How to achieve good weed control this autumn

Crop Progress

Cereal harvest was completed in September. Yields and quality were variable, mainly depending on drilling date, soil type and the degree of weed and disease pressure. Overall, yields were lower than last year, probably also attributable to a fairly wet winter, not building root mass, and not the brightest of springs. Late drilled spring or weedy crops have then brought down averages over quite a lot of farms.

Sugar beet lifting and potato harvesting are also underway in the region.

Oilseed rape drilling has also come to an end after a protracted drilling period, starting in late July all the way through middle of September. Early drilled crops had reached six true leaves by mid-September. They’ve been grazed by adult cabbage stem flea beetles, but on the whole seem to be withstanding it. Crops drilled around the August bank holiday have been mullered by a slightly later peak adult CSFB emergence this year though.

Oilseed rape crops drilled a bit later are only just emerging, and maybe with cooler temperatures in late September might get away. Slugs will be a concern, as well as getting those crops to a sufficient size to tolerate pigeons and get through the winter.

Tom’s agronomy tips for October

1. Manage oilseed rape crops appropriately

The variable nature of oilseed rape makes it quite difficult to manage, especially in the autumn. You’ve got to manage it; it needs appropriate investment, but you don’t really know if you’ve got a crop until the spring.

It’s not just cabbage stem flea beetles and slugs, rape winter stem weevil is also a risk. We’re trialling the use of Magic Traps in Lincolnshire and elsewhere around the country, which remotely monitor pests by taking photos of a trap and automatically evaluating what pests are present.

It will be interesting to see if it picks up any rape winter stem weevils and if this can support insecticide decisions.

Disease risk in oilseed rape crops will depend on variety, drilling date, weather and growth stage. Later drilled crops will have higher risk from Phoma, while light leaf spot will be more of a risk in earlier drilled crops. If you suspect light leaf spot, put leaves in a plastic bag and incubate in an airing cupboard for a few days to confirm.

There could be higher risk of Phoma this season with wetter weather, but varietal resistance on the whole is good, and larger crops should get away. There are fungicide options to protect against Phoma, light leaf spot and powdery mildew, such as Proline (prothioconazole) or Aviator Xpro (bixafen + prothioconazole), if needed.

2. Destroy the green bridge for BYDV

There have needed to be different approaches to post-harvest cultivations depending on situation. It wasn’t the driest of harvests, so some remedial cultivations have been required to remove damage from wheelings. Some have not cultivated at all while others have established catch and cover crops.

Some winter cereals will have been drilled in September. Ideally wheat drilling will be delayed into October where grassweeds are an issue, but it’s compromise especially on larger units where earlier drilling is sometimes needed just to cover the ground, or on particularly heavy land where it can be a struggle to drill after wet weather.

If you are able to delay drilling, up seed rates – crop competition is an important element of weed control.

Either way a stale seedbed is incredibly important. It’s easy to forget after harvest how much weed seed has been added to soils. Timing when to spray off might depend on priorities.

For maximum weed control the shorter the gap between Roundup (glyphosate) applications and drilling the better to minimise weed emergence before the pre-emergence. But that doesn’t provide a long enough interval for the green material to die providing a potential green bridge for barley yellow dwarf virus carry over and diseases, such as yellow rust.

For green bridge destruction, trials have highlighted applications at least 10 days before drilling reduce BYDV infection compared to shorter pre-planting intervals. Where the no insecticide option has been chosen, which may be more favourable with the Sustainable Farming Incentive, managing the green bridge is especially important.

There’s also trying to keep within resistance guidelines for glyphosate stewardship, where the Weed Resistance Action Group’s recommendation is for no more than two applications and to use cultivations between applications to kill any survivors.

There’s probably no one easy solution, so priorities might need to be chosen. It’s possible perhaps to go with an early glyphosate and then put Roundup in with the pre-emergence, which our Roundup labels permit, but some caution is required because you don’t always know you’re going to be able to get that pre-emergence spray on.

As for the application, it’s important to match dose to the appropriate species and size. Ideally wait for annual grass weeds to be 2-3 TL so there is sufficient green leaf area for spray uptake, and use a minimum of 540g a.s./ha.

Higher glyphosate rates might be needed if grassweeds have started tillering, and also in direct drill situations whether into harvest stubbles or catch crops, or after maize, where grassweeds might be bigger. If there are any perennial weeds match glyphosate to the species that requires the highest dose. Not all glyphosate is Roundup – branded products have the fastest acting formulations because of superior surfactant systems.

Finally, concentrate on the application basics. Use a fine-medium spray to get good coverage of smaller plants, and use an appropriate forward speed – not too fast – to enable a stable boom and consistent dosing. Spraying on a dry or drying leaf is important and spraying in the middle of the day can give better control as the plant should be most actively translocating the product down into the roots and delivering the best kill.

3. Use diversity of actives to help with grassweed control

There are lots of residual herbicides and active substances available in wheat and our advice is to use them. Pre-emergence chemistry is best when applied within 48 hours of drilling, ideally into soil moisture so you get sufficient root uptake.

Liberator (flufenacet + diflufenican) will make a good foundation but for most populations of black-grass, Italian ryegrass and bromes but often other actives need to be added, and likely a top-up residual to extend the period of control later into the autumn.

One option would be to add Proclus (aclonifen), which can be used at 1.4 L/ha in wheat or 1.0 L/ha in winter barley, and adds consistency to control of black-grass, Italian ryegrass and bromes. Aclonifen is primarily absorbed by the shoot, has quite a long half-life and is good in dry conditions.

Another strong option would be to use a flufenacet + diflufenican + metribuzin co-formulation (Alternator Met or Octavian Met), which the metribuzin contributes to improved grass weed and broadleaf weed control compared with Liberator. Alternator or Octavian Met can also be mixed with Proclus.

Other actives such as tri-allate, prosulfocarb and pendimethalin can be added to Liberator + Proclus in wheat, but not in winter barley. It is important that barley is not held back, to maintain its competitive benefit against weeds.

4. What about top-up herbicide sprays?

Where crops have been drilled in September in fields with grassweed issues, you may have to be flexible with your decision making. If lots of grass weeds have emerged you may even need to consider starting again or putting in a spring crop, if control is not going to be possible.

In most cases it’s likely that a top-up residual herbicide will be required during October, as in warmer conditions the pre-emergence residual activity won’t last as long. Octavian Met and Alternator Met (flufenacet + difufenican + metribuzin) are potential options for this spray – both can now be used at the full rate of 1.0 L/ha until GS25 of winter wheat or winter barley.

The metribuzin in the formulation brings bit more broadleaf weed activity, such as groundsel and volunteer beans, as well as some contact activity against grassweeds.

5. Visit one of our vegetable crop production days

Bayer remains committed to vegetable crop production, and in October we have a couple of events where you can find out how we can help support you growing those crops.

On Thursday 5 October, we will be exhibiting crop protection in carrots at the British Carrot Growers Association demonstration day being held at Huntapac Produce in Sherburn, North Yorkshire. Seminis/Vegetables By Bayer – the vegetable seeds arm of Bayer will be there to demonstrate carrot variety developments.

The following week on Wednesday 11 October and Thursday 12 October, our Brassica Innovation days in conjunction with Seminis are being held near Swineshead, Boston in Lincolnshire. We will showcase our new class leading vegetable brassica varieties, as well as our crop protection solutions.