Seedbed quality and post-emergence weed control help for southern growers

1) Seedbed quality critical for November drilled wheat 

Where fields are still to be drilled, one silver lining of the wetter weather is that there has been a flush of various weeds, including black-grass and Italian ryegrass, which can be sprayed off with glyphosate ahead of drilling. 

Keep seed rates up for November and December drilled wheat, and where possible, aim to drill to a depth of 32mm. Not all is lost by drilling now – experience from last year showed that decent yields were possible from these later drilling dates.  

Seedbed quality, though, is crucial, especially where you would like to apply a pre-emergence herbicide. Cloddy seedbeds give the opportunity for rain to wash residual herbicides down into the root zone, especially the more mobile actives and that can cause crops to go a bit yellow. While generally it will outgrow the effects, it’s not something anyone wants to see.  

Efficacy of herbicides can also be impacted by cloddy seedbeds, with clods breaking open disrupting the even barrier of herbicide across the soil surface.  

There should be lower weed pressure from a November drilling date, but there will be a value to using even a small dose of a residual herbicide rather than solely relying on spring control, especially if dealing with difficult grassweeds.  

Drilling, rolling and then spraying with a Liberator (flufenacet + diflufenican) or a metribuzin-containing product should also see crops through to March when the next germination curve for black-grass and Italian ryegrass starts to build. Adding in Proclus (aclonifen) for more difficult grassweeds, including bromes might also be beneficial.  

But if you have any doubts about the quality of seedbed and seed being covered to a depth of 32mm, consider holding off until the crop is at GS11 and use an early post-emergence option.  

2) What post-emergence options should you consider? 

There are various crop situations you may face where post-emergence herbicides might be required. These include later drilled crops where seedbed quality is poor, earlier drilled crops that had a pre-emergence spray which is now running out of steam, and October drilled crops that didn’t receive a pre-emergence spray.  

Options for all these scenarios are similar. Where the pre-emergence is running out of steam in fields with difficult grassweeds, options will likely be based around either a cinmethylin-based product or Liberator / metribuzin type options. Which you choose will depend to some extent on what was applied at pre-em.  

The metribuzin-containing products will provide some level of contact activity, but if a big flush of black-grass or Italian ryegrass has come through and it has reached the two-leaf stage, then adding in Atlantis OD (mesosulfuron + iodosulfuron) now might be better than waiting until spring.  

The challenge of waiting until spring is that a planned February application often becomes a late March / April spray when efficacy is compromised by plants going through stem extension. Sometimes biting the bullet in the autumn, even if it is the last bullet, is better than hanging on. 

All of the above also applies in the situation where a pre-em wasn’t applied. There will also be fields where there are fewer difficult grassweeds, but still require weed control for annual meadowgrass and broadleaf weeds, including groundsel and bur chervil.  

The metribuzin range is particularly useful for this as it brings a wider range of broadleaf weeds compared with Liberator. Adding in a partner, such as pendimethalin or picolinafen broadens the range of weeds controlled further.  

3) Check for oilseed rape diseases 

There’s much less oilseed rape in the ground in the region, and from talking to agronomists and farmers, it is being hammered by slugs, leading to a lot of slug pellet applications. 

Most of the oilseed rape varieties have good resistance against Phoma and light leaf spot, and there’s been no reports of disease so far.  

It’s worth monitoring for both, and if there is any inoculum present considering adding a fungicide into a propyzamide spray as necessary.  

4) Choose the appropriate dose of glyphosate for cover crop destruction 

Our research, in conjunction with RAGT, suggests that earlier cover crop destruction is beneficial for both nutrient availability and moisture regulation in seedbeds for following crops.  

Based on that, ideally, I would look to terminate over-wintered cover crops in late December or early January, at least a month before your intended spring crop drilling date. 

Most cover crops, unless we get some decent frosts or you have livestock available for grazing, will need decent doses of Roundup (glyphosate). Try to apply on a bright, sunny morning onto a dry leaf. Don’t apply in a dew. That might mean you only have a window of 1-2 hours in a day to spray, but it’s important to make every factor as conducive as possible to the cover crop taking up the glyphosate in the cooler conditions. 

Check what species are in the cover crop mix, and what growth stage they’ve reached when spraying. Last season, there were some issues around applications not being as effective as they could have been because lower doses than what was required to control the species or growth stages present. In some cases 1080g or even 1400g a.s/ha might be required.  

Using a Roundup branded formulation with their built-in surfactant system with a wetting agent will also help droplets spread across big waxy or hairy leaves and lead to better uptake of the product.  

It's possible you might need a second application because of shadowing effects from big canopies, or because once the canopy disintegrates and light gets into the base, it can encourage some germination of grassweeds.   

5) Get home-saved spring crop seed tested  

If you have home-saved spring barley seed to drill next spring, consider getting it tested for seed-borne diseases. Diseases like loose smut can build slowly over a couple of season without being obvious and then explode once infection levels have built up, as we saw a couple of seasons ago.  

A seed test helps minimise that risk and allows you to get it dressed with something like prothioconazole if necessary.   

6) Check Dekalb maize varieties 

Maize could be becoming the successor to oilseed rape in a few more rotations across the southwest this season. When you start thinking about maize varieties, which fields they should be drilled in, looking at aspect and favourability, check whether any of the new Dekalb varieties will fit your requirements. We’ve had some good results in the region with varieties in the medium maturity class on favourable sites. 

 Liberator® contains flufenacet and diflufenican. Proclus® contains aclonifen. Atlantis® OD contains mesosulfuron and iodosulfuron. Roundup® contains glyphosate. Liberator, Proclus, Atlantis and Roundup are registered trademarks of Bayer. 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 2024.