Latest resistance testing of samples collected in 2024 across Europe indicates that most grassweeds are largely susceptible to clethodim, a key herbicide for oilseed rape and sugar beet. However, resistance is present in some UK populations.

In Europe, 74 samples were collected from blackgrass, italian ryegrass and loose silky bent survivors from predominantly cereal fields in Germany, France, Spain, Denmark, Sweden, Poland and Benelux countries, says Dr Alexander Zeller, European herbicide portfolio manager for UPL.

Across those samples, Centurion Max provided an average of 98.7% control of the grassweeds grown by the specialist German company Agris42 from the seed samples collected, compared with 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,” Alexander says.

That’s not to say resistance is not present in grassweed populations, stresses Tom Wheelhouse, UPL technical specialist in the UK.

Survey work conducted by expert Dr 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%, adds Tom. In italian ryegrass, the situation was reversed with 57% of the population carrying the 2078 mutation, compared with 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,” Tom says.

No further widescale testing of grassweed populations in the UK has been conducted since that time to determine if the relative accumulation of mutations has shifted, although Tom says the expectation was that the data would still be representative of the current population 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. Not surprisingly, 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 Tom. “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 L/ha of clethodim gives around 80% control, which suggests that application or timing and not resistance are driving the issue in the field,” states Tom.

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. 

Practically, the results emphasise the need to adhere to good stewardship and practices when using clethodim. “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 L/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,” Tom adds.

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

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, Tom explains.

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,” Tom points out. “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 further adds.