6 September 2002

Resistance threatens early OSR

By Andrew Swallow

WARM, wet conditions have seen early broadcast oilseed rape get off to a cracking start – but so have grassweeds, warns Farmacy.

"Our main concern is that the grassweeds may get too big to give adequate control in some of these crops," says technical manager Pamela Chambers. "You need to get to it with a spray very quickly," she stresses.

Some early August-broadcast crops in Lincs had blackgrass at first leaf in them by last week. The weed could be past the critical three-leaf stage by mid-September, she warns.

If it is known to be resistant, or has proved difficult to control, then growers must opt for Aramo (tepraloxydim) and at the full, 1 litre/ha recommended rate, she says. "It is too much of a risk reducing rates with known difficult blackgrass."

A follow-up spray of propyzamide or carbetamide (as in Kerb or Carbetamex respectively) later in the season should be planned.

"The advantage of using these products is that no known resistance to them has been recorded, and you must look at oilseed rape as an opportunity to get on top of grassweeds for the whole rotation," says Miss Chambers.

Where older fop and dim herbicides do provide reliable control, or growers are targeting only volunteers, then Laser (cycloxydim) at 0.5-1.25 litres/ha plus methylated oil or Fusilade (fluazifop-p-butyl) at 0.2-0.75 litres/ha plus Partna maybe cheaper for the first spray on early broadcast crops.

"Rates will depend on species of grassweed present and size."

Full rate Aramo costs just under £19/ha, Laser plus oil £15-36/ha depending on rate, and Fusilade plus Partna £10-30/ha.

Growers should note that Butisan (metazachlor), Katamaran (metazachlor + quinmerac), trifluralin and Centium (clomazone) cannot be applied to Autocast or broadcast crops before crop emergence.

Later sown crops, established into cultivated seed-beds, will probably have blackgrass emerging from greater depth, and over a longer period, she notes.

In those, a well timed propyzamide plus contact material could suffice in less difficult situations, but a split approach, hitting blackgrass before it gets to any size, is preferable.

"Propyzamide activity goes down to about 1.5cm so weeds emerging from below this depth may not be controlled," she warns.

An insecticide should be included if there is visible flea beetle damage or a sizeable population of potentially virus carrying aphids. Beet Western Yellows virus (BWYV) may cause yield depression in oilseed rape. &#42

Dont be caught out by grassweeds in rapidly growing Autocast oilseed crops, warns Pamela Chambers.

&#8226 Beware rapid grassweed growth.

&#8226 Herbicide needed asap.

&#8226 Check labels for trash limits.

&#8226 Tune product to problem.