Ben Giles’ agronomy advice: smarter weed and crop management this autumn

Ben’s agronomy advice

1) Integrating glyphosate, cultivation, and drilling timings for grass weed management

Where growers have delayed drilling wheat into October, I’m hoping it is because of serious grass weed issues be that black grass, Italian rye-grass or bromes rather than the weather.

I’m not keen on using a traffic light system for field risk, but it does work. Most people will have ‘red light fields’ that shouldn’t be drilled early.

In those cases, given the early flush of grass weeds, it’s likely that at least one application of Roundup® will have been applied, although if you’re delaying drilling later into October you may still be holding off.

Where you are on to the second application, first consider whether other methods of control can be used, particularly the use of steel. We have seen resistance issues with Italian rye-grass, and a few more suspicious cases have been identified for further testing this season, the almost universal factor is the lack or very minimal use of steel. And while there are no documented cases of black grass resistance to glyphosate in the UK, the use of cultivation should not be underestimated as a pressure relief valve on glyphosate use. It doesn’t have to be before every crop, but no use of steel is a major concern for those of us who work for a company which manufactures Roundup® and tries to steward its responsible use.

If a second application is being used, it’s best used close to drilling or with the pre-emergence herbicide. Check labels carefully as there are restrictions on dose within 48 hours of drilling or as a pre-emergence for some products, typically around 540g a.s/ha, but individual products differ. If you need a more sensible dose, especially for Italian rye-grass, consider spraying outside that window.

2) Value the sequence approach for weed control

The biggest learning I’ve made from trials in the past couple of years is not whether cinmethylin should be used pre-em. or early post, or that Proclus® needs to be used in this way, it’s that the best control comes from using sequences and that includes from some very late drilled trials last season.

Now applying a sequence is much easier in a trial when you can walk across a plot with a 3m side-held boom in more or less any conditions than it is when you’re taking a 20t of sprayer and spray mix across a field. I appreciate that’s a different kettle of fish, but there’s no doubt that sequences are the way forward if you can get them on, because efficacy is better, prolonging the level of residual cover. Secondly, they tend to be safer to the crop as the mix tends to be a little simpler.

3) Are your seed rates too low?

Before I get too deep into the chemistry, for those still to drill, a reminder to consider seed rates. I think we’ve been guilty of going a little light with seed rates, and the value of varietal competition for grass weeds has perhaps been forgotten. For seed that has been overwintered in particular, it’s worth getting germination and vigour tested if you still have time, but in any case, be confident of the percent germination and level of vigour you’re going to get. As you move through October up seed rates into at least the 400-500 seeds / sqm range depending on date.

Higher seed rates will also help if plant counts are reduced as the result of the use of some of the herbicide mixes needed for the weed pressure in fields. Having a few more seeds is no bad thing.

4) Breaking down the herbicide tactics for different situations

As usual at this time of year, there are three main scenarios you could be facing with wheat crops:

• Crops drilled in September with a pre-em. applied, requiring a follow up

• Crops drilled in very late September or being drilled in early October with no pre-em. as yet and likely to require two sprays in October

• Crops to be drilled later into October and beyond that might only get one spray.

In terms of a sequence for both September and early October drilled crops, Liberator® + Proclus® followed by cinmethylin plus partner is recommended for efficacy, assuming the field needs that level of spend. For the fields with high populations, that sequence, perhaps with one of prosulfocarb, pendimethalin or tri-allate added to the pre-em. stack, will be the most consistent mix.

If there has been a low seed return and the seed bank has been depleted over the past few years, for October-drilled crops in good conditions, then you might not need that level of spend and potentially tailor the second spray accordingly. That decision is likely to be needed to be taken on field history and gut feel, because when spraying 10-20 days after the pre-em. spray you’re not really going to know how that has worked and what’s coming up.

Alternative options for the follow up include the metribuzin co-formulations, Alternator® Met, Cadou® Met and Octavian® Met rather than cinmethylin. At over £60/ha for cinmethylin-containing products, growers in this part of the world, after a less-than-successful cropping season last year, may consider using an alternative.

The metribuzin co-forms are also a good option where cinmethylin has been chosen for the pre-em. spray and / or if you’re also targeting some of the more difficult broadleaf weeds, such as groundsel or bur chervil. Our trials with these products have shown a good result against these weeds.

The trickiest situation is perhaps the later drilled October crops, where a sequence would still be best technically for grass weed control, but the risk increases of not getting the top up applied. In that situation, the balance is making sure the pre-em. has enough persistence to give good enough control if you can’t get back, without going too big with the stack and the potential challenges of crop safety.

If this all seems complicated, it’s probably because it is, and I’ve only outlined a small number of potential options. Other actives, such as tri-allate and prosulfocarb could equally have a place in some programmes and be part of a good programme.

In the end, my best advice is to remember these key principles: try to use two applications with each based on either flufenacet plus diflufenican (DFF) or cinmethylin and then add something that will last a decent amount of time with good efficacy into each timing. Aclonifen can only be used at the first timing, while pendimethalin and DFF are more flexible. Adding one of the latter three actives will help give the persistence to get through to Christmas.

5) Timing and dose for cover crop management

Looking much further ahead to cover crop destruction, this will be the last blog this side of Christmas, remember to give yourself enough time and high enough dose for the species in the mixes. It’s looking like there could be particularly high biomass overwintered cover crops, with early harvest helping growers get crops drilled in August.

If you have an underlying grass weed issue in those fields, especially Italian rye-grass, be very careful with shading and what size the grass weeds are when you come to spray off.

6) Are we using too much triazole in oilseed rape crops?

Virtually every disease control spray in oilseed rape crops includes a triazole, quite often applied alone, and it is a concern about the sheer volume we’re using. There are, albeit at greater cost, mixed products, that contain other chemistry.

Even using low doses topped up with additional prothioconazole will bring a degree of other chemistry into the crop, and that might not be a bad thing. It’s more common and perhaps easier for flowering sprays, but it is sensible to consider with autumn sprays, given Rothamsted’s research showing that both light leaf spot and Phoma may not be as easy to control as they once were.

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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