Italian Ryegrass Knowledge Hub

Bayer put the call-out to farmers and agronomists in summer 2021 and received nearly 200 samples making it probably the largest weed survey ever in the UK. Resistance testing and analysis took place during winter 2021-22, with results published in May 2022.

“Altogether, we received 197 samples, which was somewhat more than we had expected, processing the results and resistance testing took longer than we had planned,” explained Mr Cussans.

“Samples came from all over the country, but you can see from the map that there are distinct regional clusters, some were already well known such as Kent/Essex and South Yorkshire, but we can also see distinct areas in Dorset, The West Midlands and North Yorkshire / Durham.

“I think this reflects Italian ryegrass distribution and is not a quirk of how the survey was carried out. Many of the samples came individually from one farmer, not in batches from a company or agronomist.”

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A danger on any farm

“Looking at the responses to the survey, it appears to be a fairly ‘normal’ cross-section of arable farmers. This shows that ryegrass could feasibly become a problem in many more places. Most respondents are reducing cultivation intensity, growing more spring crops and have had reduced oilseed rape area which are common trends at the moment.”

Reduced cultivations and ploughing

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More spring crops

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Unsurprisingly for farmers taking part in a survey of this kind, farmers are worried about resistance and over 90% think the ryegrass problem has got a bit or a lot worse in recent years.

Interestingly, according to the survey responses, Italian ryegrass problems aren’t linked with use of ryegrass cultivars in pastures and leys. This is sometimes thought to be a prime cause of the increased ryegrass problems many farms are seeing, but it doesn’t seem to be the case.

Herbicide usage

Pre-em chemistry is an important part of weed control on many farms but, interestingly, in the survey year 18% of samples were from fields that didn’t get a pre-em. It is not clear to what extent this was planned or weather disrupting autumn applications.

The most common active used in autumn is flufenacet with 80% of all sample field treated with flufenacet as part of a mix or sequence. Given that 18% of fields weren’t treated at all, we can say that nearly every pre-em herbicide programme in the survey contained flufenacet. A testament to its value for pre-em weed control, but also an alarm bell in regard to resistance management.

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Researchers tested 197 samples for resistance to flufenacet. A subset of samples then had cross-resistance testing to other key pre-em actives, as well as an assessment of how a farm standard pre-em programme would perform against the hardest to control populations.

Good news (and some bad news)

73% of samples tested were still susceptible to flufenacet. But 17% showed reduced sensitivity and 10% were not controlled by field rates of flufenacet as a single active.

“Surveys of this kind are always likely to overstate the problem somewhat,” says John Cussans of NIAB. “The farmers most motivated to take part are those with problems, and they will send in samples from where they are having difficulty. So, the levels of resistance we see in the survey are most likely a bit higher than the true situation. But the potential is there so we need to prevent it.”

Looking across the whole survey, there is variable herbicide sensitivity, around the country and even between fields on the same farm. This means that farmers need to be prepared to resistance test and plan control programmes on a field-by-field basis.

Mix modes of action

22 samples were selected for cross-resistance testing to other pre-em actives. It showed a correlation between flufenacet, prosulfocarb and pendimethalin resistance. But we need to be careful with how we interpret this according to Mr Cussans. “If there is resistance to one active, it doesn’t automatically follow that you have resistance to the others.”

“Patterns of cross-resistance are complex, and it is important to test your own populations. This applies to the post-em products as well, where there are still a fair number of susceptible populations. Overall, while resistance is obviously an issue, it is still the case that the majority of populations are sensitive to effective herbicides.”

Importantly for many farmers, there is no resistance to glyphosate in ryegrass in the UK, unlike Australia for example. So, they still have one of the most effective out of crop controls available.

How to deal with resistance risks in the field?

Firstly, use the full range of cultural controls to manage populations before the herbicide programme. Then the aim should be to employ different modes of action within and between seasons to prevent strong selection pressure on one or two actives.

Researchers tested 7 of the most resistant ryegrass samples to see how combing modes of action stabilises control in the field. “Combining three actives, in this case Liberator + Proclus largely but not completely ‘overcomes’ flufenacet resistance.”

Chart: Control of difficult populations with Liberator + Proclus

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Many ryegrass populations still susceptible to post-ems

An important finding from the survey is the number of populations still susceptible to post-emergence chemistry. 45.6% of populations were sensitive to ALS-chemistry and, coincidentally, 45.6% susceptible to Axial (pinoxaden).

Some samples were cross sensitive to both actives, but others were sensitive to one and resistant to the other. The overall picture is as follows:

“If you surveyed people and asked how many think ALS doesn’t work on Italian ryegrass, I expect it would be a much higher number than what we found in the survey,” says John Cussans. “It’s not because those people aren’t having problems controlling Italian ryegrass, but the source of the problem is perhaps not what they imagine it is.”

Patterns of resistance 

Previous work published in 2019 looked at 50 ‘difficult’ Italian ryegrass populations. It showed a higher level of resistance to Atlantis (mesosulfuron + iodosulfuron and pinoxaden compared with results from the more random 2021 survey. Mr Cussans is keen to stress that even the results from 2021 are probably an overestimate of the actual level of resistance seen across all arable fields. “Surveys of this kind are likely to attract farmers with more problematic ryegrass.”

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“Different regions also showed variation in sensitivity to both types of post-em chemistry, but we don’t know the reasons for this variation.”

