Oilseed rape blackgrass herbicide has avoided resistance build-up

The latest research reveals that Centurion Max (clethodim) has bucked the trend of other contact-acting grass weed herbicides by still being effective on most farms in the country.

However, experts warn that all guidelines should be followed to ensure good control of grass weeds longer-term. 

This autumn marks ten years since Centurion Max was first launched to control grass weeds in oilseed rape and sugar beet.

It was billed as active on strains of blackgrass resistant to other ACCase herbicides, so it quickly found its way into herbicide programmes on most farms growing oilseed rape or sugar beet.

As part of the ACCase inhibitor chemistry group, known as ‘fops’ and ‘dims’, many farmers and agronomists expected it to follow others and its efficacy to decline quickly.

However, this has not been the case, with Centurion Max still providing good levels of control in most fields. 

Research conducted by Dr Stephen Moss analysed Centurion Max’s efficacy against a range of different blackgrass and ryegrass populations.

Fifteen samples of ryegrass and ten samples of blackgrass were tested.

First, viable seeds were placed in a petri dish treated with clethodim, and plant growth was recorded. Those plants that grew in the petri dish were transferred to pots and received a foliar application of clethodim at half and full rates. A control test was also carried out using cycloxydim, which is known to be more affected by resistance in both ryegrass and blackgrass. 

The results on Italian ryegrass showed that most of the samples were relatively well controlled by clethodim, with just four showing a reduction in seedling growth of less than 90%.

When cycloxydim was sprayed on these survivors, they were all found to be highly resistant to cycloxydim.

In contrast, they showed only partial resistance to clethodim, with the herbicide still giving 60% control even on the least susceptible sample. 

Stephen Moss © UPL Europe

“The main mutation affecting ‘fops’ and ‘dims’ in blackgrass is 1781,” says independent consultant Stephen Moss.

“Having this mutation makes plants highly resistant to most herbicides in this group of chemistry, but clethodim is an honourable exception to the rule.

“There is some resistance, but it tends to be marginal” 

The research conducted by Stephen shows that when the spray conditions are good, an application of Centurion Max will always reduce the blackgrass population to a greater or lesser extent.

“Even with the more challenging grass weed populations, you can still get good control in the field with Centurion Max if you follow the guidelines and do everything correctly,” says UPL’s head of technical services, Stuart Jackson. 

The inclusion of an acidifying water conditioner will increase the activity of Centurion Max regardless of water hardness, states Stephen.

Across the six field trials where Stephen has investigated this, a water conditioner increased blackgrass control by 11% on average.

Significantly, where the populations were more resistant to clethodim, the benefit was even greater than the average.

“We would always recommend including a water conditioner, like X-change, with Centurion Max,” advises Stuart. 

“On oilseed rape, the application timing is another critical factor in maximising the control. UPL recommends that Centurion Max is not applied after the 15th of October or when the crop goes beyond six true leaves.

“This is primarily a crop safety measure because the plant must be growing to metabolise the clethodim and avoid damage from the herbicide.

“”However, farmers will also struggle to hit the weeds due to shading if the crop canopy gets any bigger, although this may depend on the row spacing and seed rate.

The ideal timing to spray blackgrass with Centurion Max is when it is at three true leaves and has not tillered,” Stuart adds. 

Another important factor in timing is the sequence with other herbicides, believes Stuart. There should be a ten-day interval between applying any pesticide and Centurion Max and a fourteen-day window after Centurion Max has been used where no other pesticide should go on the crop.

Farmers and agronomists will need to prioritise broad-leaved weeds, grass weeds or disease control depending on what they think is the most important and then plan their sequence around the Centurion Max guidelines. 

UPL has investigated autumn blackgrass control programmes in oilseed rape and has discovered that the most effective control came from a mid-October application of Centurion Max followed by propyzamide applied in mid-December.

If the propyzamide is delayed until mid-January, then this reduces blackgrass control compared to a mid-December application by almost 10%.

Stuart thinks this is because it allows some of the blackgrass that survived the Centurion Max to recover sufficiently to develop roots below the subsequent propyzamide layer in the soil. 

Looking ahead another ten years, Stephen predicts that clethodim will still play a part in grass weed control.

“It is dangerous to predict the future, but I expect it to still be useful on most farms. It has been used for decades in Australia and is still an important part of resistant ryegrass control strategies there.” 

“It is positive news that Centurion Max is still controlling grass weeds after ten years of use,” concludes Stuart.

“If farmers use it in appropriate programmes with cultural control techniques, they should expect good grass weed control in oilseed rape.”