How to beat weeds and diseases in southern crops in October

Crop Progress

Oilseed rape crops are a mixed bag. In parts of Hampshire up to 100mm of rain has fallen in September, but the higher amounts dependent on sporadic heavy showers. Where growers have been fortunate oilseed rape crops are looking good.But where less rain has fallen in parts of west Hampshire, East Sussex and Kent many oilseed rape crops are struggling, having taken some time to get going.Establishment concerns have been exacerbated by cabbage stem flea beetle attacks. There have been more reports of the beetle in Hampshire than previously, although some areas have been seemingly unaffected. Slugs have also been a perhaps surprising threat this autumn, especially on heavy soils.Growers, aided by good conditions, have cracked on with winter barley and winter wheat drilling in the latter half of September – hopefully in fields less affected by grassweeds. Most winter barley looks like it will have been drilled in September, and with growers unlikely to stop wheat drilling unless forced, we’re heading for an earlier drilled wheat crops than for several years.The moisture in September, and low black-grass dormancy, has also brought the first flushes of grassweeds, which have been sprayed off with 540g a.i./ha of glyphosate at 1-2 leaves to good effect.

Richard’s agronomy tips for October

1. Take out flushes of grassweeds before drilling

Where grassweeds pose a serious threat, if possible, delay drilling until mid-October by when most grassweed emergence will have occurred.These can then be taken out with a second dose of glyphosate, where an initial spray has already been deployed or a first spray if not, before drilling.Monitor grassweed size for correct dose, as well as broadleaf weeds that are present in the field. If grassweeds have begun tillering increase the dose from 540g a.i./ha to 720g a.i./ha of glyphosate.Pay attention to application – this isn’t a job you should rush as using glyphosate is the most effective form of weed control. Consider your sprayer set up by choosing the correct nozzles, boom height and forward speed for the task.Finally, monitor the field after application to make sure you have achieved a full kill, and if there are any survivors use cultivation to prevent any chance of resistance developing.

2. Timely application of pre-emergence herbicides important

For winter wheat weed control, getting off to the best start with pre-emergence residual herbicides isn’t just about product choice but also timely application in the best conditions.It’s important to apply pre-emergence herbicides to fine, consolidated seedbeds wherever possible to help safeguard against crop effects. Applying within 48 hours of drilling will also maximise efficacy, as once grassweeds start germinating, they become harder to control with residual chemistry.Our recommendation for targeting grassweeds is 1.4 L/ha Proclus (aclonifen) + 0.6 L/ha Liberator (flufenacet + diflufenican). If you need additional activity you can add in one further mixture partner, typically either prosulfocarb, pendimethalin or liquid tri-allate.We’ve seen an uplift in performance where Proclus is used on the main grassweeds, black-grass, bromes and especially Italian ryegrass. The aclonifen and diflufenican elements in the mix also help give a comprehensive spectrum of broadleaf weed control.The Proclus + Liberator tank mix is also available to use as pre-emergence spray in any later-drilled winter barley crops. In barley the rate of Proclus is lower at 1.0 L/ha, and the requirement for seed to be covered by at least 32mm of soil is just as, if not, more crucial than in wheat to mitigate any crop effects.For those who have drilled in September, especially in grassweed prone fields, will need to consider a residual top up mix in October when the crop starts to emerge. Options include 0.3 L/ha of Liberator or 0.5 L/ha of Alternator Met or Octavian Met (both flufenacet + diflufenican + metribuzin) in both winter wheat and winter barley.The metribuzin-containing products can also be considered in winter barley if the pre-emergence timing has been missed. The addition of metribuzin adds a little bit of contact activity to its residual activity, and also helps control some harder to control broadleaf weeds, such as groundsel, which is a growing concern in the southeast.

3. Monitor for oilseed rape diseases

With oilseed rape drilling date being later than the past few years, so far as we approach the end of September there is very little disease around. But that could change quickly with a wet October and smaller crops could be at greater risk from Phoma leaf spot. Smaller crops means the disease has less far to travel to get into the petioles from where it can cause stem cankers.Light leaf spot will remain a threat from this autumn through the cropping period. Last year’s SpotCheck showed nearly 40% of samples analysed in October carried light leaf spot.SpotCheck, as part of our wider CropCheck service, remains a free way of getting your crops checked for all oilseed rape diseases. If you’d like a sample pack please get in touch with me or another member of the Bayer team.The threshold for treating Phoma is 10% of plants infected in backward / small crops, or varieties with a lower resistance rating, or 20% infected for crops with good resistance.With most growers likely to only do a maximum of one application in the autumn, it’s worth considering using a product with activity against both key diseases. That could be Proline (prothioconazole) at 0.32 L/ha for just Phoma or 0.46 L/ha for both diseases, or Aviator (bixafen + prothioconazole) at 0.5-0.75 L/ha.As well as giving better control of Phoma and light leaf spot, research by University of Nottingham has shown Aviator boosts rooting, which could be a benefit this season in the later drilled crops.  

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Ella and James give a tour of Bayer's latest grass-weed matrix trial.

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