Weed control help for cereal growers in the south

1. Weed control tactics for September drilled wheat 

Most growers heading into October are likely to fall in one or more of three camps, the first of which is where fields were drilled in September and treated with pre-emergence herbicides.

By around the middle of October, these fields will likely be hitting peak germination of black-grass, with numbers starting to decline subsequently. 

These will still be likely to need a top up residual herbicide in October, as activity of those applied starts to wane. Higher soil temperatures from earlier drilling will mean that decline in persistence is quicker than from a later applied application. 

There are decent options for top up residuals – it could be a straightforward Liberator (flufenacet + diflufenican). If you’ve already used that at pre-emergence a follow up rate of 0.3 L/ha would be advised. 

 We also have metribuzin-containing products, which are formulated with flufenacet and diflufenican. These fit the top up timing really well with contact activity coming from metribuzin on weeds like black-grass. They can be tailored to appropriate doses of 0.5 – 1.0 L/ha depending on grassweed pressure.  

The final option and perhaps the best as it proved to be very good in our trials last year, is cinmethylin following a Liberator + Proclus (aclonifen) pre-emergence. This brings good advantages for grass-weed control, and while costly it easily justified on the worst grass-weed fields, especially Italian ryegrass.  

2. Weed control for October drilled wheat 

The second scenario is those who are drilling at what we can probably call a more traditional drilling date of from the first week of October. These will need to lay down a residual herbicide to see you through grass-weed germination periods that run right through into November. 

Here using aclonifen, along with Liberator, for its persistency to get you right the way through that autumn germination period is probably the best way to start. These treatments may or may not need a top up depending on grass-weed pressure.   

The final scenario is in the very high grass-weed pressure fields where the best form of control is to delay drilling further into October. If you can remove the memory of last year and be brave in holding off drilling until after the peak germination passes, you win by getting the crop off to the best possible start for grass-weed control and have lower reliance on chemistry.  

This is especially the case for black-grass, but we’ve also seen delayed drilling helps with Italian ryegrass control as long as you can achieve a competitive crop. It just might not be quite as effective as with black-grass.   These crops will still need treating either with Liberator + Proclus or cinmethylin.   

3. What about winter barley? 

 Most winter barley should have been drilled in September and hopefully received a good residual herbicide in a timely manner, especially where there are grass-weeds.  

If you are still drilling winter barley in October and you’re using aclonifen, remember it must be applied pre-emergence, preferably within 48 hours of drilling. It’s a strong option in terms of grass-weed control in the crop, where choices are more limited – for example cinmethylin is not approved for winter barley. 

If your barley is on kinder ground for weeds and the crop has emerged, then an alternative option is one of the MET products. These can be mixed with products, such as Tower (pendimethalin + chlortoluron), which as a mix can be very effective against broadleaf weeds and annual meadowgrass.  

The MET products are also really helpful against groundsel and bur chervil. In a survey we did of growers about weed pressures, groundsel was the one that had increased the most, while bur chervil was a growing concern. A litre of a MET product will give very good control of groundsel, while a Scottish Agronomy trial suggested that a GS12-13 application of a metribuzin-containing product brought over 50% reduction in bur chervil, which was significantly greater than any other residual option in the trial.  

4. Resistance warning for azoles in oilseed rape 

 A reasonable number of growers held off planting oilseed rape into September to try to mitigate against flea beetle migration damage. Ironically perhaps, it’s turned into a year where we’ve been looking at each other trying to understand where the flea beetles are – up until now there has been very little pressure.  

It’s not too late as this is written on 10 September for the migration to occur as it can swing from one week to another, and last year some data from our demo site in Herefordshire using MagicTrap saw a spike around 19 September, so it is too early to say we’re out of the woods yet.  

The oilseed rape that has been planted though looks largely unaffected and is very forward for the time of year. That might mean some growth regulation will be required during October, if not before. 

One other area of interest is the finding in western Europe of resistance being found in Phoma leaf spot to azole fungicides. This is at low levels and hasn’t been detected in the UK, but overreliance on azole products such as prothioconazole and tebuconazole could lead to problems. 

A good strategy to mitigate that, if you need to apply a spray this autumn, would be to drop in an alternative mode of action, either a SDHI or strobilurin fungicide. Fortunately we have good varietal resistance for Phoma leaf spot, and the more forward crops will be less at risk of infections moving from lesions into the stem petioles.  

Later drilled crops might be more at risk though, while monitor the forward ones for light leaf spot, as conditions have bene favourable. 

Liberator® contains flufenacet and diflufenican. Proclus® contains aclonifen. Tower® contains pendimethalin and chlortoluron. Liberator and Proclus 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 2024.