16 August 2002


HARDER-TO-CONTROL grassweeds, plus higher populations in crops following last years surge into earlier drilling, mean growers must take a fresh look at stale seed-beds this autumn.

Grassweeds have stolen a march in some fields over the past 18 months, forcing up the level of herbicide spending, but still resulting in disappointing levels of control, says Steve Portas, technical manager for Dalgety Midlands.

The rush to get crops established last year also meant growers failed to clean up problem fields.

"Last year, in the rush to get early wheat in, many growers found to their horror that the main flush of autumn germinating blackgrass came in the crop. A wet half of October meant grassweeds soon got well established, and resistant blackgrass was impossible to control."

Pointing to a Dalgety survey this April, Mr Portas says stale seed-beds are now the most critical part of grassweed control. Where resistance was a problem two sprays of glyphosate pre-drilling reduced grassweed numbers around 80% prior to a full programme of carefully timed blackgrass sprays. The combination gave very high levels of control.

"Stale seed-bed production starts with the combine. Ensure a good chop and even spread of straw. Next, disc to get some straw/soil mixing. Then, consolidate to preserve moisture and give good soil to seed contact to encourage weed germination."

For bad blackgrass situations, patience is needed because at least one burn-off is essential. He also advises growers to reconsolidate after ploughing, because soils may be drier, so weed germination may take longer – especially if unweathered clods are brought up.

As for herbicides, Mr Portas recommends Touchdown (glyphosate trimesium) plus novel adjuvant Hornet, a new product tried on farm last autumn. Speed of kill is increased and rainfastness is down to 45 minutes – quicker than glyphosate formulations alone. Both are crucial for quick turn around, he says.

Advised Touchdown dose is 1-1.75 litres/ha. But many standard glyphosates have insufficient wetter at these rates and hard water also reduces activity.

"Adding Hornet first to a spray tank full of water before Touchdown has given the highest weed control in the shortest time in trials at Throws Farm." &#42

Glyphosate or paraquat?

Pick your stale seed-bed herbicide wisely and you could end up with better weed control, faster kill and a possible reduction in BYDV, says Syngenta brand manager Alison Bosher.

While Touchdown (glyphosate) is better against perennial weeds or annual grasses with two or more leaves, Gramoxone (paraquat) suits smaller annual grasses and broadleaf weeds.

Gramoxones faster activity also gives a quicker burn off in catchy weather and when growing conditions are poor, and could help where there is risk of BYDV.

ADAS research showed an 80% reduction in aphid numbers 5 days after paraquat application, compared with a 20% increase in numbers 5 days after glyphosate.

"Use Touchdown for perennial and large annual weeds, and where theres no problem with the weather and BYDV," says Miss Bosher.

"Gramoxone has a cultivation interval of only 4 hours and is rainfast in 10 minutes. So use it in catchy weather," she adds.

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