Time to test herbicide-resistant blackgrass

Blackgrass control this season has proved patchy and growers are being urged to test seed from plants showing some form of herbicide resistance in the next fortnight, to aid decisions for the coming season.

New guidelines issued by the Weed Resistance Action Group highlight that timing is critical in getting the right results from resistance testing.

Testing for blackgrass resistance is a valuable management tool, says Stephen Moss of Rothamsted Research, who is also a WRAG member.

Last autumn’s spraying campaign was difficult. “The dry conditions were not conducive to the key pre-emergence herbicides working, putting more reliance on post-emergence sprays,” he says.

“If pre-emergence sprays are falling down, more reliance is put on the post-emergence applications and some are becoming less effective because of increasing resistance.”

Fast forward to the present and the level of blackgrass infestation is clear to see, and the recent spell of poor weather is not helping the farmers’ cause.

“In addition, the recent wet conditions have encouraged tillers to produce more seed heads.”

“Herbicide resistance testing lets you know what you are dealing with and how severe the problem is,” says Dr Moss.

The WRAG guidelines offer advice on making certain sample collection, testing and interpretation are carried out correctly. Resistance tests are only as good as the seed sample collected and the extent to which it represents the suspect population, he says.

In order to get the best possible seed sample, growers should collect blackgrass seeds in winter cereals from early July to early August and on a dry day.

But collection should not happen until most seeds are mature and 10-20% have already shed, say the guidelines.

There are several different tests available that give varying amounts of information to the grower.

The key choices are two glasshouse methods – one growing seeds in pots and the other using plants that are collected from the particular field and then allowed to regrow.

Alternatives include a petri-dish germination test, molecular tests in a laboratory and radio-labelled lab assays.

“It is important to establish exactly which test best serves your particular problem,” says Dr Moss.

“For blackgrass, I believe a test with Atlantis (iodosulfuron + mesosulfuron) in pots plus Laser (cycloxydim) and Stomp (pendimethalin) in petri-dishes is a good option, giving a good resistance profile,” says Dr Moss.

WRAG seed collection guidelines 

• Collect seeds on a dry day by rubbing heads over a bag or tray so only ripe seeds fall off. Aim to collect a cup full of seeds.

• Make sure samples are representative of the problem field by collecting seeds from many plants in an area of about 100m by two to three tramlines, unless the problem occurs in a smaller, distinct patch.

• Allow seeds to air-dry in a shallow, open container for a few days and then transfer to paper envelopes for storage and transport (never store in polythene bags).

• Send samples to the testing centre as soon as possible after collection.

See the new guidelines at http://bit.ly/KHUU62

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