Weed control advice for early drilled wheat in the South West

Matt’s agronomy tips for October

1) Why 7–10 days before drilling is the sweet spot for Roundup^® applications

Trials with NIAB have shown that the interval between spraying Roundup^® and drilling is key, and the sweet spot seems to be around seven to 10 days before drilling to break the green bridge for barley yellow dwarf virus infections.

In the South West, it’s particularly important to kill off volunteers and grass weeds so the aphids run out of a food source before the new crop emerges, and the data shows that timing reduces risk significantly.

If weather prevents applications, it’s still worth doing the Roundup^® application for the weed control benefits alone before the pre-emergence, especially if you’re in a no-till system, to reduce the risk of transplanting survivors.

Always check the growth stage of the weeds you’re applying to in the stubble, as it is easy to think 540g a.s./ha of glyphosate will do the job. But part of the reason resistant Italian rye-grass has developed in the UK is because of persistent use of low doses regardless of weed size and expecting it to work. The minimum dose I would recommend is 720 g a.s/ha, except if you are applying it with the pre-em., when labels limit applications to 540g a.s/ha.

Treat glyphosate as a high value product and make sure the boom height is kept to 50cm above the target and travel at forward speeds of 12-14 km/h. If you haven’t got pulse width modulation on your sprayer or auto boom shut off, be really conscious of corners, tight angles and sweeping points in the headlands where the outer part of booms can travel a lot faster than the sprayer and it’s easy to end up sub-optimally dosing.

2) How to boost grass weed and broadleaf control this autumn

Where cereal crops have been drilled in September, it is asking a lot of any pre-em. chemistry to suppress weeds all the way through autumn. If you know you have difficult grass weeds, such as black grass, Italian rye-grass and bromes, an additional post-em. herbicide with residual activity and some contact will help with reducing weed pressure and see a better level of control through the autumn and part of winter.

In that situation, I’d suggest looking at one of our metribuzin co-formulations, Alternator® Met, Cadou® Met or Octavian® Met, if one wasn’t applied as a pre-em. Another option, again depending on what was used at pre-em. could be to use a cinmethylin product.

For October drilled crops, where you’ve followed best practice cultural controls, delaying drilling, spraying off the big spike of emerged grass weeds before drilling, I would base pre-em. programmes on Proclus® and Liberator®.

Alternatively if you think you might only get one application in the autumn, would be to partner Proclus® with a metribuzin co-form, which has been a really effective start to programmes in our trials for black grass, Italian rye-grass and bromes. For the latter, it might also be worth adding some tri-allate or pendimethalin to further boost control.

A metribuzin co-form could also be an option early post-em where you haven’t applied a pre-em. in “easier” grass weed situations and know you have annual meadow-grass and broadleaf weeds to deal with in both winter wheat and winter barley. In barley, which is a softer crop, I would play safe and use 0.5 L/ha, and then tank mix an alternative product if you need to broaden out weed spectrum.

Metribuzin is helpful in situations where more difficult broadleaf weeds, such as groundsel or bur chervil are present. Trials have shown good control of these weeds from metribuzin co-form products, which will take the pressure off applying either an autumn or spring sulfonylurea herbicide.

3) Weed control in winter oats

Liberator® now has a label recommendation for use in winter oats as a pre-em only application. It used to be allowed under an EAMU (extension of authorisation for minor use) at different rates, but the maximum rate on the label recommendation is 0.3 L/ha.

The new rate is because flufenacet is hot on the crop. Best practice would be to use 0.2-0.3 L/ha pre-em. and be very careful with overlaps when spraying and ensure good seed depth coverage to protect the seed. Watch the weather forecast before spraying, and if heavy rain is forecast for the next couple of days after proposed spraying then don’t apply it.

4) Optimising glyphosate and timing for cover crop termination

It should be a decent year for cover crop growth this autumn after most growers will have been able to drill in August following the early harvest.

Termination date will depend on when you plan to drill your following crop. Trials with NIAB a few years ago found that for spring barley drilled in February, which a few growers will do on the higher ground down in the South West, the cover crops need to be burned off around a month before expected drilling date to avoid any yield drag on the following crop.

That’s partly because you want time for the biomass to start to degrade into the soil, which not only helps make drilling simpler, but also protects the soil as a mulch and if it is dry, prevent the cover crop sucking up too much moisture out of the ground and affecting the following crop.

Rates for glyphosate applications will depend on how much biomass and especially what species you’ve grown. Some species may well need at least 1080g a.s/ha or even 1440 g a.s/ha to make sure of good coverage and uptake to get good control. Use higher water volumes (200 L/ha) and the right nozzle type to help with canopy penetration.

Mowing or grazing with sheep before Roundup® applications can help with removing biomass, which could help avoid shading of grassweeds or smaller cover crop species under the canopy and prevent the need for a second spray, which will increase resistance development risk. If you graze, allow at least three weeks for the plant to recover so it will take up the chemical.

-----

Alternator® Met, Cadou® Met and Octavian® Met contain diflufenican, flufenacet and metribuzin. Liberator® contains diflufenican and flufenacet. Proclus® contains aclonifen. Roundup® contains glyphosate. Alternator®, Cadou®, Liberator®, Octavian®, Proclus® are registered Trademarks of Bayer. Roundup® is a registered Trademark of the Bayer Group.All other brand names used are Trademarks of other manufacturers in which proprietary rights may exist. Use plant protection products safely. Always read the label and product information before use. Pay attention to the risk indications and follow the safety precautions on the label. For further information, including contact details, visit www.cropscience.bayer.co.uk or call 0808 1969522. © Bayer CropScience Limited 2025