From weeds to blight, navigate farming challenges this September

Aleks’ agronomy advice for September

1) Balance grass weed control and BYDV risk in stubble fields

Effective stubble management is a key part of controlling the difficult grass weeds that are very much the priority in our region. Two of the best tools in any weed control programme are cultivation and Roundup® application.

A back-to-basics strategy of spraying off volunteers and early germinating grass weeds with glyphosate, cultivate, whether shallowly or perhaps even ploughing, followed by a second round of glyphosate as close to drilling as possible is likely to be a decent approach this season.

Using two applications increases the likelihood target grass weeds are smaller, which improves efficacy and speed of activity, with the chemical translocating through the plant quicker.

This approach also helps kill off the green bridge for aphids that transmit barley yellow dwarf virus, at the earlier timing. With high levels of aphids seen this spring and summer, BYDV is a potential risk for cereal crops this autumn and needs to be considered. Ideally, volunteer cereals and grass weeds should be sprayed off at least 10 days before drilling, which can create a tension between optimum grass weed management and BYDV control.

That’s particularly the case where growers use a no-till system. But in that situation a one spray approach might be necessary because of the need to follow stewardship guidelines of not applying two applications of glyphosate without some form of mechanical cultivation. There’s also a time element where no cultivations are used to improve seed-to-soil contact meaning germination is potentially slower, especially in dry weather.

In this situation growers might have to balance what the key priority is – grass weed or BYDV control.

Either way it’s important to make glyphosate count. If grass weeds are tillering by the time of application, rates of glyphosate should be increased to 720g a.s./ha to ensure good control. Application technique is important – use a sensible forward speed, the correct nozzles and water volumes for the target, and keep booms stable at 50cm above the target.

Don’t add things like citric, fulvic or humic acid to the spray tank. These can lower pH of the spray solution, lowering efficacy and are not a supported tank mix by Bayer.

2) Early drilling: bigger yields or bigger weed bills?

With an early harvest and experience from the previous difficult autumns for establishing crops still fresh in the memory, it’s difficult to expect farmers to sit on their hands this autumn for too long, especially if the weather is good for planting.

The risk factor from delaying drilling is increasing and understandably growers are weighing up the risk of not getting in a crop is worse than the technical challenges earlier drilling can cause. Earlier drilling also potentially brings lower seed costs, and a higher yield potential assuming you can manage the higher risks of BYDV, Septoria and grass weeds effectively.

Some of those risks can be mitigated through variety choice: BYDV-tolerant varieties, higher Septoria resistance and super-competitive varieties could all be considered, although it is unlikely you will find the perfect variety for all three, plus high yield and your chosen end market.

The largest risk is probably from an increase in grass weeds, and drilling in September almost certainly commits you to a higher herbicide spend and at least two applications pre-Christmas. No matter how good the chemical, it’s not realistic to expect one application in September to last through the autumn.

Persistency, though, is essential. Higher soil temperatures in September and early October, which break down herbicides quicker than if used later in the autumn, reducing effectiveness. Unfortunately, both our two most impactful actives against grass weeds, cinmethylin and flufenacet, have relatively short half-lives meaning their effectiveness will probably only last around a month from use in mid-September, which is unlikely to be long enough to cover the grass weed germination period.

With flufenacet that can be alleviated to some extent by mixing a metribuzin co-formulation, such as Alternator® Met, Cadou® Met or Octavian® Met or Liberator® with Proclus®, which has a longer half-life providing an extended period of activity.

It’s also less affected by dry weather than most other residuals, as it takes up the active through the shoots growing through the layer of herbicide. Many other residuals rely on root uptake and moisture plays a key role in moving the chemical to zone where roots are active.

Where grass weeds are particularly tricky you could also add tri-allate or prosulfocarb for additional activity.

3) Cercospora on the move: why a third spray might be needed

Cercospora spores have been found at most, if not all, of the BBRO CropWatch monitoring sites around the sugar beet region in July and the first part of August, with active lesions found at least five sites over that period.

If the warm weather forecast for the second half of August comes with high humidity or showers, risk will likely only increase.

Combined with slow growth in the beet crop delaying lifting of some crops, crops might benefit from a third fungicide spray more than most other years, which is where Caligula® fits. It is strong on rusts, with a real strength against Cercospora, can be applied from 1 September and a latest application cut off of just seven days before lifting.

In trials, we’ve seen a 6.1t/ha yield response from using Caligula® as a third spray following two sprays of Angle.

4) Protecting Conviso® Smart sugar beet: stop weed beet before it starts

There’s a decent area of Conviso® Smart sugar beet in East Anglia this season, with some new growers.

To maintain its effectiveness for the future, it’s important to manage the system, including groundkeepers and bolters effectively – otherwise you risk having weed beet challenges all over again.

Ideally plough after beet crops to make sure beet tops don’t regrow, monitoring following crops in those fields to make sure no volunteers emerge. In cereal crops, treating with a hormone herbicide is the best option for control.

In areas where you temporarily store beet, consider leaving this area uncropped so it’s easier to see any bolters emerge, and treat with glyphosate as soon as you see any.

5) Protect late season potato crops from tuber blight

Late blight pressure this season has been extremely low, but it typically ramps up later in the season with cooler nights, warmer days and more frequent dews combining with dense canopies increasing risk.

Tuber infection risk also increases, which is where Infinito® can help, providing you haven’t used your total maximum dose of 6.4L/ha for the season. It provides good control of both foliar and tuber blight with activity against zoospores, making a good option for one of the last sprays in the programme.

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Alternator® Met, Cadou® Met and Octavian® Met contain diflufenican, flufenacet and metribuzin. Angle® contains azoxystrobin and difenoconazole. Caligula® contains fluopyram and prothioconazole. Infinito® contains fluopicolide and propamocarb. Liberator® contains diflufenican and flufenacet. Proclus® contains aclonifen. Roundup® contains glyphosate.

Alternator®, Cadou®, Caligula®, Conviso®, 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