From flea beetles to weed control: September’s agronomic advice for the north

Emma’s agronomy advice for September

1) MagicTrap can help monitor for cabbage stem flea beetle migration

While there was enough moisture in late July and early August for oilseed rape establishment, the current hot spell and dry forecast through to August bank holiday weekend might have put a stop to oilseed rape drilling.

Those that have drilled already are probably happy, particularly with the low incidence of cabbage stem flea beetle in harvested crops giving some encouragement.

But the warm weather might encourage more pressure, particularly as we reach what is usually the peak migration period around the bank holiday. Installing a Bayer MagicTrap in your field can help monitor if that is happening – and provide insight into when the peak has passed to help inform when to drill, moisture permitting.

For September plantings, a variety with good autumn vigour will be best, which tends to point towards a hybrid variety. DEKALB® varieties, such as DK Exstar, DK Exsteel and DK Excentric all meet that requirement. With later drillings it’s worth upping seed rates a little to help compensate for potentially slightly higher crop losses.

2) Maximise pre-drilling weed management

Grassweed control from pre-emergence herbicides last autumn was generally strong, as growers were able to apply good herbicide programmes. That should minimise weed seed return in crops and lessen the need for any major cultivation reset this autumn.

The dry weather has slowed germination of weed seeds so far this summer, although hopefully it will have increased predation. If and when there is any rainfall, a light cultivation will help improve seed-to-soil contact to encourage germination as part of a stale seedbed.

When spraying off with Roundup® rates will be dictated by weed species and size. Focus on applying using good application technique for best results, considering boom height, water volumes, forward speeds and nozzle choice.

Monitor carefully for efficacy and contact your local Bayer technical manager if you’re concerned about performance. Where there are any survivors of a glyphosate application, make sure these are destroyed by a mechanical cultivation.

Timing is dependent on your perception of barley yellow dwarf virus risk. Aphid pressure has been high this spring and summer, not just in cereals but other crops, which is alluding to potential for a higher pressure BYDV year. With this in mind, it emphasises the importance of controlling grass weeds and volunteers to reduce the risk of a green bridge for infection in the next cereal crop. Ideally that needs to be done around a week to 10 days before drilling, although it does compromise timing for weed control to some extent.

3) Delaying drilling likely to fall on deaf ears

Given how dry it has been, I think most growers are now expecting that it might be a difficult autumn again – when it rains, it won’t know when to stop. With an early conclusion to harvest, there will also be plenty with time on their hands and eager to get on with drilling.

That’s making it unlikely that many growers, understandably, will want to delay drilling for grassweed management, even if technically it is the right thing to do for that problem. The upshot will be that pre-emergence residuals take on even more importance, and that a post-emergence follow up, possibly even two, are likely to be required before Christmas.

Think about the half-life of different actives that can be used pre-emergence. Earlier drilling in usually warmer conditions reduces the half-life and thus persistence of products. Actives, such as aclonifen and diflufenican have longer half-lives than cinmethylin and flufenacet, and thinking through how to use that in sequences across the autumn is important.

Proclus® can be used pre-emergence only and is one to consider where you have difficult grassweed challenges, such as black grass, Italian rye-grass or bromes in combination a metribuzin-based co-formulation such as Alternator® Met, Cadou® Met or Octavian® Met or Liberator® to give a really good start to programmes.

The metribuzin-based co-formulations give a little extra efficacy against grass weeds, plus widens the spectrum of broadleaf weeds, adding control of weeds like groundsel, crane’s-bill and bur chervil.

You can also consider adding in tri-allate for wild oats and brome, and prosulfocarb for Italian rye-grass and black grass to the base mix where required.

4) Low blight pressure still requires monitoring

Late blight pressure in potatoes has been low all season, thanks to the dry weather, helping to take some pressure off programmes.

There’s always a risk in irrigated crops, or if the weather changes, and programmes need to be maintained until the end of the season. Stick to intervals and for the last couple of sprays look for products that have efficacy against both foliar and tuber blight, such as Infinito®, while making sure you don’t exceed any label restrictions on numbers of applications or maximum total doses.

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Alternator® Met, Cadou® Met and Octavian® Met contain diflufenican, flufenacet and metribuzin. Infinito® contains fluopicolide and propamocarb. Liberator® contains diflufenican and flufenacet. Proclus® contains aclonifen. Roundup® contains glyphosate.

Alternator®, Cadou®, DEKALB®, Infinito®, Liberator®, Octavian®, Proclus® and Roundup® are registered Trademarks of Bayer. 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