When do you think about applying weed resistance management strategies on-farm? For too long the answer, at least in practical terms, was when resistance had already gained a foothold in fields on the farm.
That has changed on UK farms with messages around cultural weed controls hitting home, but increasing challenges with resistant Italian ryegrass, as well as the omnipresent black-grass threat means there is no time to be complacent.
Flufenacet resistance in Italian ryegrass is the latest challenge.
Of 50 Italian ryegrass samples taken in 2019, 14% had significantly reduced flufenacet efficacy, says John Cussans, weed biology specialist for NIAB.
“You are talking rates of up to 960g/hectare of flufenacet not having an impact. It is a problem in one or two populations, coinciding with the pattern of use of flufenacet getting worse, with overuse partially driven by the straight becoming available.”
Fortunately, in black-grass it is not resistance but reducing sensitivity to flufenacet which is the issue.
“Resistance development is happening in the field in different ways with different species. With black-grass it is a shift in sensitivity, with Italian ryegrass you are starting to see populations where there is no utility in the field at all from realistic field rates of flufenacet.”
As a result of that survey a more extensive survey of 200 Italian ryegrass samples from 2021 is currently underway.
Each of the 50 samples from 2019 was also tested against the contact herbicides, Axial Pro (pinoxaden) and Atlantis (mesosulfuron + iodosulfuron), which highlighted there were nearly as many populations that were resistant to flufenacet alone (4%) as there were populations resistant to both flufenacet and ALS-inhibitors such as Atlantis (10%).
In total, 76% of populations were resistant to Atlantis, with 66% to Axial Pro, which leaves a third to a quarter of populations where those contact herbicides would be still contributing to control.
That could mean some growers might have been too quick to move away from contact herbicides, Mr Cussans suggests, with resistance testing playing a crucial role in understanding individual populations across a range of herbicides.
“In practice, there are farmers who have stopped using a group of herbicides because they mistakenly believe they are no longer useful and by just using a lot of pre-ems now have flufenacet resistance but no ALS resistance.”
Generally once resistance is in a population it does not leave, says Harry Strek, scientific director for Bayer’s Weed Resistance Competence Center in Frankfurt, although what order it arises is driven entirely by farm practice.
That was confirmed by a large-scale study in south west Germany, where fop/dim target site resistance was still present in black-grass populations 15-20 years after the last application to a particular field.
“The results showed that each field had a different resistance status, sometimes significantly different, depending upon the history for herbicides and agronomic practices for that field. Fields with a large proportion of resistant blackgrass could be located directly next to fields with low numbers or none with minimal border buffer areas,” Dr Strek says.
The study, which sampled 90 fields across the landscape, found that agronomic practices influenced resistance development as much and sometimes more than chemical practices, he adds.
Agronomic factors which had a higher impact than the chemical factors include the number of different crops in the crop rotation, delayed seeding and tillage regimes.
“Of course you have to take agronomic and herbicides together. Farmers who understand that are doing much better in their overall weed control.”
Economically, employing good weed resistance management stacks up, he says.
At a Bayer Integrated Weed Management platform site in France where a wheat, oilseed rape, wheat rotation was developing resistance to Atlantis a standard system of earlier drilling and fewer herbicidal modes of action, sequences and mixtures employed was compared with more integrated weed management (IWM) friendly practices.
These included periodic tillage, using stale seedbeds with delayed drilling using a low disturbance drill and more varied herbicides supplemented with mechanical weed control.
“At the start of the programme there was a medium to high infestation of Italian ryegrass. By the end under the standard system, it grew to a high population which was also highly resistant to ALS chemistry.
“Under the IWM programme, the infestation decreased and ALS resistance remained low.”
While the IWM programme cost €83/ha (£70/ha) more to implement than the standard system, over the four years of the project it almost doubled the return, Dr Strek says.
“Put another way the farmer would have lost €504/ha/year [£425/ha/year] by not adopting integrated weed control measures and will benefit in the future from driving down weed populations and keeping resistance risk low.”
HERBICIDE RESISTANCE WAS INEVITABLE
IT was inevitable weeds would become resistant to herbicides. Long before herbicides existed weeds were adapting to detoxify naturally-produced substances encountered in typically a hostile environment for success.
Research has shown black-grass with non-target site resistance confers higher fitness in soils contaminated with oil or heavy metals.
That means weeds were likely pre-adapted to be able to detoxify herbicides, Mr Cussans suggests.
“The very nature of the environment for an arable weed being disturbed and difficult habitat to exist in gives rise to a high background prevalence of traits that give an ability to detoxify herbicides.
“That also means it happens much quicker – it is odd the rate at which herbicides select for herbicide resistance. It is why we need to talk about resistance management before we get resistance rather than afterwards.”
For the past 10 years, generally UK growers have been better at recognising that weed management is the best resistance management and resistance management is best by focusing on good weed management.
“2012 was our annus horribilis with poor conditions around drilling and poor control from contact herbicides. Since then, growers are looking at rotations, prioritising fields for delayed drilling or growing spring crops.
“The single biggest thing that has changed to transform grassweed management is that growers will look at autumn conditions for a winter cereal and if they
think drilling the crop won’t work, they do not do it,” Mr Cussans adds.
PINPOINTING ROLE OF MULTIPLE MODES OF ACTION
TRIALS set up by Bayer and NIAB are aiming to demonstrate the impact of mode of action diversity rather than herbicide loading on Italian ryegrass and black-grass control.
That is slightly more complicated than it sounds as usually when adding a mode of action you also add to the herbicide loading, Mr Cussans says.
“These trials adjust for that so there is a constant herbicide load and you either deliver it with one, three or five different modes of action.”
While Mr Cussans stresses he would not reduce herbicide rates in the field, in a trial it provides a way of testing the hypothesis that using more modes of action will increase control.
“Last year, we were able to demonstrate in the short term you can improve herbicide efficacy in Italian ryegrass with a mode of action diversity strategy and we are doing more work this season to reinforce that message.”