Factor) will help to control wild oats, blackgrass and bromes. Use all herbicides at recommended label rates and avoid having to spray older or larger plants.
Where resistance is present, you must consider your best options for autumn versus spring grass weeds and use this to guide on whether to plant autumn or spring crops in a specific field.