Herbicide resistance in grass weed populations: the challenge continues to grow

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​Teagasc has just published an overview of the herbicide resistance status of grass weed populations within tillage systems during 2024, writes Richard Halleron.

Significantly, the work relates to tests carried out on suspect plant samples submitted by commercial farmers.

This work has confirmed that resistance in meadow grasses is arising due to the increased reliance on spring ALS herbicides and less use of autumn-applied residual herbicides.

Farms with ACCase (eg. Axial) resistant wild oats only have one herbicide type option available (ALS type: eg. Pacifica Plus or Broadway Star) which can only be used on wheat.

Herbicide resistance in grass weed populations is now a major challenge for arable farmers in Northern Ireland. And the scale of the problem is increasingHerbicide resistance in grass weed populations is now a major challenge for arable farmers in Northern Ireland. And the scale of the problem is increasing
Herbicide resistance in grass weed populations is now a major challenge for arable farmers in Northern Ireland. And the scale of the problem is increasing

Although no fully herbicide-resistant brome has been detected to date, our previous studies have indicated that loss of sensitivity in brome populations is widespread when herbicide rates are reduced.

Teagasc research scientists are now stressing the importance of resistance testing to determine which herbicide will work for a specific weed population.

Moreover, there is an immediate need to develop robust integrated weed management practices to reduce reliance on herbicides which are becoming more restricted and much less effective.

Annual meadow grass

Winter wheat remains a key driver for the local arable sector.Winter wheat remains a key driver for the local arable sector.
Winter wheat remains a key driver for the local arable sector.

Eight annual meadow grass populations grown along with a sensitive population were sprayed at the 2-to-4 leaf stage with ALS, ACCase and glyphosate herbicides.

Two populations were susceptible to all herbicides tested with six resistant to the ALS-Pacifica Plus.

Target-site mutations were the main mechanisms that conferred resistance in these six ALS-resistant populations.

Four of the populations had Pro-197 ALS gene mutations while two had Trp-574 ALS gene mutations.

All six populations that were resistant to Pacifica Plus showed cross-resistance to Broadway Star (not labelled to control meadow grasses).

ACCase-Centurion Max (used in winter oilseed rape) and glyphosate herbicides were found effective on all eight populations.

Rough-stalk meadow grass

Two rough-stalk meadow grass populations grown alongside a sensitive population were sprayed at the 2-to-4 leaf stage with label rates of ACCase (Axial, Falcon, Stratos Ultra, Centurion Max), ALS (Pacifica Plus, Broadway Star) and glyphosate (Roundup Flex) herbicides

Both populations were resistant to ALS-Pacifica Plus.

The main resistance mechanism was target-site mutations. The two resistant populations showed cross-resistance to Broadway Star, even though Broadway Star is not labelled to control meadow grasses.

Although, rough-stalk meadow grass control is not listed on most ACCase product labels, widely-used cereal (Axial) and graminicide (Falcon, Stratos Ultra or Centurion Max) herbicides and pre-sowing glyphosate were all effective at recommended label rates.

Spring wild oats

A total of 20 different wild oat populations were sprayed at the 3-to-4 leaf stage with label rates of ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica Plus and Broadway Star) herbicides.

Trials conformed that 14 out of the 20 populations were ACCase-resistant, with none resistant to ALS type herbicides at this point.

All 14 populations were resistant to Axial and Falcon, with 7 of those resistant to Stratos Ultra also.

Bromes

A total of 15 different bromes (including sterile, great and soft) were sprayed at the 2-to-4 leaf stage with label rates of ACCase (Falcon, Stratos Ultra), ALS (Pacifica Plus, Broadway) and glyphosate (Roundup Flex) herbicides.

ACCase, ALS and glyphosate herbicides were effective on all populations.

Dealing with the threat of herbicide-resistant grass weeds

Blackgrass and other herbicide-resistant grass weeds are now a reality within local arable systems, according to Lemken Ireland’s Derek Delahunty.

“But thankfully, the problems are not as large as those being faced by tillage farmers in Great Britain,” he added.

“The herbicide resistant blackgrass that have in this country probably arrived on combine harvesters and other items of machinery imported from Britain.”

In order for farmers to deal with problems associated with herbicide resistant weeds, they need to know the extent of the challenge that exists in the first place.

Derek Delahunty again: “This means being able to recognise these weeds in the field at all their growth stages.”

The Lemken representative cites whole cropping as a means of reducing blackgrass infestations.

He said: “It’s an option that can be considered in cereal crops that are at an advanced stage. Blackgrass is an annual plant. So, taking this approach can help to reduce the weed burden, assuming the crop is ensiled prior to seed set taking place within the blackgrass population that exists.

“Some blackgrass seeds may well survive this process. It’s important then to ensure that the dung in which the seeds can be found is not spread back on to tillage ground.”

Putting tillage ground back into grass is another way of controlling blackgrass.

“Cutting silage for a number of years will act to significantly reduce weed population numbers,” confirmed Delahunty.

