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Crop Advice & Expertise

Four detailed insights into cereal weed control and oilseed rape disease threats this October in the Midlands

Ben Giles provides his advice on how to control grassweeds and oilseed rape disease in the Midlands

Crop Progress

Oilseed rape establishment is highly variable again ranging from crops with 7-8 leaves to stuff that was written off in early September. It’s partly drilling date, but mostly the lottery of moisture. Even on the same farm, there are fields that haven’t made it, a few miles away from those that have.

The earlier drilled crops have generally escaped adult cabbage stem flea beetle, although some succumbed to slugs. Some of the later drilled crops both hit a dry spell and that warm weather at the end of August when flea beetles came out in force.

But there’s more this season that’s looking good than perhaps even last season – probably because a greater percentage was drilled earlier.

Some people had already ventured out to start drilling wheat by the third week of September, but relatively few. Where they have it is more in the Cotswolds where grassweed pressure is generally lower than in Bucks and Northants.

But most growers were holding their nerve, although probably not for much longer. I think the 10th October rather than the 15th might be a common point to kick off. Luckily most have been saved by reasonable yields and prices last season so it’s not like the awful October drilling window last autumn has been as detrimental to business as it could have been. Hopefully that will mean growers see sense as they know what will happen with grassweeds if they drill early.

 

Ben’s agronomy tips for October

1. Cover early drilling weed control windows

Anything that was drilled in September you would hope was only drilled then because of lower grassweed pressure. But it does mean a longer emergence window in the crop, so how do you cover that window effectively when temperatures are warmer, and it is drier? It might mean slightly higher herbicide spend or splitting the spend not all in one go. That’s becoming more popular not only because it prolongs the period of residual cover but also because you’re not hitting the crop hard by having a heavy stack all going on together.

Splitting becomes more of a challenge as drilling becomes later as there are fewer opportunities to travel on fields , which is why one hit residual mixes tend to be more popular as drilling progresses. If you can still split, my feeling is that trials show that it is the approach that gives the most consistent results.

I’d be looking to apply up to 360g/ha of flufenacet in mixture across the programme. Some growers will be able to get away with less depending on what you are mixing it with but using high doses of straight flufenacet is lunacy.

Our recommendation would be to partner flufenacet with diflufenican and aclonifen (Liberator + Proclus). Other additions are available on top if you have high populations of difficult grassweeds – so triallate helps with autumn bromes and wild oats, while prosulfocarb or pendimethalin are preferred options for ryegrass. All three will give useful additional activity on black-grass. With these mixtures you do need to trade-off efficacy against potential crop effects and cost.

 

2. Follow up pre-emergence applications within 10-14 days

If you are splitting applications or need another residual application then you should be following up ideally within 10-14 days after the pre-em to get the most out of it.

That often means you’ll need a different flufenacet brand to what you applied at pre-emergence.

I would also try to use different mix partners to what you used previously just to deliver a little more active and different modes of action.

Before you all starting jumping up and down about excessive chemical use, that’s top end for the most difficult situations. A lot of growers will get away with a lot less than that through all the work they have been doing using cultural controls, spring cropping, etc., to bring down populations and rely a lot less on chemistry, which is definitely the right way forward.

 

3. Concentrate on pre-emergence in any later drilled winter barley

Winter barley is a more difficult animal for grassweed control, partly because of the lack of post-emergence options, and partly because of its lack of tolerance to big mixes compared with wheat.

Compared with wheat there’s no aclonifen, so you’ve lost one of your big modes of action, so Liberator plus pendimethalin or triallate are good options. Liberator + prosulfocarb is another option if you have ryegrass or a mix of weeds. In general, not going quite as high on the stacks is the rule.

Follow up residual applications are less common in winter barley – you’re using barley’s competitive nature to do what half the herbicide in wheat is doing for you, particularly if you’re growing hybrid barley or very vigorous two-rows. In reality, if you want to grow barley and you know you have a bad grassweed problem, you’re probably better off waiting until the spring and growing spring barley. Winter barley is a good entry for oilseed rape, but if you have very high black-grass problems is barley followed by oilseed rape the best option for getting good control across a rotation?

 

4. Assess whether you need an autumn disease control spray in oilseed rape

With more early crops there is the potential for a longer disease window in the autumn, but because a lot of them have got quite big quickly you’re also reducing the threat from Phoma. That’s because if you have big leaves it takes longer for the Phoma infection to get through the leaf into the petiole and down into the stem.

Most varieties have very good Phoma resistance, which will also help out potentially, so it is a case of walking crops and assessing risk.

On the flip side early drilling increases risk from light leaf spot and if it is warm and wet enough you could see that disease pre-Christmas, although it is rare to visually see it in fields this side of Christmas.

Our free SpotCheck service, where you send leaf samples to ADAS to assess disease levels will be available from early October, so why wouldn’t you make use of it? You can register for a sample pack online at cropscience.bayer.co.uk/bayer-spotcheck. Last year it showed the earliness of the infections, giving an early warning of the threat.

The dilemma is sometimes whether to spray pre-Christmas. While it is easy to show improvements in disease control in trials, the yield effect is trickier to prove, unless you get bad infections and light leaf spot gets up onto the pods or Phoma causes stem cankers.

If SpotCheck found light leaf spot or you’re visually seeing it in the field this side of Christmas I would be applying a fungicide. Typically, you would only want to spray once. The exception might be if you find Phoma early on, especially on smaller crops above the 10% threshold.

But barring that the later one-spray option fits in nicely with propyzamide applications, which will likely be prothioconazole-based. Using alternative chemistry, such as in Aviator (bixafen + prothioconazole) because of reported shifts in light leaf spot sensitivity to azoles, will mean both actives are working on the disease.

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