More than just a spring weed control “tidy up” – Farmers Guide – Cam Murray, Toby Kellie
“Once day length and temperatures start increasing and weeds begin growing, we could see undesirable levels of grassweeds coming through in some crops.” ...
Spring herbicides are likely to be more important than usual for keeping winter cereals weed-free this season, given wide variations in autumn drilling and residual programmes, according to agronomy firm Hutchinsons.
While many growers managed to drill early into favourable conditions for crop establishment and pre-emergence sprays, some were hampered by record breaking rainfall during October – resulting in crops going in late into sub-optimal conditions, potentially without any residual herbicide.
It is these later-drilled, patchy crops that are at greatest risk from weed competition come spring, as underlying weed pressure is likely to be greater and they lack the competitiveness of better established, earlier-sown crops, says northern regional technical manager, Cam Murray.
But wheat drilled early into high-risk grassweed situations could also come under more pressure in spring, especially if there was not sufficient time between harvest and drilling for the usual cultural controls, such as stale seedbeds, he notes.
“Once day length and temperatures start increasing and weeds begin growing, we could see undesirable levels of grassweeds coming through in some crops.”
Attention often focuses on black-grass but, while this remains a big issue in many areas, ryegrass is also a significant threat, he warns. “Ryegrass proliferates more widely, is more competitive, lasts longer in the soil and is highly evolved to develop cross-resistance to multiple active ingredients.
“It can be a beast to control if it’s allowed to spread and establish, so if you see any ryegrass, you have to take a zero-tolerance approach.”
Oxfordshire-based Hutchinsons agronomist Toby Kellie adds that, even if residual chemistry was applied last autumn, there could be question marks over its efficacy given heavy rain in October which may have washed actives deeper into the soil.
Where fluefenacet-based residuals were applied for grassweeds, he recommends topping levels up with additional flufenacet as soon as conditions are suitable for travel in early spring.
“By maintaining residual levels, we can hopefully tackle the problem at the root before weeds emerge. But remember, applying a flufenacet top-up of 120g a.i/ha is only effective if there’s some residual left in the soil after autumn treatment; that amount won’t do much on its own.”
If young black-grass is already present, Mr Kellie says contact-acting products based on iodosulfuron-methyl-sodium and mesosulfuron-methyl are generally most effective. It may be worth including a residual product in the mix to pick up any later emerging weeds, he adds.
Mr Murray says crops should be monitored closely over coming weeks allowing growers to respond with the appropriate herbicide programme.
“You’ve got to understand what species you’re fighting, as well as its biology, when it germinates, how it grows, and whether there are any resistance issues. It is incredible how well weeds can evolve and adapt, so don’t just keep doing the same things.”
He agrees that iodosulfuron and mesosulfuron are the main option for black-grass control, although growers must be wary of grassweed resistance issues to these actives which could limit their effectiveness.
Iodosulfuron is also stronger than its counterpart on ryegrass, but an alternative is pinoxaden, which is one of the most effective when applied at full rate. The main options, featuring the number one active for brome control, are products based around pyroxsulam, he says.
For all grassweeds, Mr Murray says the best results come from applying herbicides early to small weeds, before tillering, and by making sure they are actively growing. “One way to check whether active growth is happening is to pull up a plant to see if the small white root hairs are starting to develop.”
Good application technique is also vital to ensure contact-acting chemistry hits the target. “You’ve only got one chance, so slow down and get boom height right to maximise spray coverage on weed leaves.”
Broadleaf weed control
Mr Murray says broadleaved weeds tend to be less of an issue in cereals where effective residual programmes have been applied, and many species will be controlled by solfonylurea chemistry applied for grassweeds.
Their later emergence also means crops should be better established and more able to outcompete emerging broadleaf weeds, although there can be issues in gappy and thin crops, or where there is a field history of particular weed problem.
Where targeted broadleaf weed control is required, there is a wide range of chemistry, based on ALS and hormone actives, so it is all about matching the herbicide to the weed spectrum.
One broadleaf weed that is becoming a particular concern, especially on lighter soils, is bur chervil, which is extremely competitive and can cause combining difficulties if allowed to establish, he says.
“Bur chervil must be controlled early, at the seedling stage. If you do, then 90-100 per cent control is possible, but work done by leading research organisation NIAB TAG has shown in trials that once bur chervil gets to stem extension, then control will drop to just 32 per cent.”
A sulfonylurea-based herbicide, such as metsulfuron-methyl, with added hormone chemistry such as dicamba or mecoprop-P generally gives best control, he advises. “Iodosulfuron and mesosulfuron are not as effective on bur chervil.”
One weed problem wheat growers may have little control over this spring is volunteer cereals, notes Mr Kellie. Given the large area of spring cropping grown last season, disruption to harvest 2020 and rush to get winter crops sown in rain hit areas, some growers may not have had chance to fully remove all volunteers before drilling, he notes.
“There could be a lot of volunteers in some crops and there aren’t many options we can turn to in cereals. However, it remains to be seen how many volunteers survive the winter, so we’ll assess the situation in spring and see what products might offer some control.”