Weed control, oilseed rape disease and cover crop destruction advice for growers in the west

1) How to deal with these three possible weed control scenarios 

Most wheat fields are likely to be in one of three potential situations: the crop was drilled earlier in September or October, received a pre-em and is now requiring a top up; crop drilled but didn’t get any herbicide as yet; and the fields still to be drilled. 

Starting with the fields that are drilled and received a pre-em, top-up sprays will depend on what you’ve done at pre-em and what are the weed targets. 

The good news is that you do have options. From the Bayer stable, there are the metribuzin-containing products, which give you a good dose of residual from the flufenacet and diflufenican, plus some contact activity from the metribuzin. They also give you a decent broadleaf weed spectrum, including groundsel and bur chervil. 

Other options include using cinmethylin-based products, unless you used that at pre-em. 

This timing fits well with mixing in with barley yellow dwarf virus sprays if applicable. 

In the second situation, where the crop is drilled, but no residual herbicide has been applied because of weather or because you’re concentrating on drilling, the first step should be to assess weed germination. 

No one will do this given how little crop has been drilled in some places, but in difficult grassweed situations, at least a passing thought should be given to spraying the field with Roundup (glyphosate) if high numbers of grassweeds are emerging. Technically that might be the right thing to do as otherwise you will be living with the legacy of that seed return for years to come. 

Assuming that the decision is made to press on, if the field has emerging difficult grassweeds, such as black-grass and Italian ryegrass, the metribuzin products are again an option, along with cinmethylin. 

If the grassweeds are reaching the two-leaf stage by the time of application, I’d consider mixing in Atlantis OD (mesosulfuron + iodosulfuron) as a proper contact-acting herbicide with the metribuzin products. It won’t be the cheapest option, but it has provided success in the past.   

Where your weed spectrum is “easier” with weeds like annual meadowgrass and general broadleaf weeds, the metribuzin products will also provide cover.

Finally for those fields still to be drilled – weather permitting – then establishing a competitive crop should be the first target. Keep seed rates high and do everything possible to achieve a decent seedbed quality, as difficult as that might be. 

If the crop has gone in well, and seed is covered by at least 32mm of soil, then there is the possibility of using a good pre-emergence, such as Liberator (flufenacet + diflufenican) + Proclus (aclonifen) where a reduced grassweed burden still justifies it. That kind of mix will give persistence of activity over winter and into spring, and mean you’re not dealing with some of the more difficult broadleaf weeds in the spring. 

Where the risk of crop damage is higher because seed is shallow or even uncovered in places, or not at a consistent depth, then you probably have to leave any residual herbicide until the crop has 1-2 leaves. These residual products, especially some of the more soluble ones, can get washed down to the rooting zone in those situations and that’s going to cause problems.    

2) Look out for Phoma and light leaf spot in oilseed rape

According to the AHDB , all the monitoring locations for Phoma in oilseed rape in the West Midlands were forecast to reach potential treatment thresholds during October.  

With most oilseed rape crops in the region on the smaller size, it could mean that an autumn fungicide is justified, particularly in varieties rated 7 or below for Phoma. 

If you are planning to treat during November, I would use a fungicide with activity also against light leaf spot, such as Aviator (bixafen + prothioconazole) with its dual modes of action, to help protect against both diseases.  

3) Advice for cover crop destruction 

I’m not sure there is a proven time for when it is best to terminate a cover crop. If you go too early, soils can end up sitting bare before the next cash crop, and you can feel you’ve missed out on some of the benefits of growing it.   

Too late and there’s evidence that it can leave conditions wet and / or lock up nutrients for the following crop and cause a yield penalty.  

Personally, I think it is probably better to be a little early than too late and risk that yield penalty, so I’d suggest terminating around four weeks ahead of your intended cash crop drilling date.  

When using glyphosate to kill the cover crop, the most important aspect is thinking about what species are present and their size, and what dose will be required. For covers with a large amount of biomass it could be at least 1080g, possibly 1440g a.s./ha. 

Consider glyphosate stewardship guidelines as well, which advise not using two sprays on the bounce without some form of cultivation / alternative method of control in between. Consecutive doses to open up the cover crops and then another dose to kill any re-growth are probably not ideal.  

Roundup® contains glyphosate. Atlantis® OD contains mesosulfuron and iodosulfuron. Liberator® contains flufenacet and diflufenican. Proclus® contains aclonifen. Aviator® contains bixafen and prothioconazole. Roundup, Atlantis, Liberator, Proclus and Aviator are registered trademarks of Bayer. 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 2024.