22 June 2001

Seed test important to beat blackgrass

HERBICIDE resistant blackgrass is now widespread across England, says Stephen Moss of IACR-Rothamsted.

"We are not quite so certain of the frequency of the different resistance mechanisms," he says. "Enhanced metabolism is the most common form, but we are detecting more target site resistance than before."

That can make a crucial difference to grassweed control techniques. So investing a little time and money in sampling and testing now could be well worthwhile.

Resistance testing

The Rothamsted Rapid Resistance Test is offered by ADAS and Oxford Plant Sciences, as well as most agrochemical distributors and some crop consultants. Cost is set individually, so check locally for details.

As seed is germinated in the presence of herbicides the results are back by the end of September, so can be used to help with herbicide choice.

"The test identifies the resistance mechanism present, and the degree of resistance, so advice on herbicides can be quite specific," says Dr Moss.

Seed collection

For blackgrass, the best time to collect seed for testing is usually the second and third week in July, when about 10-20% of seeds have already shed.

A common mistake is to collect too early before seeds are viable. In spring sown crops, seed collection may need to wait until August.

"Collecting ripe seeds should be done by gently rubbing the heads over a polythene bag," he notes. "Wild oats are most easily collected by holding the panicle inside a bag and shaking vigorously."

The best time to collect wild oats for testing is in the second half of July or early August, while Italian ryegrass should be done in early to mid-July.

Aim to collect seeds over an area of about 100m by 2-3 tramlines, within the main problem area, advises Dr Moss. "The sample needs to be representative of the problem field, so a few seeds from lots of heads should be collected."

Although only a few hundred seeds are required for the test, growers should collect a mugful to allow for drying and cleaning losses. Avoid collecting in very wet conditions, as this can cause seeds to become dormant, and do not store them in polythene bags.

"Once collected, transfer seeds to paper envelopes for transport and storage. Label the envelope with the name of the field, farm and date of collection, and air dry the seeds as soon as possible.

"Small samples can be dried in the envelopes by standing them on one end with the flap open. But larger samples are best dried in trays placed in a dry, ventilated situation. They should be dry within a week."

Test results

Growers receive results with an interpretation.

When resistance is absolute, which tends to be the case with target site resistance, the interpretation is relatively easy. "But more commonly resistance is partial, making interpretation and advice more difficult," says Dr Moss.

"Plants may also have one or both types of resistance and can show complex patterns of cross-resistance to different herbicides." &#42