Charting the expanding footprint of different grass weeds is a difficult task and slowing it or preventing it is an even greater challenge.

Resistance to herbicides is adding to the challenge and this may be partly imported and partly homegrown.

It is critical that we recognise and identify resistance development in our fight to tame nature.

Herbicide-resistant wild oats

Most Irish fields have developed a problem with wild oats in recent decades.

Where herbicides are the sole control option, resistance can and will develop

Once a population becomes established in a field, it results in a long-lived seed bank, which shows sporadic germination.

This limits cultural control methods and forces growers to rely mainly on herbicides.

Where herbicides are the sole control option, resistance can and will develop.

So, knowledge of resistance levels and cross-resistance patterns among the main herbicide types is critical to developing integrated weed management strategies to eliminate or control resistant populations, to prevent resistance development and to reduce reliance on herbicides.

We have resistant wild oats

Following reports of weed control difficulties in 2019, seven wild oats populations (R1 to R7) were collected prior to harvest.

These were tested for herbicide resistance with ACCase and ALS herbicides at full recommended rates. Initial screening tests revealed reduced sensitivity in all populations to at least one of the herbicide actives.

Herbicide effectiveness was measured by estimating the effective dose rates required to kill 90% of the treated plants – the ED90

Further evaluation of the R1 to R7 populations was conducted to ACCase herbicides and compared to four susceptible populations.

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 and Falcon.

Herbicide effectiveness was measured by estimating the effective dose rates required to kill 90% of the treated plants – the ED90.

Results to date

The recommended label rate of 30g/ha of Axial did not kill 90% of the plants in six of the seven populations – this needed between 1.2 to three times the recommended rate.

This confirmed that these six populations are Axial-resistant.

With Falcon, the effective dose rates required to kill 90% of the plants in the seven populations (R1 to R7) were between 2.5 and > 8.0 times the recommended rate of 100g/ha.

Populations R1, R2, R3, R4, R5 and R7 were cross-resistant to Axial and Falcon, while R6 was resistant to Falcon only.

There was no evidence of ALS resistance found.

The consequences

Cultural or non-chemical control measures should always be used as the first form of defence.

If you have different cross-resistance within a field (for populations R1 to R5 and R7) this limits the options to change or alternate herbicide actives within the ACCase group.

While ACCase-Axial may control R6 for a while, ALS-Pacifica Plus at label rate was still highly effective on all seven ACCase-resistant populations when used at the correct growth stage (but it can only be used on winter wheat).

Where wild oat herbicides are used, all surviving plants must be hand-rogued to capture the initial appearance of resistant plants before seed is returned.

Herbicide-resistant blackgrass

Increasing reports of blackgrass in Irish tillage fields are extremely worrying and a zero-tolerance approach is essential to prevent its spread.

Blackgrass heads of varying sizes can be seen in more and more tillage fields around the country.

The occurrence of herbicide resistance in these populations suggests that they may have been introduced from the UK via seed, straw, or machinery contamination in recent years. It is also possible that we had existing populations which developed resistance, if they were regularly treated with herbicides that had efficacy against blackgrass.

Knowing the herbicide sensitivity status of a population will guide the methods that can be used as part of an integrated weed management (IWM) approach. Where chemicals can be used, they must be followed by the removal of all above ground seed sources that enable seed return by rogueing, early conservation or destruction.

Resistance in Ireland

Four suspect populations (R1 to R4) were collected in Cork, Meath and Waterford prior to the 2020 harvest. These were tested for their response to two herbicide classes, compared to a susceptible population (S2).

Effectiveness was measured by estimating the dose rates required to kill 90% of the treated plants

Plants were sprayed at the two- to three-leaf stage, with rates ranging from 0.25- to eight-times the recommended label rates of ACCase (Falcon) and ALS (Pacifica Plus) herbicides.

Effectiveness was measured by estimating the dose rates required to kill 90% of the treated plants (ED90). For non-resistant weeds, the ED90 values should be much lower than the label rate.

These evaluations showed that Falcon at label rates (100g ai/ha) was virtually ineffective in all populations, notably R1 (ED90 > eight times the label rate (Figure 1A). Pacifica plus at label rates (500g product/ha) was ineffective in populations R2 to R4, but R1 was sensitive (Figure 1B).

Figure 1: Symptoms of susceptible (S2) and resistant (R1 to R4) populations of black-grass following application of ACCase-Falcon (A) and ALS-Pacifica Plus (B) at dose rates from 0.25 to 8-times the recommended label rates (highlighted in red).

The implications

There are no chemical control options available for multiple herbicide-resistant populations such as R2, R3 and R4 populations.

The R1 population may still be adequately controlled with an ALS herbicide.

So, drastic approaches would be needed to reduce and eliminate the soil seedbank, including putting a field down to grass for a minimum of five years.

The most important lesson from this work is to prevent the establishment of blackgrass on your farm. Correct identification and herbicide resistance testing are also critical.

In brief

  • Resistance to herbicides seems to expanding at nearly the same pace as the weeds themselves.
  • Some populations are already resistant to all useful chemistry.
  • So far, resistance seems to be confined to blackgrass and wild oats, but problems may also occur in other weed grasses.