Both short- and long-term wins are on the horizon for growers with grassweed problems, providing the fundamentals of resistance management are adhered to. CPM shares the news.
“Herbicide effectiveness improves the later crops are drilled.” JOHN CUSSANS
By Janine Adamson
After the past two seasons of immense challenge, any agronomic win, no matter how small, should be worthy of celebration.
And while some might struggle to get excited over a simple change in rate or dose split, with drilling continuing to err on the earlier side of proceedings for some, news from Bayer could help to mitigate risk when it comes to grassweed management.
Albeit it is a simple update for aclonifen (as in Proclus and Bandur) – whereas previously its total dose of 1.4 l/ha had to be applied in one hit in winter wheat, it can now by split – 1 l/ha at pre-emergence timing followed by 0.4 l/ha at early post-emergence. There’s also been an update for winter barley, with growers able to utilise a full 1.4 l/ha total dose compared with the previous 1 l/ha.
EXTENDED TIMINGS
Bayer Cropscience’s Darren Adkins says it’s a minor change but incurs advantages. “Splitting the dose means you can extend the timing of autumn-applied herbicides to protect against later germinating weeds, which is useful in early drilling situations,” he explains.
According to Darren, this is even more useful because of aclonifen’s longevity and mode of action. “As an active ingredient, it’s less reliant on soil moisture and sits on soil surface to be taken up by emerging weed shoots only. This is certainly an advantage if drilling has taken place in September and it’s dry.
“With the split dose, it gives the flexibility to factor in different modes of action into early post-em sprays, and create greater diversity in the partner programme.”
So from Darren’s perspective, what could an autumn weed control programme look like with this in mind? He says the fundamentals of herbicide planning won’t really change. “That’s thinking about what grassweeds or broadleaf weeds are being expecting based on the farm’s history, and the resistance status of those weeds.
“Then it’s considering the timing of drilling and understanding how the different modes of action behave in the soil and their persistency. Those are the variables that really inform programme choice,” he comments.
When drilling earlier, it’s likely two if not three autumn applied sprays will be required, whereas planting later will increase the chance of having to apply all products in one go, notes Darren.
LAYER OF CONTROL
Thinking again about aclonifen, because the product doesn’t enter the soil profile, it forms a layer on the soil surface, he says. “This is useful because if conditions are dry but with enough moisture for weed seed to germinate, then there’ll be activity from aclonifen before more soluble actives get down to the roots.
“It’s pretty much insoluble in water – it doesn’t go into soil and sticks to surface incredibly well. Once bound, it won’t move.”
A bonus is that it isn’t broken down by UV activity either, adds Darren. “A lot of chemistry will break down in sunlight, but aclonifen has been used widely in tropical regions purely because it’s so good at remaining stable in dry conditions. It also has a useful half-life of around 80 days for long-lasting activity.”
Considering all of the benefits of aclonifen, he doesn’t shy away from the need to protect it. “If we look at the cereal herbicide modes of action available to play with, we have a good range. In fact, there are 10 if you include glyphosate and the post-emergence options.
“We don’t struggle, but products should be chosen wisely to ensure diversity in programmes and longevity of the chemistry,” he urges.
As for this year specifically, in high pressure situations, Darren raises that Bayer is supporting the use of cinmethylin plus aclonifen at 1.0 l/ha.
With so much talk about drilling date, ADAS’ John Cussans agrees that it doesn’t just have an impact on the success of stale seedbeds and weed seed germination. “In the past, we perhaps hadn’t appreciated what an effect drilling date has on herbicide performance.
“Bigger picture data suggests that later drilled crops have a lower requirement for herbicides and follow-up contact sprays. Simply, herbicide effectiveness improves the later crops are drilled,” he raises.
SEED-BANK CONTROL
He also highlights that there’s much more to weed management than ‘getting away with it’ for a year or two. “You must proactively manage the weed seed-bank in the background all of the time.
“Okay, in some situations in some years, drilling later doesn’t give you the results you want due to less competitive plants undermining what you’re trying to achieve. And then, cultural strategies stop working well if weed numbers are really high. But in the long-term, usually, delayed drilling will yield results.”
For Agrovista’s Mark Hemmant, it’s been another difficult year to hold off. “With the spring droughts we’ve been experiencing, we known early drilled crops are more resilient to arid conditions. Consequently, this will mean advising the use of more herbicides, but if cultural controls really fail, it’ll be applying glyphosate and starting again.”
John highlights the reasons behind why stale seedbeds may not be effective. “The true benefit of a stale seedbed with shallow cultivation isn’t seen for 3-4 weeks after that initial cultivation; you have to allow time and for glyphosate to work, and two weeks isn’t enough. Time is our most valuable currency.
“Equally, what’s required is different between dry and wet conditions. In wet conditions you want to get the blackgrass seedlings to chit and reduce return in the following crop. But in dry situations it’s far better to leave the freshly shed seeds on the surface so they’re vulnerable to predation.”
Mark raises that there may be a misconception of what a successful stale seedbed actually means. “Growers achieve a good weed flush and could assume there’s the potential to then reduce spend on a herbicide programme, particularly when evidence shows there’s no benefit beyond two stale seedbeds – a third or fourth doesn’t increase control.”
In agreement, John says it’s easy to over-estimate what can be achieved with a stale seedbed. “Remember, the flush is only 5% of the seed-bank that’s waiting dormant below the soil surface.”
Returning to the conundrum of how to devise an effective herbicide programme, John proposes that the order of application is variable depending on conditions. “If you’re sequencing it’s best to do that with a 10-14 day gap, apply the best herbicide actives first followed by the less effective ones. Always prioritise controlling the early germinating weeds as these are the ones that produce the most seeds, certainly don’t let those individuals come up in the crop.
“Then in drier conditions or where there’s a possible crop safety risk, always apply the safer material first (aclonifen, bixlozone, tri-allate etc) and the less safe options later. However, whatever you do, do not skip the pre-emergence stage.”
Hutchinsons’ Dick Neal agrees that a sequence of actives rather than a single, large pre-em stack should offer the best blackgrass control. “The aim of the sequence is to overlap herbicide active dose with blackgrass emergence.
“We have to make sure we have herbicide active in the soil from 1 October through to the middle of November,” he urges.
With blackgrass emergence increasingly protracted, he suggests the focus should also shift towards building a competitive crop from the outset. “This could include placing phosphate where roots can access it, using variety blends to strengthen crop competition and sequencing herbicides to match the weed’s emergence pattern, with the aim of leaving as little opportunity as possible for survivors to replenish the seed-bank.”
Casting much further ahead, Bayer’s Richard Phillips says there’s another new herbicide mode of action due on the horizon. “Icafolin-methyl will arrive next – a new mode of action from HRAC Group 23 for post-emergence control of grassweeds and broadleaf weeds, including blackgrass and oats.
“How it works is treated weeds stop competing but keep their structure, forming a mulch layer. It’s EU launch is due at end the of the decade for potatoes, pulses and veg, and we’re currently looking at how authorisation can be achieved for the UK following EU alignment next year. For use in cereal crops, we’re aiming for authorisation in the early 2030s,” he comments.
This article was taken from the latest issue of CPM. Read the article in full here.
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