With oilseed rape back in the spotlight, strategies for weed control remain hotly debated – with cultural methods and new chemistry both in the mix. CPM finds out more.
“A clean crop isn’t just a nice to have – it’s the foundation for both yield and quality.” TOM MONK
By Charlotte Cunningham
Following years of uncertainty for oilseed rape, this season’s harvest has delivered a shot of confidence for some UK growers – with yields tracking ahead, pest pressure easing, and an earlier-than-usual combining window.
But for Hampshire grower Tom Monk, the standout 5.8t/ha from one field of Attica isn’t just down to good luck. It’s the product of a long-term, methodical approach to weed control that starts well before drilling and continues right through to harvest.
Farming 1350ha at AF Monk (Rookley) Farm, Tom and his family specialise in seed production, making seedbed hygiene non-negotiable. “We’re growing crops for seed, so the tolerances are incredibly tight,” he explains. “That mindset carries over into our commercial OSR; a clean crop isn’t just a nice to have – it’s the foundation for both yield and quality.”
Tom’s programme begins the moment the previous crop comes off. “We stubble rake straight away to encourage volunteers – often winter barley – to chit, then we go in with glyphosate. That takes out a huge amount of competition before we even think about drilling,” he says.
Looking in more detail at the 2025 crop, a disc cultivator with a seeder unit was used to establish buckwheat as a companion crop, in some cases incorporating sewage sludge for early nutrition, and the ground was rolled to retain moisture ahead of OSR drilling a few days later.
“The discs leave some stubble at various angles which helps with soil protection and moisture retention,” says Tom. “The buckwheat’s there to provide canopy cover against potential cabbage stem flea beetle, but it also helps to smother any weed flush.”
Turning focus to chemistry, Tom doesn’t believe in a blanket pre-emergence spray. “We wait until we know the OSR is up and even, before we commit to spending. In a clean, thick crop, chemistry might be minimal; in a gappy field, we’ll step in.”
This season’s control strategy included two graminicides to control grassweeds and volunteers. Meanwhile, Korvetto (halauxifen-methyl+ clopyralid) has been a go-to for broadleaf weed control in the spring he explains.
“In one field, after the two graminicides, charlock was the only thing popping up – that tells me we’d done the groundwork right,” he adds. “The less you have to tidy up in-crop, the better the canopy you’ll have by spring.”
Long rotations are a central component in the plan, too, he highlights. “We aim for at least eight years between OSR crops in a field. That’s not just to help control pests like cabbage stem flea beetle, but to let the weed seed-bank decline naturally. Fields with a known charlock problem are simply taken out of the OSR plan entirely.”
One of Tom’s most consistent decisions is to avoid a blanket pre-harvest glyphosate. “If the headlands are messy, we’ll treat them, but we leave the main crop alone. I’ve seen around 0.5t/ha uplift in some years by not using it,” he says. “Instead, a pod sealant is applied to lock in seed and reduce shedding losses.”
Although this strategy has proven successful for Tom, the debate regarding whether or not to invest in weed control until a crop is ‘safe’ is one that divides opinion – with some arguing this sets the crop up to fail before it’s had a chance.
“Ten years ago, two- or three-way pre-emergence herbicides were used regularly, but now, growers are reluctant to invest until they know the crop will survive – whether against cabbage stem flea beetle, pigeons or slugs,” says Rob Adamson, head of technical development at ProCam.
“Additionally, some residuals can reduce the vigour of the OSR itself, so there are many factors that mean usage has dwindled. Where it is still done, in many cases pre-emergence applications are pared right back to metazachlor – sometimes with clomazone,” he suggests.
Rob says the challenge with a ‘wait and see’ approach is that the weed burden often builds unchecked. “The weed burden hasn’t changed compared with what we were targeting 10 years ago. There are some weeds such as hedge mustard which if not targeted pre-emergence, will be very difficult to control. Other broadleaf weeds such as chickweed have a rapid growth rate, and if left until you’re applying Astrokerb (propyzamide+ aminopyralid) in November or December, these weeds can be far too big to get effective control.”
