Testing for blackgrass resistance will take on increased importance this season with the confirmation target-site resistance to both the fop/dim group and grassweed sulfonylureas can occur in the same plant.

The concern is the possibility of this “double target-site resistance” building up quickly with the continued use of high-risk herbicides, Rothamsted Research’s Stephen Moss says.

Most arable farmers are aware of target-site resistance to the fops and dims, as at least 40% of the farms that spray regularly against blackgrass have got the problem lurking in their fields, he notes.

“But there’s another type of target-site resistance out there, which gives resistance to grassweed sulfonylureas, such as Atlantis and Lexus.”

Much less is known about the extent of this resistance.

“It seems to be on the increase.

And it’s worrying, because there aren’t any herbicides with new modes of action coming through to help.”

Where these two types of target-site resistance occur in different plants, there are still opportunities to control them by rotating or mixing fops/dims and sulfonylureas.

But if both mechanisms are found in the same plant, the options are limited.

“It’s the nightmare scenario,” he says.

“Neither fops/dims, sulfonylureas or even mixtures or sequences of both of these will be able to control these plants.

If such a population builds up, it leaves you with very few herbicides to choose from.”

Fortunately double target-site resistance has not been a problem on a field scale yet, but blackgrass plants with both types of resistance have been found in commercial fields.

Tests done at Rothamsted Research have confirmed the existence of both target site mechanisms.

“We don’t yet know why this has occurred as the sites concerned don’t appear to have atypically high historical use of sulfonylureas.

Possibly the use of chlorsulfuron-based products in the 1980s may have affected the weed plants, but there’s little hard evidence to support this theory at present.”

Dr Moss is certain about one thing.

“This double resistance will only occur where you’ve already got fop/dim target site resistance.

That’s why seed testing is important – you can identify any fields at risk.”

Identification matters because the only herbicides which are likely to work once double target-site resistance builds up are IPU and the pre-emergence materials.

“And as there are question marks over the continued use of IPU, it’s likely to come down to the pre-emergence products.

And by their nature, they are variable.”

Any suspicions of such a problem should be followed up immediately, he advises.

“Get a seed test done.

Don’t wait until the following year to try and confirm your suspicions.

If you can pick it up early, you are more likely to deal with it successfully.”

He stresses there’s no need to panic.

“Double target-site resistance isn’t giving growers a major problem at present.

But it’s important they appreciate it already exists out there and using high risk herbicides, such as Atlantis, is only going to hasten its onset.”

The Rothamsted Research stand at Cereals 2006 will have a demonstration on double target-site resistance, and Dr Moss will be available to answer queries and talk to growers about the problem in grass weeds.

fwarable@rbi.co.uk