How to manage weeds and diseases in cereal and oilseed rape crops in the west in October

Crop Progress

We’ve had 35mm of rain in August, albeit in one event towards the end of the month, and another 36mm in September so far.

Oilseed rape is, as usual, all over the place. It’s a bit ironic that the crops drilled early on, when there was concern over lack of moisture, did actually get some moisture and are looking alright.

Other oilseed rape crops are struggling – there have been more slug problems than growers probably expected. Cabbage stem flea beetle has also been worse than last year.

Cereal drilling has started and continuing apace. But it’s been good to hear that growers are drilling the fields with fewest grassweed problems first.

Gareth’s agronomy tips for October

1. Choose glyphosate brands carefully

Where you are spraying off stubbles pre-drilling it’s important to use the correct dose of whatever glyphosate brand you are using for the weed target and size. For grassweeds at 2-3 leaves use a minimum of 540g a.i./ha, while if you have tillering grassweeds up that to a minimum of 720g a.i./ha.

While we don’t have any resistance issues in the UK, be mindful that we don’t want any repeat doses to surviving weeds, so if you’re in that situation do a cultivation in between applications to remove survivors.

Coupled with that formulation of product does make a big difference. I’ve had clear feedback that, because of the glyphosate availability challenges, growers have used different formulations and they are seeing clear differences between brands, with some of them not very good at all.

Where the application will be the last one pre-drilling, ideally leave it as late as possible before drilling, especially if you’re not going to do any cultivation before or when drilling.

The alternative is to drill and put the glyphosate in with the pre-emergence application but check tank mix recommendations as different brands have different recommendations.

2. Be cautious with pre-emergence stacks in winter barley

The first question around winter barley where you have significant grassweed issues is whether you should be even growing it because you are limited in the options for control, and the crop is more sensitive to those products too. Stacks, therefore, need a bit more caution than in winter wheat.

But where you are drilling winter barley in October with grassweeds likely to be present, this year there is the option to use Liberator (flufenacet + diflufenican) + Proclus (aclonifen) as a pre-emergence only treatment.

The Proclus rate is lower than in wheat at 1.0 L/ha, and we don’t back any additional tank mix partners. It brings a new mode of action into the barley crop.

Where the crop has already been drilled, or later in the month, a follow up residual herbicide spray might be required. It’s better to do this at peri- or very early post-emergence – perhaps a couple of weeks after the pre-emergence spray, as the only true contact post-emergence option is Axial Pro (pinoxaden) to which there are significant resistance issues.

Options for a residual top up include Octavian Met and Alternator Met (flufenacet + diflufenican + metribuzin) at 0.5 L/ha. The Metribuzin gives a little bit of contact activity on top of the residual activity.

Those products, along with straight Liberator, are pretty good in annual meadowgrass and broadleaf weed situations in both wheat and barley. You might need to add in a partner to broaden the broadleaf weed spectrum in some cases.

3. More modes of action useful for wheat weed control

In wheat you have a few more options where grassweeds are challenging. I’d use Liberator + Proclus as the base – Proclus can be used at 1.4 L/ha in wheat – and then add other actives if you need to add more into it for black-grass, Italian ryegrass and bromes. Pendimethalin, prosulfocarb and tri-allate will all bring something to the party.

We see a real benefit in mixing different actives rather than just relying on one active. The one thing to remember is the more you put on the more risk there is of crop damage. The biggest risk factors are applying complicated tank mixtures just as the crop is about to emerge, on light stony soils, with heavy rain forecast for just after applications. Add in overlaps on your ins and outs when spraying you have a good recipe for causing damage.

Drilling date and timing of pre-emergence will determine how likely and urgent it is whether you need a follow up. The pre-emergence activity won’t last as long on early drilled / sprayed crops.

If you have a mat of grassweeds coming up in the crop then while you probably don’t want to hear this, you should consider pressing the reset button and starting again. If it is not many weeds then topping up with the metribuzin-based products is an option, with the addition of Atlantis OD (mesosulfuron + iodosulfuron) a possibility if you need more contact activity.

4. Monitor for oilseed rape diseases

Look at the forecasts for the key autumn oilseed rape diseases – Phoma and light leaf spot, or use our SpotCheck leaf sample service in conjunction with ADAS to check leaves for disease.

The risk should be quite low for Phoma currently as it is all based on rainfall, although plants are quite small which will increase risk of spores getting into the stems and causing stem cankers later in the season.

Phoma tends to come in a bit earlier than light leaf spot, but most growers will only want to do one autumn spray. I think it is important to use products that have activity on both diseases – too often growers apply something for Phoma and forget about light leaf spot, which is potentially the more damaging disease.

Both Proline (prothioconazole) and Aviator (bixafen + prothioconazole) have activity against both diseases, and there is also research by the University of Nottingham that shows rooting benefits from using Aviator.