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With cold weather putting a stop to herbicide programmes, two agronomists have advised growers to adopt a ’rapid reaction’ attitude to be ready to spray as soon as temperatures rise.

Black-grass pressure has mounted as emergence and growth continued throughout December and January. Now, plummeting temperatures have halted progress.

’This weather has put back Atlantis WG (iodosulfuron + mesosulfuron) and Broadway Sunrise (pyroxsulam + cloquintocet + pendimethalin) applications,’ says independent agronomist Patrick Stephenson. ’The problem is that we’ve a lot of black-grass out there and the temptation is to go and spray it.

’Daytime temperatures in January were often over 8oC, which meant grass-weeds were growing and getting bigger by the day. The cold snap will dampen growth but as soon as temperatures rise they’ll be off. By March or April we’ll be dealing with a different animal altogether,’ he says.

Sean Sparling agrees, ’When soil temperature is below 4oC, growth is negligible, plants are simply staying alive. Metabolism doesn’t stop but it does slow down a great deal. Currently soils are below 1.4oC.

’Atlantis WG has always performed better going into a cold snap than coming out of one and that’s what we are facing now. Grass-weeds were actively growing before atmospheric and soil temperatures fell, so it’s likely that Atlantis applied during that period will have been absorbed by those black grass plants,’ he says.
’Coming out of a cold spell, we need to be sure that target plants are waking up. It’s in those few days that we will achieve the best results.

’New products such as GF2070 (pyroxsulam + cloquintocet + flupyrsulfuron) and Broadway Sunrise prefer warmer conditions, as they require more active growth.

’I’ve always been more comfortable going on with Atlantis WG in colder conditions but active growth is still needed and I can’t see that happening within the next two or three weeks

’Once soil temperatures have risen night time frosts are less significant,’ he says. ’It’s diurnal temperature and sunshine that will warm up the soils prompting active growth. When plants get moving again it’s going to be slow ’ which means it’s important to get the other application variables right.

’A dry black-grass leaf and at least three hours of dry conditions following application will secure rainfastness. Nozzles and water volumes are significant factors, because it’s important to get good coverage.

’To tackle the grass-weeds when they are small the first seven to ten days of active growth are critical,’ says Mr Sparling.

’Bigger grass-weeds produce more amino acids and it’s these that work against ALS inhibitors,’ he adds. ’If the weather warms up then black-grass will get going and any comprising variables will have an even greater impact.

’By taking out bigger plants as early as you can, the crop becomes more robust and can then deal with any second flush itself, crowding out germinating weed seeds.’

Mr Stephenson and Mr Sparling agree that active growth is more important than atmospheric or soil temperatures. ’It has become evident as we’ve gained experience with sulfonylurea-based products that efficacy is more closely related to active growth of target weeds rather than temperature. Weather has an influence through impacting growth," explains Mr Sparling.

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