He added: “Blackgrass is not the problem in countries like France and Germany as is the case in the UK and Ireland.

“This is because farmers in continental Europe can successfully grow a number of crops on th same ground each year.

“In France, farmers will establish a cover crop as soon as they complete the annuakl cerelam harvest, most likely in late June or early in July.

“When it comes to establishing the subsequent winter cereal crops, they chop the cover crop using a front mounted machine with the drill attached to the rear of the tractor.

“This means the freshly planted grains have the immediate protection of the previous cover crop. As this organic material decays, it acts as a valuable fertiliser for the newly emerging cereal.

Delayed planting is another blackgrass control option.

“As soon as fields are clear, following the previous harvest, growers should go in with a light harrow or tine, simply to disturb the top few centimetres of soil,” Delahunty commented

“This encourages the germination of blackgrass weed seeds that are lying close to the surface. This process provided very effective weed control options prior to planting the subsequent crop.”

He concluded: “I am very aware that delayed planting is a very weather-dependent option. And there’s no doubt that the weather last autumn and the year previous was not working in farmers’ favour in any sense.”

Transitioning to direct drill systems can take up to 10 years

Transitioning to min-till and direct drill production systems can take up to a decade, according to Derek Delahunty.

“And this is very much a gradual process.” he added.

“Taking account of factors such as the enhancement of soil quality and key physical challenges, includng compaction, is at the very heart of this progression.

“Given the complexity of this transition, the full conversion from plough: till: sow to a direct drill based-production system may take varying amounts of time across the different soil and land types that make up a typical arable farm in Northern Ireland.

“Significantly, Delahunty, regards ploughing as a critically import technique both at the start of a commitment to min-till or a reset button at any stage thereafter,” he explained.

“Historically, the plough was the go-to option when it came to securing effective weed control within Irish cereal production systems.

“And we have to learn from history. Chemical weed control systems have only been around for a relatively short period of time, relative to the overall impact made by production agriculture, going back centuries.”

The Lemken representative questions the long-term availability of chemical-based herbicides into the future.

“And this includes glyphosate,” he stressed.

According to the Lemken representative, ploughing will remain at the heart of the agronomic systems practised in Northern Ireland.

He further explained: “But the technique must be carried out properly. And this means achieving the complete inversion of the soil being ploughed.

“If it’s a case of only half turning the soil, problems will arise. For example, under these circumstances many weed seeds will still have the opportunity to germinate.

“So instead of only having weed seeds scattered on the surface of a field, they become distributed throughout an entire soil profile.”

Delahunty also recognises that climate change is acting to deliver new challenges for tillage farmers when it comes effectively establishing and management crops.

“As ambient temperatures rise, weed seeds will have an opportunity of germinating much more quickly and effectively,” he said.

“Back in the day, farmers would have been able to plough: till and sow their crops. The next time they would have visited the fields was at harvest time.

“Obviously this is no longer the case. And fundamentally, this is because weed populations are changing their growth habits. This reflects their ability to adapt to changing circumstances.

He concluded: “So farmers here in Northern Ireland must learn from history and change their crop management systems in a corresponding manner.”

Enable Conservation Tillage

Meanwhile, Teagasc is confirming that the challenge of ‘uncontrollable grass weeds’ within crops remains a major challenge for all arable farmers.

This is particularly so, where the use of minimum cultivation techniques, is concerned.

Teagasc’s recent Enable Conservation Tillage (ECT) project was developed to communicate the latest thinking on how these techniques can be applied in ways that respond to the grass weed challenge.

In the past, a number of Irish growers experimented with min-till, direct drill or no till systems, but were unsuccessful.

Many identified the difficulty of controlling of grass weeds as one of the primary reasons for reverting to plough-based systems.

The ECT project, however, highlighted a number of monitor farmers who had successfully made the change.

One of the big take-home messages from the project was the fact that that the establishment system is only part of the puzzle, more integrated pest management (IPM) tools must be incorporated into the farm system for the practice change to work.

According to Teagasc arable specialists, good rotations, avoiding compaction, learning from mistakes and reducing grass weed problems through cultural control methods, as well as herbicides, are all important parts of a successful transition.

Herbicide-resistant blackgrass populations are already creating challenges for Irish tillage farmers. Experience at Teagasc Oak Park in Co Carlow is already confirming that that blackgrass can become an issue unless growers are vigilant.

In 2020, as part of a plan to increase biodiversity on the farm, wild flower margins were planted. Significantly, the Oak Park farm manager identified blackgrass plants in one field.

A plan was put in place to eliminate the blackgrass. Glyphosate was used to burn off the affected margins, multiple stale seed beds were applied and the margin in question was ultimately planted with grass.

Italian ryegrass is another grass weed that is starting to become an issue on many Irish farms.

The problem can be traced to the autumn of 2018 when arable farmers were paid to establish forage crops after harvest to produce much need forage for livestock farms.

Some of this Italian ryegrass has now become a problem with many herbicides struggling to control it.

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