As such, he believes residual chemistry still has a place – even if growers fear early losses. “Metazachlor on its own is still worth applying – it has a level of flexibility with a pre or post-emergence label (up to 4 leaf), so you can still wait for the crop to emerge, while also taking some of the pressure away from post-ems to rescue the situation – which is much harder when weeds get away.”
Broadleaf species in particular not only compete for nutrition, but also physically hold the OSR back, he continues. “I’ve seen trials where untreated plots didn’t just have more weeds, the OSR itself was smaller and less vigorous. In other words, by feeding weeds you’re starving the crop.”
This vigour is crucial in the battle against other establishment challenges, stresses Rob. “The sooner you can take weeds out, the greater the biomass and leaf cover. That vigour makes the crop more resilient to pigeon grazing and flea beetle; it’s all interlinked. Leaving untreated weeds is a self-fulfilling prophecy – you weaken the crop and then wonder why it doesn’t survive.”
For those who do want to hold off until the crop is up and avoid the potential knocks of residual actives, Corteva’s Belkar (halauxifen-methyl+ picloram) has provided a flexible option, says Rob. “Belkar is a useful post-em contact herbicide with rates from 0.25 l/ha on 2 leaf crops up to 0.5 l/ha once OSR is beyond the six-leaf stage,” he explains. “It gives broadleaf control, but again, requires growers to act before weeds are too big. You can’t just wait until November and expect Belkar to clean everything up.”
This season, a new development enters the toolbox in the form of LaDiva (aminopyralid+ halauxifen-methyl+ picloram) from Corteva. “It’s effectively a stronger Belkar,” says Rob. “Where Belkar can struggle with pansies, chickweed, mayweed or speedwell, LaDiva fills those gaps.
“It also has activity on hedge mustard – a notoriously tricky brassica-type weed in OSR – which until now has relied on clomazone pre-emergence to really control it. But the main benefit is being able to use aminopyralid earlier, rather than waiting for propyzamide-based mixtures later on.”
That separation is valuable, he adds. “Astrokerb as a product has always been a compromise – waiting for soils to cool sufficiently to optimise persistency and blackgrass activity while also requiring a dry leaf for broadleaf weed kill. By using aminopyralid earlier through LaDiva, you can hit broadleaf weeds when they’re smaller, saving propyzamide for grassweeds when conditions are right.”
One caveat is rotation, as aminopyralid carries restrictions in following crops. Therefore, if potatoes or certain legumes are in the mix, it’s not an option, points out Rob.
While chemistry can often dominate the conversation, cultural practices shouldn’t be ignored either, he adds. “How you establish the crop makes a big difference – the more soil you disturb, the greater the flush of weeds. With OSR, you don’t have the luxury of a long stale seedbed, so if you’ve moved a lot of soil, you can expect broadleaf weed flushes to come through thick and fast. By contrast, direct drilling tends to bring fewer weeds with it.”
Rotational position can also play a part in overall control strategies. “With OSR grown less frequently than it used to be, growers can take the opportunity to control key weeds elsewhere in the rotation. Hedge mustard, for example, should be managed in cereals rather than left until OSR,” he adds.
Looking ahead, with strong OSR prices tempting more seed into the ground this summer, Rob believes a more proactive approach to weed control is warranted. “There are growers who drilled in late July with moisture and both the OSR and weeds have a long growing window ahead. . Without an early plan, the weeds will win.
“If you keep on top of broadleaf weeds, OSR has the vigour to push through everything else the season throws at it. If you don’t, you’re asking a weak crop to survive pigeons, flea beetle and slugs, on top of weeds. That’s not realistic – in OSR, if you set it up to fail, it will fail.”
This article was taken from the latest issue of CPM. Read the article in full here.
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