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Talking Agronomy with Chris Martin: carefully targeting oilseed rape management


 They say fortune favours the brave and that has certainly been the case for growers in the region suffering with grass-weed issues who delayed wheat drilling until the middle of October.


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The weather stayed kind enough to allow drilling into almost perfect seedbeds during this period, allowing the peak flushes of black-grass and rye-grass to be controlled prior to drilling the crop. There was also plenty of moisture giving residual herbicides the best possible chance of working. In some earlier drilled fields, however, black-grass is already present at alarming populations, which is going to put incredible pressure on our ever diminishing chemistry set.


Slugs have been extremely challenging on notorious fields, to the extent where some redrilling has occurred. As we move through the autumn, most crops will have received their maximum total dose from August 1 to December 31 of 210g/ha of metaldehyde recommended in metaldehyde stewardship guidelines, and growers should now move onto ferric phosphate-based pellets.


See also: Wet weather proving ideal for slugs


While you may not see the same degree of slime trails and dead slugs on the surface as you would expect to with metaldehyde-based pellets, ferric phosphate pellets such as Derrex are a very effective slug killer, and really proved themselves as the industry-leading pellet in the last bad slug autumn of 2012.


Oilseed rape seems to be in two camps. The early crops have never looked back and are proving both useful game cover and a boot cleaning service for the shooting parties. Crops which are going to reach 25cm in height by the end of autumn growth are likely to respond well to autumn growth regulation, and this should be applied between the 4 to 6 leaf stage for maximum effect.


Many backward crops, however, continue to struggle and have been plagued with cabbage stem flea beetle despite multiple insecticide applications with different modes of action. Attention now turns to flea beetle larval invasion, which can occur any time between late-October and early spring. If damage exceeds the thresholds of more than 50% of leaf petioles damaged or more than five larvae per plant, then a pyrethroid spray should be applied.


Peach potato aphid

Peach potato aphid

Oilseed rape crops should also be inspected for peach potato aphid (myzus persicae), which are typically found close to leaf veins on the underside of the leaf. Due to peach potato aphids displaying high levels of resistance to pyrethroids, where they can readily be found products with a different mode of action such as Biscaya (thiaclopid) or Plenum (pymetrozine) should be used to reduce the risk of turnip yellows virus.


Light leaf spot tends to be the big potential disease ‘yield robber’ in oilseed rape across the region and with few varieties in the ground having solid resistance to the disease, crops are worryingly vulnerable, and if the traditional ‘bonfire night’ light leaf spot spray has not yet been applied, it should be applied soon as a dry leaf is available to control the early epidemics which are the most damaging.


For optimal grass-weed control in oilseed rape, applications of propyzamide should be made when soil temperatures have got down to 10degC and falling, and there is sufficient soil moisture in the soil for plant uptake. Both these criteria are now met in most soils, so applications should be carried out at first opportunity; however, they must only be made after taking all necessary precautions to avoid contaminating surface waters.


If heavy rain is forecast the responsible course of action would be to delay the application. The longer the period of time, between application and a severe rainfall event the less likely it is propyzamide will be lost through surface run-off and drain flow, allowing it to remain in the zone of activity for longer optimising efficacy.

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