FURTHER EVIDENCE that grassweeds can develop resistance to new sulfonylurea herbicide Atlantis has prompted calls for growers to use the product with care.

Of six ryegrass populations tested, one from Essex has shown clear evidence of resistance, while two others showed more marginal evidence, says grassweed expert Stephen Moss of Rothamsted Research.

Each sample came from fields where Atlantis under-performed. In one field quite a lot of ryegrass plants survived treatment with the full field rate of Atlantis, he notes.

Enhanced metabolism resistance, which affects most herbicides, could be to blame, Dr Moss suspects. “There is quite good correlation in the tests between the insensitivity to Atlantis and that to diclofop, as in Hoegrass.”

But on-off target-site resistance is also feared in blackgrass, after detailed tests on seed from another Essex field (Arable, Aug 6 2004). “Everything from the biochemical assays and genetic studies we are doing points to it being target-site,” says Dr Moss.

Field trials are checking whether the glasshouse finding can be replicated in the field. “I am pretty confident it will be,” he says.

But five more recent blackgrass samples from other problem fields have shown no lack of performance from Atlantis, he notes.

Manufacturer Bayer CropScience says its tests echo that. “There is no case in the handful of poor field performances by Atlantis against blackgrass that can be attributed to resistance,” stresses the firm”s Gordon Anderson-Taylor.

Poor performance could stem from other causes, including poor application and bad weather, he notes.

But, in the light of the latest findings, growers should not be complacent, says Dr Anderson-Taylor. “Ryegrass probably presents a greater challenge for Atlantis than blackgrass. It is harder to control difficult ryegrass than it is difficult blackgrass.”

Timing is also more of a problem. “Hopefully, growers will have got on some residual herbicide this autumn, but they must not get complacent even if they have. The message about early applications where there is enhanced metabolism resistance still apply, although you can”t go too early, otherwise you are not using the contact activity of Atlantis to its best,” he says.