Target-site resistance to the ALS herbicides as seen in blackgrass has been confirmed in ryegrass for the first time in the UK, raising fears about a new grassweed threat.
This latest discovery could elevate ryegrass to the same status as blackgrass in the future unless growers in affected areas take early action to prevent its spread, warns Stephen Moss of Rothamsted Research, who identified the resistance mechanism in ryegrass seed collected in 2012.
“We already knew there was resistance to the ACCase herbicides in ryegrass, but we hadn’t found any evidence of ALS target-site resistance until now,” he says.
His finding has been confirmed by both Syngenta and Bayer CropScience in samples originally collected in 2012, as well as by researchers in France.
“This type of resistance is very common in Australia and other countries further afield, but Europe hasn’t reported the same difficulties,” says Dr Moss.
However, in France it was found at a low frequency in a small number of plants back in 2007, but remains an uncommon resistance mechanism there.
Back in the UK, Dr Moss liaised closely with Syngenta, who carried out further tests on his samples and identified ALS target-site resistance in two out of the 14 leaves sent to the company’s laboratory.
“All of the leaves were from plants that showed resistance to Atlantis, so the implication is that other mechanisms, such as enhanced metabolism, are more common. This supports previous work which indicates that target-site resistance, both to ACCase and ALS inhibitors, is currently less common in ryegrass than it is in blackgrass.”
Likewise, Bayer tested six resistant ryegrass populations from 2012, finding two of them had ALS target-site resistance.
“All of these tests confirm we do have this resistance mechanism in ryegrass. Fortunately, it is at a much lower frequency than is seen in blackgrass and the situation is still some years behind that of blackgrass. But this could change.”
The practical implications of the new mechanism are that products such as Atlantis, Unite and Broadway Star will no longer work as effectively where it has been detected in ryegrass, leaving growers with limited chemical options, he explains.
“In our work, we used Atlantis on the ryegrass sample with resistance and achieved only 52% control.”
His advice to growers who are having difficulties getting good control of ryegrass is to have samples tested for resistance. “There’s a danger that growers have assumed they have enhanced metabolism resistance, which is much more common.”
Where the weed is present at very low densities, there’s the option to hand-rogue or patch-spray it, he suggests.
Other measures include reducing reliance on high-risk herbicides and making better use of the pre-emergence materials. “Growers will still have the same conundrum as they do with blackgrass regarding Atlantis and Broadway Star use, as these will still give some control if only a small proportion of the population is ALS resistant.
“Axial can give good control, but it is affected by ACCase target-site resistance, which also occurs, although less commonly than in blackgrass.”
Cultural control methods also apply, says Dr Moss. “As with blackgrass, the use of techniques such as ploughing, stale seed-beds and delayed drilling can help, since the majority of the seed germinates in the autumn.”
|From the field|
Independent agronomist Sara Harrison-Osborne of Prime Agriculture has been dealing with an escalating ryegrass problem on her clients’ farms in Essex.
Until quite recently, an autumn application of Atlantis gave good control of the weed. But a decline in efficacy has been very obvious in the past few years, especially in certain fields, with resistance being suspected, she reports.
“Ryegrass is a difficult weed because it can germinate every time the soil surface is disturbed,” she says. “As a result, although switching to spring cropping does help, it’s not as effective as it is with blackgrass.”
The other consideration with a move to spring cropping is that fops and dims have to be relied on for control, she points out. “So you need to have the seed tested for resistance to this type of chemistry before you go down this route.”
In winter crops, she has tried all the different pre-emergence herbicides, alone and in stacks, to see which approach works best.
“The most successful is Crystal, providing it is applied pre-emergence. The pendimethalin seems to be a better pre-emergence partner to the flufenacet than diflufenican is, even if there’s confirmed resistance to pendimethalin in the ryegrass population.”
Adding Defy at this timing has also given good results when conditions are favourable and there’s moisture in the seed-bed, she says. “If the conditions aren’t quite what we hoped for, we use Defy at the peri-emergence stage, or in a mixture with clodinafop, as in Auxiliary,” she says.
That gives 70-80% control and is then followed up with a post-emergence spray of Atlantis while the weeds are still small. “But the pre-emergence treatment is essential. If that doesn’t get applied, the crop can be overwhelmed.”
Other measures used in the battle against ryegrass include rotational ploughing and growing first wheats only. “A two-year break from wheat is even more effective, if it can be included.”
Stale seed-beds and delayed drilling haven’t worked quite as well, she reveals. “The weed’s prolonged germination makes it difficult to get the desired results.”
She refers to ryegrass as the new blackgrass. “It often starts off as patches around gateways and field edges, but it can very quickly become a problem. One plant can produce up to 30 or 40 tillers.”