A KEY herbicide has now seen reduced efficacy due to sensitivity issues – especially with blackgrass and ryegrass.

Growers are now being warned to continue to actively use integrated weed management programmes after Bayer's grassweed resistance testing has confirmed reduced sensitivity to key active ingredient, flufenacet.

The testing has found, in a few isolated cases, black-grass from farms in Germany with reduced sensitivity to flufenacet. More troublingly, results show that rye-grass with enhanced metabolism resistance to flufenacet is present on a small number of UK farms.

There are no results indicating flufenacet resistance in black-grass in the UK or overseas, but reduced sensitivity in Germany should act as a wake-up call to farmers.

Bayer pointed out that this did not necessarily mean significant changes in weed control methods, because growers are now well used to integrated control programmes to manage black-grass. However, these will be even more important for stewardship of the active for future years and remain crucial for overall success of programmes.

Farmers with challenging rye-grass should have more concern. The first step is to understand the field situation with a resistance test, said a spokesman for Bayer. Regardless of the situation, but particularly if resistance is confirmed, integrated weed management programs which reduce weed pressure in the seedbank will be crucial to reduce further development of the problem.

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


Despite reduced sensitivity being identified in black-grass samples from Germany, there is no immediate concern for pre-emergence herbicide programmes, said Bayer’s Dr Gordon Anderson-Taylor.

“When testing for resistance we determine the amount of active substance needed to provide 50% and 90% control of a population. The researchers noticed that, in some samples of black-grass, a larger dose of flufenacet was needed to reach the 50% and 90% control level compared with sensitive type black-grass.

"Hence, we describe this black-grass as having reduced sensitivity. But, in all samples from the UK, applications at the commercial use rate of 240g/ha still provided 90% control in lab conditions despite variability in sensitivity," he said.

In practical terms, Bayer does not anticipate these results will lead to a radical change in black-grass 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. 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.

More worryingly, the research indicated that that the efficacy of two other important actives were also affected by reduced sensitivity. When applied as single actives, both pendimethalin and prosulfocarb had lower efficacy against black-grass with reduced sensitivity to flufenacet. The research did not look at the efficacy of these actives when applied in mixtures and sequences.

“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,” pointed out Dr Anderson-Taylor.

Eliminating survivors of the herbicide programme is one important technique growers should try to use, where possible, byand-roguing and eliminating thick patches with Roundup (glyphosate) both prevent black-grass populations getting out of hand and stop the shed of seed to cause problems next season.


The situation with rye-grass is more concerning.

In the study, some UK populations required more than 300 g/ha flufenacet applied pre-em for 50% rye-grass control and more than 3 kg/ha flufenacet for 90% control. These figures are well above the label rate of 240g/ha, hence the resistant classification for these rye-grass strains.

Researchers also found rye-grass samples from France and the USA with resistance.

“The results are worrying for farmers with rye-grass problems, but I would like to emphasise that resistance is unlikely to be widespread,” added Dr Anderson-Taylor. “Once again, the research is focusing on the most resistant samples available.

“Understanding the rye-grass population on your farm is essential. A resistance test can tell you the status of rye-grass and help you manage it more effectively. But there are three things all farmers dealing with rye-grass need to do to mitigate the risk: cultural controls, crop rotation and varying herbicide modes of action.”

Cultural controls should aim to reduce rye-grass populations before crops go in the ground. This includes delayed drilling as the majority of rye-grass germinates in the autumn. Rotational ploughing, higher seed rates and aggressive patch management will also be important. Like black-grass, the aim is to get plant numbers down to manageable levels so herbicides can be effective, he said.

Wider rotations, particularly more spring crops and more non-cereal crops are also essential. Non-cereal crops are particularly important because they allow a wider range of herbicide modes of action to feature in the herbicide programme across the rotation.