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Pinoxaden tends to be applied late

Data from the survey shows that farmers using pinoxaden tend to apply it later than ideal in April, May and even June. This implies it is used as a ‘fire brigade’ treatment rather than as a planned final step in the programme. ALS-based products such as Pacifica Plus (mesosulfuron + iodosulfuron + amidosulfuron) tended to be applied earlier in spring, when control is usually better.

For all post-ems, timing, conditions and application technique have a huge impact on efficacy. If you have a sensitive population, make sure that herbicides are getting the best possible control to help prevent resistance developing. Resistance testing will identify the best chemical control options in a particular field.

Resistance test to confirm status

Weed control is too important to base it on guesswork. With such variable patterns of resistance in Italian ryegrass, a resistance test can help identify the right chemistry for the job. Once you know which actives are still effective, plan programmes that rotate different modes of action and integrate several cultural controls.

Italian ryegrass can develop resistance to a number of common herbicides. A resistance test is a valuable tool for farmers managing the situation on their farm because each farm is unique.

Farmers have a huge influence on the resistance situation on their farm. Machine hygiene aside, resistance is a result of management history, unlike insecticide and fungicide resistance which can quickly spread between farms. Consequently, national surveys and discussions with other farmers can only take you so far. Testing problem weeds is vital because you can even find different resistance profiles between fields on the same farm.

“If you hear somewhere about a resistance trait, it doesn’t mean you have that resistance trait on your farm,” says John Cussans. “There is no substitute for understanding the population you are trying to control in a field. There is no substitute for resistance testing.”

Various types of resistance

Resistance develops to each active separately. While there are some patterns of cross-resistance, they are not entirely predictable, so a specific resistance test is needed. For example, the recent Italian ryegrass survey found all conceivable combinations of resistance to the key post-em and pre-em actives.

Black-grass is slightly different, most farmers have faced resistance in the same order of ACCase followed by ALS and then possibly reduced sensitivity to pre-ems.  As far as we know, there is no fundamental reason for this order except for how and when these actives were used. So, it could be the case that as reliance on pre-em chemistry increases with fewer post-ems, we could see different resistance patterns.

Moreover, there is also the difference between target site resistance and enhanced metabolism resistance to ALS-inhibitors in black-grass. In practical terms, target site resistance is an on/off switch, once it’s there, farmers can no longer get useful control. Metabolic resistance is different, applications at early growth stages in conducive conditions can ‘overcome’ metabolic resistance and deliver helpful weed control.

Long-term planning

In many ways, the most useful aspect of a resistance test is telling you which products are still effective. Once there is RRR resistance, the decision is straightforward to change products. But where plants are still susceptible or showing some signs of resistance, there is still time for farmers to act.

Treasure any active which is effective by following the familiar stewardship guidelines. Apply at the right growth stage, in good conditions, with good application technique. Don’t rely on any one active too much in the programme and look at how diversifying cultural controls and rotation can take pressure off a limited number of herbicide options.

Collect a good sample

Watch the videos below for guidance on how to collect a seed sample for ryegrass and black-grass resistance tests:

Black-grass 

Italian ryegrass 

Remember resistance tests are from survivors

 When you collect seed in July, it is by definition from weeds which have survived the herbicide programme, susceptible plants have already been controlled. If viable, look to manage these survivors before they shed seed. Patch-spraying with Roundup (glyphosate), hand rogueing and seed destruction can all help prevent very high seed return. Meaning the population size stays manageable with all remaining cultural and chemical control.

Field experience shows that effective Italian ryegrass control requires several cultural controls and chemical controls stacked together. “Cultural control methods such as delayed drilling and spring cropping help,” says John Cussans. “But they don’t make quite as much of a difference compared to black-grass. More cultural and chemical methods need to be stacked together to ensure durable control.”

Ryegrass is often said to ‘keep on germinating’, but research has not fully backed this up. AHDB Research published in 2010 found that 80% of ryegrass seeds germinated in October, 7% in November, 6 % in December and very little in spring. This would suggest that delayed drilling and spring crops should be effective.

The same study also notes ryegrass’ ability to produce tillers thus seed heads even at small weed densities, this perhaps goes some way to explaining why ryegrass can appear so problematic despite crops being drilled after the peak germination window.

“Much like the situation with post-ems, there is some aspect of ryegrass’ life cycle that we haven’t got to grips with,” says Bayer’s Tom Chillcott. “Resistance tests and emergence studies tells us that post-ems or spring crops should be effective, but it is not always the case, and it is something we need to understand as an industry if we are going to rein in ryegrass.”

Diversity in control

Experience globally shows that ryegrass is highly adaptable to chemical and cultural control methods, hence diversity is essential in all control methods. In chemical programmes, this means using different actives, ideally from different HRAC mode of action groups within and between seasons.

In cereals, several key pre-emergence herbicides belong to HRAC Group 15 (Inhibition of Very Long-Chain Fatty Acid Synthesis) such as flufenacet, prosulfocarb and tri-allate. Using actives from other groups such as aclonifen in Proclus (Group 32) or metribuzin in Octavian Met and Alternator Met (Group 5) will improve the resilience of herbicide programmes. As will post-em chemistry, which still has good efficacy against many ryegrass populations.

Diverse cropping is important as it allows more modes of action to be used across the rotation, helping to prevent the development of resistance. There is also a direct effect as different methods and times of establishment are used for different crops. Patch spraying with Roundup (glyphosate), hand roguing and weed destruction can all be used to good effect against ryegrass too.