Weed control advice for East Anglian growers in October

Crop Progress

Some oilseed rape growers took advantage of moisture in early August to get the crop planted, while others have waited until early September as it became trickier later in August. It’s about 50:50 between where it has emerged and looks okay without too much damage, and crops getting annihilated by either flea beetles or slugs.

A decent amount of groundwork has been done in August and September ahead of cereal drilling with it seems more growers deciding this year to rotationally plough or do more cultivation to bury weed seeds.

Rachel’s agronomy tips for October

1. Decide on aim of glyphosate pre-drilling sprays

A lot of data suggests the closer to your drilling date you apply glyphosate to stubbles or stale seedbeds, the better weed control you get as obviously more weeds emerge, and as long as you are using the correct dose of glyphosate, that will kill everything off pre-drilling.

But we also have newer research that suggests if you’re looking for more barley yellow dwarf virus protection by stopping aphids feeding and removing the green bridge for carry over, then applying glyphosate 10-14 days beforehand will help as well.

That means it can be a bit of a balancing act because it’s not good practice to make repeat applications of glyphosate because of the resistance risk. Ideally, you’re looking to use it once or twice at most, using the correct dose to avoid leaving any survivors, and potentially going back in with cultivation to manage any survivors before any subsequent applications.

So in East Anglia, I think the focus will likely for many be more on weed control as that’s the greater risk, with applications closer to the drilling window.

Check what weeds are in each field and their growth stages to help adjust Roundup (glyphosate) rate for the target. Once grassweeds start tillering they need a higher dose – a minimum of 720g a.s./ha compared with 540g a.s./ha while still 2-3 true leaves, and it could be as high as 1800g a.s./ha if there are perennial weeds present. Broadleaved weeds, such as volunteer oilseed rape or clovers, also don’t take up glyphosate quite as effectively so need higher doses.

Keep an eye on spray application technique too, all the obvious things such as optimum water volume, using the correct nozzles and spraying in weather which will minimise drift, as drift essentially lowers your dose, and leaves you more at risk of hitting non-target crops or areas.

2. Leave wheat drilling as late as you dare where there are difficult grassweeds

As with oilseed rape drilling date for wheat can be a toss-up between going earlier or later. The wetter weather in the past couple of months will be helping weeds chit, so it has been good weather for black-grass control. Waiting for as long as possible to get as many flushes of weeds within reason is probably more important than targeting any particular drilling date.

Later drilling dates will not only help with grassweed control but also reduce Septoria and barley yellow dwarf virus risk. But as late as you dare always sounds better when writing because it can be practically very difficult to actually to stick to in some years.

3. Base pre-emergence sprays around weed germination windows

The main basis for weed control programmes obviously starts with knowing what weeds you’re trying to control. It’s also worth considering when the main germination windows will be – for example, with black-grass we know the germination window typically sits very heavily with cereal germination, while Italian ryegrass and bromes tend to be more extended or vary from autumn cereal drilling dates.

With black-grass that synchronicity helps explain why delaying drilling is more effective, especially in combination with using some strong pre-emergence residual chemistry, such as Liberator (flufenacet + diflufenican) plus Proclus (aclonifen).

Each of those active substances are in a different Herbicide Resistance Action Committee (HRAC) mode of action group, helping as a resistance management strategy and also increasing efficacy and bringing in control of other weeds.

The more active substances you add the more expensive the stack becomes, but prosulfocarb or tri-allate can also be useful additions in black-grass and Italian ryegrass situations, while tri-allate is also useful against bromes.

For black-grass situations, delayed drilling and a strong pre-emergence treatment potentially could see you through the autumn without requiring any top-up treatment.

But with Italian ryegrass where germination might be more protracted through autumn, then a top-up of something like Alternator Met or Octavian Met (both flufenacet + diflufenican + metribuzin) or flufenacet could be required.

The label for the metribuzin-containing products has changed for this autumn with it now be permissible to apply the full label dose of 1.0 L/ha up until GS25 of the crop, rather than there being a date and dose restriction. That means it could be a good option for later drilled crops as well as a top-up.

Many of these residual options can also be used on any winter barley crops remaining to be drilled, or if they are still pre-emergence when you read this. Winter barley is more sensitive, so the maximum dose of Proclus is only 1.0 L/ha compared with 1.4 L/ha in wheat, and we don’t support the use of any additional tank mix partners other than Liberator.

In both crops remember the rules about drilling depth and covering seed. These residuals are designed to create a layer of chemistry over the soil that the weeds pick up as they emerge, and are not meant to come into contact with the seed. Crop damage risk increases where crops have been drilled more shallowly than the recommended 32mm, especially where applications are followed by heavy rainfall, or where slots have not closed properly exposing seeds, for example.

4. Monitor for oilseed rape diseases

Deciding whether to apply an autumn fungicide in oilseed rape can be a difficult decision. You don’t want to apply one where the crop isn’t going to make it, but we do see a benefit from applying one in protectant situations.

Last year after the dry early autumn we didn’t see a huge amount of disease but this year it’s been perhaps more of a normal year so it will be worth monitoring for both Phoma and light leaf spot, considering varietal resistance, crop health and forecast risks to help make a decision over whether to apply something like Proline (prothioconazole).