6 October 1995

Conditions good for autumn weed control now, but…

GOOD conditions for autumn weed control could leave a sting in the tail for growers using some of the newer contact-acting products.

Moist but generally warm soils mean weeds are germinating fast in both oilseed rape and cereals.

Dense swathes of cereal volunteers in rape need tackling "without delay", urges ADAS. Carbetamex (carbetamide) or Kerb (propyzamide) can help where blackgrass resistance is a problem, says Cambs-based John Garstang.

Both products have wide application windows. But this years conditions tend to favour earlier rather than later treatment, he says. "Where youve had a good flush of volunteers after the combine most will have emerged and theres no point in waiting."

In the absence of resistance an appropriate "fop" or "dim" herbicide should be applied, he advises.

Rapid germination makes weeds in cereals vulnerable to early post-emergence treatment, but Mr Garstang says it is essential to time sprays correctly.

The problem this year is that the window for sprays which need to go on before weeds have germinated, such as Avadex (tri-allate), is quite tight. "Youve got to be fairly quick," he comments. Avadex is becoming part of many control strategies for blackgrass and sterile brome.

Ciba Agricultures Charles Paice agrees that timing is vital. But it is particularly important for products containing the contact-acting clodinafop such as Amazon, Hawk and Unite. "All our work shows the optimum timing for control of blackgrass is GS13 – the three-leaf stage. A useful guide is that in a normal year this comes six to eight weeks after drilling."

There should be little further weed germination after then, he believes. Where blackgrass control is known to be difficult, pre-emergence Avadex (tri-allate) should be part of a "total approach", he advises. "Where the blackgrass is entirely sensitive one application of clodinafop at GS13 should be enough."

The danger this year is that many growers were tempted to sow early, says Mr Paice. That means much of the blackgrass is already approaching the critical GS13 stage. "You have to hit it at three leaves," he says. "One possibility then is to use Hawk and follow up with full rate IPU in December/January."

Peter Taylor of Herts-based Samco says it is "still early days" as far as weed control in wheat and barley goes. "Where there is a carpet of blackgrass well probably use a contact material, clodinafop or Cheetah, at a reduced rate with IPU for some residual effect."

&#8226 Steve Cook of Hampshire Arable Systems says the main difficulty at the moment is obtaining product from merchants. "Most have only had a proportion of what they ordered." With spraying needs likely to come "a week to 10 days" early that poses a potential problem, he says.

Fast crop and weed emergence demands an early herbicide to hit weeds hard. But using a contact material could mean a second spray is needed to control weeds emerging later in the season, warn advisers.