Plant protection product manufacturer Bayer Crop Science has warned of early signs of grass weed resistance to the key herbicide active ingredient flufenacet developing in Europe.

The company’s grassweed resistance testing programme has found a few isolated cases of blackgrass on German farms with reduced sensitivity to flufenacet as well as confirming that ryegrass with enhanced metabolism resistance to flufenacet is present on a small number of UK farms.

The research has also indicated that the efficacy of pendimethalin and prosulfocarb herbicide actives was also affected by reduced sensitivity. When applied as single actives, both had lower efficacy against blackgrass with reduced sensitivity to flufenacet. The research work did not explore the efficacy of these actives when applied in mixtures and sequences.

Bayer stresses that there is no indication of flufenacet resistance in blackgrass in the UK or overseas but warns the reduced sensitivity in Germany should act as a “wake-up call” to farmers and their agronomic advisers. While responsible growers are now used to integrated control programmes to manage blackgrass, the company underlines that these techniques will be even more important for stewardship of this active for future years.

UK growers with a challenging ryegrass situation are advised to undertake resistance testing. Bayer advises that integrated weed management programmes to reduce weed pressure in the seedbank will be crucial to reduce further resistance development in all situations, but especially where resistance is confirmed.

“We always knew there was a risk to herbicides such as flufenacet of enhanced metabolism resistance affecting performance,” comments James Clarke, who chairs the Weed Resistance Action Group. “It will be particularly challenging on farms with ryegrass and it serves as a very timely warning for those with blackgrass. However, along with other active substances, flufenacet will remain a part of weed control programmes to deliver effective control.”

Bayer’s Dr Gordon Anderson-Taylor says the company does not anticipate these results leading to a radical change in blackgrass management. “A large number of growers have already successfully adopted measures such as delayed drilling, higher seed rates and spring cropping to manage black-grass populations down to lower levels before using herbicides,” he notes.

“But product stewardship continues to be of crucial importance. Most importantly, herbicides should be used in mixtures and sequences with other actives. The research focused on straight flufenacet but also showed that control of reduced sensitivity populations improved considerably by using flufenacet + diflufenican. This is good news for growers as co-formulated products like Liberator (flufenacet + diflufenican) have been the bedrock of pre-em herbicide programmes for many years.

“No active is safe from the risk of resistance, so it is always important to vary modes of action throughout the herbicide programme to slow down the development of any resistance,” Dr Anderson-Taylor continues. “Achieving the best possible results with herbicides is also important, using pre-emergence herbicides like Liberator at the true pre-em timing will get more consistent results.”

He also recommends growers eliminate any survivors of the herbicide treatment by hand-rogueing or the use of Roundup (glyphosate) to prevent populations growing and shedding seed.