Growers with blackgrass hoping to continue winter cropping must strive to get background populations of the weed down to no more than 10-15 plants/sq m, NIAB TAG‘s Jim Orson said.

“Blackgrass is now the master,” warned Mr Orson, who suggested that some growers already had to act on the assumption they could not rely on Atlantis (mesosulfuron + iodosulfuron) to control the weed.

Using an updated model developed by Rothamsted Research‘s Stephen Moss, Mr Orson highlighted the differences in the levels of control needed to stop blackgrass building up within fields.

Growers using shallow (5cm) cultivations needed 99% control of plants. However, those ploughing to 20cm required only 90%; and with more cultural measures added, such as adopting higher seed rates, the control could be allowed to drop to 80%, he explained.

All other things being equal, current pre-emergence and early post-emergence herbicides could just about contain the weed in all-winter cropping rotations. But to prevent yield loss, the background levels of the weed needed to be low. That was because any plants surviving such treatments had the potential to produce plenty of seed because of compensatory tillering, particularly when crop competition tended to be low, he explained.

“For anything surviving a pre-em treatment survival is complete, and it’s able to tiller as if unsprayed.”

That made it particularly important to ensure such applications were made under conditions that encouraged their effectiveness. “You’ve got to get them on in the right conditions – not when the soil remains dry long after application.”

Colleague Ron Stobbart, examining a wide range of stacked and sequential pre- and peri-emergence herbicide regimes, said the most effective generally consisted of three to five active ingredients and tended to include flufenacet.

There were also signs, linked to a feature known as Henry’s constant, that some were more tolerant of dry conditions than others, tri-allate being more so than flufenacet. “We maybe need to consider more which products to use under particular conditions, especially when it’s dry,” he said.