Atlantis resistant blackgrass causes concern

Research uncovers new Atlantis resistant blackgrass fears and highlights the importance of cultural control measures. James Andrews reports

Blackgrass populations with enhanced metabolism resistance to key herbicide Atlantis, could be as difficult to control as those with target-site resistance, a research project has revealed.

The findings painted a gloomy picture for the continued efficacy of the product, Rothamsted Research’s Stephen Moss said at recent WRAG/Rothamsted Research Association herbicide resistance meeting.

Traditionally, enhanced metabolism was thought to be less of a concern than target site resistance, but it seemed to be building faster than expected, said Dr Moss. “It has surprised me that you can get such a high level of resistance from enhanced metabolism.” This meant growers would have to rely increasingly on pre-emergence herbicides and cultural control measures, he said.

The study compared Atlantis (mesosulfuron + iodosulfuron) efficacy in blackgrass that had not been subjected to the product with the same population that had been sprayed for five consecutive years.

When the two populations were sprayed with Atlantis last autumn, most of those that hadn’t received Atlantis before were killed, while most of the selected plants survived, he said. “We are 95% sure this is due to enhanced metabolism.”

It showed enhanced metabolism resistance could develop a robust resistance to Atlantis in a short time, he said. In populations with enhanced metabolism resistance, control was only marginally better than with target site resistant populations. Atlantis and pyroxulam both gave a low level of control, although adding another herbicide such as Liberator could enhance control of enhanced metabolism resistant populations, he said.

Limiting Atlantis use would help prevent further resistance build up, but there was no evidence to suggest resistance would decline. “It will be very difficult to persuade farmers not to use it as long as they are getting some control, but the key thing is to question whether you really do need to apply Atlantis.”

Growers would have to start adopting more non-chemical controls, while pre-emergence herbicides such as flufenacet would be increasingly important, he said. “Cultural methods offer limited control and are an expensive option, but we need to face up to the fact that they are necessary.”

Ploughing, lengthening crop rotations, spring cropping and delayed drilling would help reduce blackgrass populations, as would planting higher seed-rates and leaving ground fallow, he said. “But you need to do some sort of cultivation or spray glyphosate to control the blackgrass plants that germinate.”

Case study: Cultural control in Oxfordshire

Oxfordshire farmer Richard Davey was upping seed-rates to tackle Atlantis resistant blackgrass, said Dr Moss. “He has four fields with Atlantis resistant blackgrass and one field gave zero control due to target site resistance.”

Traditionally, he used low seed-rates and could achieve wheat yields of 11t/ha, but increasing resistant blackgrass populations meant he needed to consider cultural controls.

Trials with 150, 250, 350 and 450 wheat seeds/sq m were planted this autumn and sprayed with pre-emergence herbicides, said Dr Moss. “This has given an excellent 96% control so far, but there are still 20 plants/sq m.”

Previous trials on this field showed 25 plants/sq m could result in 5t/ha yield loss, so Mr Davey was still planning to use Atlantis as he has few other options, said Dr Moss. “He recognises that more drastic measures may have to be introduced in future, and to his credit, he and his consultant are thinking ahead.”