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13 Sep 2022

Italian ryegrass survey shows scale of control challenge

As featured in Arable Farming Magazine

Italian ryegrass survey shows scale of control challenge

by Arable Farming Magazine August 2022 issue

With herbicide resistance in ryegrass detected outside the weeds traditional hotspots, growers are being advised to combine chemical and cultural approaches to control.

A survey to measure the extent of the Italian ryegrass weed control problem on UK arable land suggests growers who have even a slight suspicion that weeds are affected by herbicide resistance should more widely adopt testing, extended chemistry and cultural strategies to raise control levels.

Commissioned by Bayer and conducted by NIAB, the survey is believed to be the largest study conducted of Italian ryegrass populations across the UK.

It identified significant incidences of resistance to some commonly used grass-weed control active ingredients, in a weed which on some farms is becoming an issue to rival black-grass.

However, combinable crop growers already have many of the tools needed to tackle the problem, supplemented by a now-broader herbicide options list.

That was the message from speakers at a recent joint Bayer/ NIAB meeting at which the latter revealed the results of its survey, and the former released details of a new approval for use of its aclonifen active ingredient in winter barley, to add to its listing for use as a herbicide in winter wheat, in both circumstances alongside Liberator (flufenacet + diflufenican).

Ryegrass is becoming ever more evident as a concern for farmers across a wider area than previously thought, said NIAB weed biologist John Cussans.

Unlike some other parts of the world even relatively close to home in Italy and Spain we are fortunate in that we have no known cases of Italian ryegrass resistance to glyphosate here, so it remains a useful clean-up tool ahead of establishment of the next crop.

But the UK is not immune to resistance development.

As an industry we cant be complacent and growers need to be aware of the risks of being too heavily dependent on chemistry for control and not using integrated management with other control measures.

Assessing 197 Italian ryegrass samples from across the UK, the survey tested each for sensitivity to Liberator , Axial (pinoxaden) and Atlantis (mesosulfuron + iodosulfuron), with a smaller sub-set of 22 tested for cross-resistance.

Correlation The latter showed strong correlation between pre-emergence products flufenacet, prosulfocarb and pendimethalin, but little between aclonifen and other established pre-emergence products.

While surveying to estimate the true frequency of herbicide resistance is almost impossible, we were surprised to receive as many samples as we did, said Mr Cussans.

We know there are areas with particular Italian ryegrass issues, including the South East and west Yorkshire, but we assessed samples from all over the UK, and while there were massive variations in herbicide sensitivity, there are clearly other regions with problems.

Pot tests showed no control from flufenacet on 9.6% of ryegrass plants tested.

Thats much more significant than the sensitivity shift to flufenacet on black-grass.

Control was significantly reduced in a further 17.3% of cases.

This shows clearly the need to rely on more than chemistry for control, and within chemistry to use a broader approach to active ingredient groups.

While a national study can provide a broad picture of the situation, farmers need to know the resistance status of their own individual fields, and the efficacy of pre- and post-emergence chemistry on populations, said Mr Cussans.

Status You cannot respond to general resistance issues if you dont know your own fields status and the ryegrass populations susceptibility to each active group.

When comparing populations collected from two fields on the same farm, often well over half are significantly different in herbicide sensitivity.

This is important to understand when planning resistance testing/monitoring its a very different science to the more national and global issues of fungicide and insecticide resistance.

Italian ryegrass has a strong ability to survive and is happy on poor soils and in drought conditions.

But it would seem some farms are not taking into account the need for control beyond chemical and are using chemistry such as pinoxaden in April-June as a fire brigade treatment.

There is a need to ensure farmers are not dropping perfectly effective herbicides without identifying the cause of a weed control issue.

Control difficulties can occur for many reasons, and resistance may be only one in a particular field population situation.

But the vast majority of people are using only three actives to control their grass-weeds, and there is enormous scope to bring in others, beginning with pre-emergence treatments.

We are actually in quite a good place for availability of effective chemistry, with a new product approval for barley from Bayer and more on the horizon from other manufacturers.

We have the chemistry needed to keep grass-weeds in general under control, when used alongside cultural control measures.

Whats important is to adopt products with four or five modes of action over the course of a programme to minimise resistance development risk, just as the industry has grown used to with fungicides.

Care needed to protect glyphosate resistance development

Shifts in the sensitivity of Italian ryegrass to glyphosate have been recorded in some difficult-to-control UK populations of the weed, underlining the need for good glyphosate stewardship to prevent the resistance development seen in some other parts of the world, suggests John Cussans.

Glyphosate resistance is not a UK issue, but we have been growing closer to it, he warns.

We have been able to select for glyphosate resistance under high-pressure experiments in glasshouse trials.

That underlines the importance of ensuring any survivors of glyphosate applications are not allowed to set seed.

Rates Roger Bradbury, technical specialist at Roundup manufacturer Bayer, says the correct rates and timings and application technique are central to good glyphosate stewardship.

Weed Resistance Action Group guidelines say post-harvest glyphosate applications ahead of the next crops establishment should be limited to two at most, with their efficacy monitored closely and any poor control investigated.

Such stewardship is essential if we are to protect the usefulness of glyphosate and prevent the development of resistance to both selective and non-selective herbicides.

Aclonifen now approved for winter barley

Bayers grass-weed active aclonifen, well-established as a pre-emergence weed control tool in winter wheat as Proclus and supplied as a co-pack with Liberator (flufenacet + diflufenican), is now approved for use in winter barley, broadening growers grass-weed control options in a crop where they have been more limited than for wheat.

The addition of Proclus to the tank would add £10/hectare to the spray programme, calculates the firm.

While the label rate for Proclus use in wheat is 1.4 litres/ha, Bayer has lowered this to 1.0 litres/ha for barley, reflecting the crops sensitivity, although the accompanying Liberator rate remains at 0.6 litres/ha.

Bayers Tom Chillcott says trials work has shown no crop impacts from well-made pre-em applications.

While flufenacet is still very effective in most situations, our research has indicated incidences of flufenacet-resistant ryegrass, and to a lesser degree black-grass.

That underlines the needs to look after it if we are to maintain its usefulness to the industry long-term.

Higher loads are not the answer to grass-weed control issues, as this will simply further raise the risk of resistance development and degrade the actives efficacy.

This is why we have been working to broaden the available modes of action in grass-weed control for barley.

Extend The presence of diflufenican in Liberator already helps protect the efficacy of the flufenacet, and we have been able for some time to extend that further with the addition of aclonifen via Proclus as a partner product for grass-weed control in wheat.

Now the same is available for those growing barley.

When used in conjunction with cultural controls and weed patch management, combinable crop growers have a further in-crop option for grass-weed control in crops other than wheat.

Weve seen uplifts of 5-7% in black-grass and ryegrass control in barley from the use of Proclus alongside Liberator, and the combination of the two products also provides a wider broadleaved weed control spectrum.

As with the combinations performance on wheat, the aclonifen application sits on the soil surface waiting to act as weeds grow through it, while the flufenacet element of the tank mix works on weed roots.

As a result, users benefit from both improved grass-weed control and a wider broadleaved weed control spectrum.

As with the conditions for success in pre-em weed control for wheat crops, seedbed preparation and drilling quality are important, adds Mr Chillcott.

Drilling into an even, consolidated seedbed at a 32mm depth is necessary to ensure emerging barley does not come into contact with aclonifens activity zone.

The sprayer must follow within 48 hours, and weather forecasts checked to ensure no heavy rain is likely in the 48 hours after application.

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