Herbicide presents farmers with resistance breaking opportunity

• Good effectiveness in UK crops

• Timing and technique important

• Remember to abide by guidance

Most grassweeds are largely susceptible to a key herbicide for oilseed rape and sugar beet – although resistance is present in some UK populations, according to the latest testing of samples collected in 2024.

The herbicide in question is clethodim – the active ingredient in Centurion Max. In trials, it provided 98.7% control of grassweeds compared to just 66% control from mesosulfuron + iodosulfuron and 79% with pyroxsulam + pinoxaden.

“I think it proves that using clethodim in oilseed rape or sugar beet, can be a resistance-breaking opportunity within a predominantly cereals rotation,” says Alexander Zeller, European herbicide portfolio manager for UPL.

But that’s not to say resistance is not present in grassweed populations.

Gene mutations

Survey work conducted by expert Stephen Moss when he worked at Rothamsted Research from 2011 to 2014 found two gene mutations that allowed grassweeds to potentially overcome the inhibition of the enzyme acetyl-coenzyme A carboxylase (ACCase) by ‘fop’ and ‘dim’ herbicides, including clethodim. Inhibiting that enzyme disrupts lipid biosynthesis, leading to plant death.

One of the mutations, I-1781-L, was found to be present in 85% of the blackgrass populations at the time, with the second mutation, A-2078-G, present in only 8%, says  UPL technical specialist Tom Wheelhouse.

In Italian ryegrass, the situation was reversed with 57% of the population carrying the 2078 mutation, compared to 31% for the 1781 mutation.

“Clethodim is much more affected by the presence of 2078, and less so by 1781, whereas the other ACCase products, such as cycloxydim, are heavily affected by 1781,” explains Mr Wheelhouse.

No further widescale testing of grassweed populations in the UK has been conducted since that time to determine if the mutations has shifted. But Mr Wheelhouse says the expectation was that data would still be representative in 2024.

Sampling in the UK last season took a different approach to that in mainland Europe, with samples targeted from fields with a known problem with clethodim control. Unsurprisingly, resistance tests using these seeds showed more issues with control, although not for all samples.

One blackgrass sample from the seven collected clearly showed the population was susceptible to clethodim, says Mr Wheelhouse. “That points to the clethodim issue being either poor timing or application technique rather than resistance.”

Two populations showed similar levels of control to a known population carrying the 1781 mutation. “Using the equivalent dose in the lab test as a field rate of 1 litre/ha of clethodim gives around 80% control, which suggests that application or timing and not resistance are driving the issue in the field,” says Mr Wheelhouse.

The other four populations were likely to carry the 2078 mutation and be the cause of the poor performance in the field, he suggests. The difference highlights the importance of testing populations, particularly after poor performance in the field.

The results emphasise the need to adhere to good stewardship and practices when using clethodim.

Application technique

“In particular, making applications in the right conditions, paying attention to application technique, such as operating at the correct boom height and not going too fast, always include a water conditioner and use a minimum water volume of 150 litre/ha.

“Don’t fit application rate to the field size to avoid having product left over – use the correct dose, and make sure the target weed is at the correct growth stage and is actively growing,” adds Mr Wheelhouse.

In oilseed rape, clethodim should not be applied after 15 October or to crops with more than six true leaves (TL), he explains.

“If you push the boundaries, that’s when there is potential to cause injury to the crop, in particular if the plant has progressed into the reproductive phases of growth and is beginning to shut down for winter, it doesn’t metabolise the chemical quickly enough, and that’s why you eventually see fused flowers or abortion.”

Timing challenge

That can cause a challenge with timing with the current practice of drilling the crop either very early in August or before or into September.

Early drilling could reach that 6 TL stage quickly, and potentially before the weed reaches the ideal growth stage of two leaves to one tiller, or when conditions are such that the weed isn’t actively growing.

Later-drilled crops might not have grassweeds at the target growth stage before the 15 October cut-off, especially where growers have taken advantage of the extra time to create a stale seedbed and spray off emerged grassweeds with glyphosate before drilling.

“Understandably, most growers are not balancing decisions about drilling oilseed rape relative to when grassweeds will emerge,” says Mr Wheelhouse. “But it is something to consider.”

Centurion Max should only be used as part of a programme, which starts with cultural control, with follow-up applications of propyzamide at least 10-14 days after clethodim applications, he adds.