A further 52 cases of blackgrass resistance to Atlantis (mesosulfuron-methyl + iodosulfuron-methyl-sodium) have been confirmed by glasshouse pot tests.

Fortunately most have not been associated with complete field control failures, Rothamsted Research's Stephen Moss says.

In total 133 confirmed cases of resistance have now been identified across 21 counties using the test*, which uses seed collected before harvest from winter wheat fields treated with Atlantis.

"The two latest counties to be affected are Buckinghamshire and Rutland," he says. "Worst hit is Lincolnshire with 22 farms affected, Cambridgeshire has 16 cases, Suffolk 14 and Oxfordshire 12."

Generally good results

But considering the extent Atlantis is used the number of cases of resistance remains low, he notes.

All the samples are collected from fields where some blackgrass has survived, but in the vast majority of cases there wasn't a complete control failure. "The overall results in the field were generally good. But any resistance found in seeds collected from surviving plants should act as an early warning."

Testing helps growers keep up-to-date with evolving resistance, he stresses, as well as allow practical advice to be formulated.

LINK and HGCA-funded research is continuing into the resistance mechanisms involved, he notes.

"Finding out whether resistance is due mainly to the ALS target site mechanism or enhanced metabolism is important. It will tell us the speed at which resistance is likely to build up and if other graminicides might be affected."

ALS target-site resistance has been confirmed in eight of the populations, but most of the others haven't been tested for their precise resistance mechanism yet. "For those eight, and probably many others, it means that cross-resistance to other ALS graminicides is likely."

Where resistance is due to the target site mechanism, the effects develop rapidly and there's further selection with ALS graminicides, he explains.

"In contrast, enhanced metabolism gives a lower degree of resistance and may build up more slowly. This gives growers the opportunity to overcome it to some extent by applying Atlantis in the autumn, when the plants are small, in conjunction with a residual herbicide."

Cultural control methods

About three quarters of the Atlantis resistant samples also showed resistance to fops and dims (ACCase) herbicides, Dr Moss adds.

"This shows that two different types of target site resistance can occur in the same populations. It's a concern, as the number of alternative, lower resistance risk herbicides is declining."

Cereal growers should use cultural control methods wherever possible to reduce dependence on herbicides, he advises. "A robust pre-emergence herbicide should also be used, to take the pressure off the post-emergence sprays."

The post-emergence sprays should be applied when the weeds are small and more vulnerable to herbicides, he adds. "Try not to rely solely on high-risk herbicides, such as fops, dims and ALS inhibitors.

"And have a seed sample tested if you have any concerns. Detection of resistance at an early stage allows more time for formulating alternative control strategies."

Good stewardship of products

James Clarke, chairman of the Weed Resistance Action Group (WRAG), points out the latest resistance figures strengthen the need for good stewardship of products.

"They also demonstrate resistance is an ever increasing risk that is only likely to get worse, especially as there are fewer and fewer alternatives available."

Growers and agronomists have tried to reduce the pressure on Atlantis, Bayer's Gordon Anderson-Taylor says.

"We've seen how they've made good use of stale seedbeds and delayed drilling, and how the use of pre-emergence flufenacet-based products in sequence with Atlantis has been adopted."

The switch from spring to autumn use of Atlantis, with an effective residual partner, has also continued, he reveals.

"These latest results show that growers must continue to adopt appropriate resistance management strategies," adds Mr Anderson-Taylor. "Resistance to Atlantis is still uncommon, which is a testament to the efforts made to date. Everything must be done to keep it for the longer term."

* Testing is carried out by Rothamsted Research, Bayer CropScience and Syngenta.