Arable farming: the contents of the chemical toolbox are fast diminishing

Throw in the recent upsurge in Barley Yellow Dwarf Virus (BYDV) activity within both spring and winter cereals – an event that had not been predicted by crop scientists or entomologists – and we are really witnessing the reality of nature now outpacing the rate of endeavour that can be achieved by our best scientists.

It all adds up to a scenario that could see the end in sight for chemistry-based crop management regimes.

Blackgrass

Blackgrass is symbolic of this challenge.

Thankfully, it is not widely found across the island of Ireland at this stage.

But the weed now represents a major challenge to arable farmers in many parts of England.

It is now, more or less, totally resistant to all selective herbicides.

The need to keep blackgrass out of Northern Ireland is obvious.

However, the future management of the weed within crop rotations is of more than significant interest to local cereal growers.

Hence, the relevance of the blackgrass trials, which are now taking place at Stow Longa in Cambridgeshire.

The projects have been developed and managed by agronomists with the Agrii organisation.

The work has one, overriding objective: to develop rotation and cultivation-based techniques as a means of controlling herbicide-resistant grass weeds.

This list includes blackgrass, bromes and ryegrasses.

The trial work to date has identified a number of factors that will impact on blckgrass plant numbers.

These include: planting date, the rotations followed and min-till versus ploughing.

But driving all of this is a fundamental knowledge of the soil that is actually being worked.

At the Stow Longa site the non-organic fraction of the soils have a 51% silt content.

This is an extremely mobile component within any profile and plays a key role in developing soil pans if zero or min-till cultivation options are practised on a consistent basis.

The work at Stow Longa has been supported by Lemken from the get-go with the results generated from the work used by the company to develop its own views on the future of tillage machinery design.

Blackgrass is now endemic in many parts of England. Plant numbers in excess of 200/ha can reduce overall cereal yields by up two 2t/ha.

The Stow Longa work has consistently confirmed the impact of planting date on blackgrass control.

Where winter cereals are concerned, delaying drilling dates into October provides growers with the opportunity to maximise the control of autumn germinating blackgrass plants.

The inclusion of spring cereals within a cropping rotation is another factor that helps to drive down blackgrass populations.

Driving all of this is the fact that growers going in with a spring crop have the opportunity to control blackgrass plants that have germinated over a six month period.

Significantly, hybrid barleys also perform well under high blackgrass pressure.

The work at Strow Longa has consistently confirmed that this winter cereal option can out-compete the weed under most growing conditions.

Cultivation

But the big question, from a cultivation point of view, comes down to the following choice: the plough or some form of min-till option.

The results achieved to date indicate that continuous min or zero-till does very little to aid the control of blackgrass.

However, coming in with the plough after five years acts as a re-set button.

The highest yields and greatest margins generated at Stow Longa have been consistently achieved from crops that were established using a plough-based system, following a period of min-till.

The crop margins generated by the Agrii trials also confirm the tremendous impact that heavy infestations of blackgrass can have on final crop financials.

In 2022, which was an excellent year for all cropping systems, the range in overall gross margins came in at £1,821/ha.

A Teagasc perspective

Teagasc – the Agriculture and Food Development Authority – is the national body providing integrated research, advisory and training services to the agriculture and food industry and rural communities.

Teagasc is confirming 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.

This is a crisis situation.

These products should be carefully used in conjunction with hand rogueing to reduce the seed bank number.

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 and ACCase-bwes herbicides 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.

BYDV

Recent developments in the Republic of Ireland have confirmed the changing nature of the threat posed by Barley Yellow Dwarf Virus (BYDV) to cereal crops.

Barley Yellow Dwarf Virus (BYDV) is running rampant through winter and spring cereal crops at the present time.

This development is bad enough in its own right.

However, adding to the concern is the fact that Teagasc research scientists don’t know how the disease-causing aphids are getting on to crops in the first place.

These were two of the key messages delivered at a recent Teagasc Crop Walk, held in North Co Dublin

Teagasc tillage specialist, Shay Phelan, commented: “BYDV is very apparent in spring and winter cereal crops south of Dublin.

“So, the possibility of a similar reality being the case across the rest of the country is very real.”

Phelan admitted that Teagasc scientists had not predicted these developments.

“Suction traps in places like Teagasc Oak Park had not caught significant numbers of flying aphids over recent months,” he explained.

“So this leaves open the possibility of grain aphides physically walking through fields and/or over wintering on individual cereal plants.

Phelan continued: “This type of behaviour on the part of the aphids has not been recognised.

“In reality, we are talking about a very fluid situation, which will be a key focus of new research within Teagasc.”

In the meantime, cereal growers are being strongly advised to spray their spring barley and wheat crops with an insecticide as a matter of priority.

Shay Phelan again: “The best time to put on an insecticide is when crops are the four-leaf stage.

“One application of an active product will suffice.

“Products should be applied at full label rates.”

According to Teagasc, BYDV can have differing levels of crop impact with yield losses of up to 80 per cent a possibility.

However, this would only be in the case of very severe infections. The average loss figure is in the region of 30 per cent.

The risk periods for BYDV infection coincide with milder temperatures and the associated migration/reproduction of aphids.

Normally, earlier sown crops are at the highest risk of infection and it is difficult to avoid insecticide application in these scenarios.

In-crop monitoring is essential before an insecticide is applied to check if aphids are present.

Growers should inspect headlands first, because aphids are three times more likely to land near headlands than further out the field.

For the record, aphids were identified on spring wheat plants by a number of the participants taking part in the North Dublin crop walk.

AHDB

The Agriculture and Horticulture Development Board (AHDB) is a statutory levy board, funded by farmers, growers and others in the supply chain to help the industry succeed in a rapidly changing world. The international team develops export markets in new countries such as China and the Far East, as well as looking after key markets for UK exports such as France and Belgium.

According to the AHDB, BYDV is the most economically important virus in UK cereals.

The scale of yield loss depends on aphid activity, BYDV presence and strain, growth stage at infection and environmental conditions.

Although various species of cereal aphid transmit BYDV, two are particularly important in the UK and Ireland.

In the south, the bird cherry–oat aphid (Rhopalosiphum padi) is the principal vector.

However, in the midlands and the north, the grain aphid (Sitobion avenae) is usually more important.

As aphids do not pass the virus on to their offspring, they must acquire BYDV through feeding on infected host plants.

The time between acquisition and the aphid being able to transmit the infection is 12–48 hours.

In addition to feeding on the foliage, aphids can also feed on crop roots, especially in warm, moist conditions.

Initially, only a small proportion of aphids is likely to carry BYDV. However, because of the way in which virus spreads from plants to aphids, even initial small populations of infected aphids can lead to significant economic damage.