Weed control options for southern growers this October

Crop Progress

Virtually the last of the oilseed rape was drilled by mid-September, with a handful of fields already needing re-drilling where flea beetle damage was too much or the crop hadn’t established well enough. Cover crops have also been established in some fields following cereals, while there are main crop potatoes to be lifted and maize harvest to come.

Rain in the southwest has meant minimal cultivations in August and into September, but most can knock it over pretty fast and get it turned around quickly when conditions are right.

Matt’s agronomy tips for October

1. Destroy aphid green bridge 10 days ahead of drilling

Incidence of barley yellow dwarf virus last year was fairly normal in Devon and Cornwall, but the further east into Somerset, Dorset and Wiltshire agronomists suggest It was the worst year for a long time.

I think that was a little bit down to the fast turnaround of spraying stubbles off within a couple days of drilling. That’s good advice for weed control and one of our label benefits for Roundup (glyphosate) is that you can cultivate within six hours in certain conditions.

But if you’re planning to be an insecticide-free farm, for example, under the Sustainable Farming Incentive (SFI), you have to use integrated pest management (IPM) techniques to control aphids. If they are feeding on the cover crop or on cereal crop stubble volunteers while they are dying following Roundup treatment, they can just migrate straight into the new crop as it emerges.

That’s something to be conscious of, especially when there have been four-figure bird-cherry aphid suction trap catches in September as there was in Exeter. The potential is for BYDV to be worse this year, because usually around a third of aphids carry the virus.

Our trials last year found that spraying off with glyphosate at least 10 days before drilling to destroy the green bridge effectively made a difference to BYDV incidence. So ideally, I would suggest going 10 days before drilling, and then monitoring for weeds before drilling and making a decision whether you need some form of mechanical disruption either before or during drilling to help minimise resistance risk before applying Roundup post-planting with the pre-emergence, or as a separate application.

In the end you have to weigh up all these elements to make the best start for both weed control and to reduce BYDV.

When using glyphosate, trials work with spray application expert Tom Robinson highlighted the optimal forward speed was 12 km/h, which is slightly slower than maybe people would consider with glyphosate which can be seen as a simple spray as the perception is that it effectively works no matter what you do. But that’s not quite right as you can see reductions in efficacy and survival the faster you go and also at higher boom heights.

Spraying at a steady speed at 50cm boom height gives a consistent pattern across the width of the boom, which obviously then means you are delivering the optimum dose. The trials work also found the 3D nozzle was the best choice for applications giving the right droplet size and lower the potential for drift.

2. Delay drilling where difficult grassweeds threaten

It’s always a challenge to know when the weather might allow later drilling, but from a weed control point of view, especially in the warmer coastal areas of the southwest, holding off drilling until at least the 10October helps both with BYDV and weed control.

We’ve all seen in trials how much of a reduction you can get in grassweeds, especially Italian ryegrass and black-grass by holding off and maximising the benefit from glyphosate sprays.

But on the higher elevation chalky ground in the south through the Downs it’s harder to hold off because soils cool down quicker and slows growth down, and there’s the consideration of how much area you need to drill.

3. Think about active substance groups for pre-emergence sprays

You’ll all know that knowing which weed species you have will affect your programme and cultivation strategies, but we’ve been talking a lot about modes of action and rather than just putting more herbicide on, thinking about which active substances you’re using and which groups they belong to – how they actually work in terms of interrupting various metabolic pathways.

That’s important as if you mix modes of action you’ll reduce than chances of both resistance development and also improve control because you’re not allowing any populations to improve how it metabolises any particular mode of action, reducing efficacy.

For example, if you’re building your pre-emergence spray around flufenacet, which is in the Herbicide Resistance Action Committee’s group 15 – inhibition of very long chain fatty acids – by adding diflufenican, as formulated in Liberator, you’re adding an active substance in Group 12 (inhibition of phytoene desaturase).

If you have difficult grassweeds, such as black-grass, Italian ryegrass or bromes, adding Proclus (aclonifen) adds a third mode of action (Group 32 – inhibition of solanesyl diphosphate synthase) and more importantly anything from 10-30% control depending on grassweed.

My advice is to build a programme around flufenacet and DFF, and then add in other modes of action to diversify modes of action while covering other weeds within the field spectrum.

In addition to Proclus, other options such as prosulfocarb and tri-allate – both in a sub-group of Group 15 – are also good options to consider. They have been the strongest actives to add to Liberator + Proclus for black-grass and Italian ryegrass control in wheat.

For sterile brome, which is probably one of the most difficult to control and features most heavily on surveys in the southwest, a trial in Somerset on a particularly difficult headland patch suggested adding pendimethalin was the best option. I think that’s because of the long half-life the combination of actives gives, providing a long stable layer of residual across the soil surface and a constant barrier for weeds as they germinate.

4. Will early drilled crops need a top-up for weed control?

Any early drilled crops where grassweeds are a threat may need a follow up as early as October. A good option could be one of the metribuzin-containing co-formulations Octavian Met or Alternator Met, which also contain flufenacet and diflufenican.

We’ve received a label extension for both which means you can now apply the full rate of 1.0 L/ha up until GS25 of winter wheat and winter barley, rather than there being a calendar restriction on rate.

These products also make effective options for the control of annual meadow grass and broadleaf weed populations post-emergence at the 1-2 leaf stage of the crop.

5. No Liberator option in winter oats

Unfortunately the emergency use authorisation for Liberator in winter oats has expired as the area of the crop grown now means it is classified as a major crop. Bayer and other industry partners are actively working to find a way to either get winter oats on the Liberator label or other emergency use alternatives, but at the moment nothing has been approved.

Obviously we will let growers know if and when that changes.

6. Consider an autumn fungicide in oilseed rape

Oilseed rape diseases were low last autumn, but with significantly more rain this August and September that might not be the case this season. Phoma usually occurs first, and we do have strong varietal resistance so it’s important to monitor for whether thresholds have been breached.

Fungicides, such as Proline (prothioconazole), can play a useful role in keeping crops healthy through to spring and preventing the need to firefight if either Phoma or light leaf spot get out of control during the season.