Herbicide-tolerant OSR beats weeds

Growing a herbicide-resistant oilseed rape to overcome a severe charlock problem has enabled one grower to reintroduce the crop into his rotation after a decade’s absence because it was not economically viable.

An escalating charlock problem prompted business management consultant John Harmer to introduce a test area of the Clearfield system on one of his client’s farms.

In 2011, he took the decision to drill one field with a candidate Clearfield variety, so that its in-bred herbicide tolerance to imadazoline chemistry would allow the post-emergence use of Cleranda (metazachlor + imazamox), the broad-spectrum herbicide developed specifically for use with the tolerant varieties.

The aim was to be able to control the charlock, as well as other problem broad-leaved weeds, so that the crop would be competitive and oilseed rape could be grown as a break crop in the rotation again, rather than having to fallow the worst affected fields.

What is Clearfield?

The Clearfield system consists of herbicide-tolerant hybrid oilseed rape varieties, produced using conventional breeding techniques, and a specific post-emergence herbicide from BASF, Cleranda, which contains metazachlor and imazamox.

The registered Clearfield varieties all carry a CL suffix, making their identification more straightforward, and Cleranda can only be applied to those varieties. Application to non-Clearfield crops will result in complete crop loss.

At least four plant breeders are involved with the system, but just one winter variety, DK Imagine CL, has been described on the HGCA Recommended List. A further two winter types, both from Pioneer, are candidates and will be considered later this year.

The seed and herbicide are marketed separately. Careful use of the system is advised, as Cleranda belongs to the group of herbicides known as ALS inhibitors and the control of subsequent volunteers must employ different chemistry. It is estimated that around 1% of the UK oilseed rape area will be Clearfield this year.

Through Agrii agronomist Charlie Prince, seed of a coded Clearfield variety, CWH 1580 (DK Imagine CL), was obtained. The weather then made it impossible to drill the crop until 13 September – some three weeks later than Mr Harmer had planned – putting it under pressure right from the start.

“It was slow to establish, as you would expect from any mid-September drilled oilseed rape crop,” recalls Mr Harmer. “And when it did finally come up, so did the weeds. It was swamped by charlock and hedge mustard.”

Pre-emergence weed control wasn’t attempted. “The conditions were against us and we knew from experience that there aren’t any herbicides that will touch charlock anyway. So we put all our faith in Cleranda’s activity.”

While a fungicide/trace element spray went on in the autumn, as planned, the Cleranda wasn’t applied until 25 February.

“That was also later than originally intended, but it has to go on in good growing conditions for best results,” he continues. “And even at this late stage, it did take out all of the weeds.”

Otherwise, the crop received the same agronomy as the non-Clearfield oilseed rape, he notes.

At harvest, a final yield of 3.4t/ha was recorded. “That was very good considering the year that we had. Interestingly, it outyielded the conventional Sesame being grown on the same farm, which was drilled a month earlier on 14 August.”

Such a stern test has convinced Mr Harmer that the Clearfield system has a place, especially where cruciferous weeds are prevalent, and he will oversee the drilling of 39ha of DK Imagine CL on the same farm this autumn.

“It’s proved itself, it doesn’t cost us any more to grow and it has allowed us to reintroduce rape in problem areas,” he summarises.

But the control of Clearfield volunteers does have to be managed, as their in-built herbicide tolerance means that growers can’t rely on ALS inhibitor herbicides, or sulfonylureas, to control them in following crops, he acknowledges.

“We actually didn’t have many volunteers, but a Liberator/DFF treatment in the subsequent wheat crop did the trick on the few that were present.”

And good rotational planning is also required, so that there’s no misapplication of the herbicide to conventional crops, he remarks.

Clare Tucker of BASF stresses that Clearfield allows a wide range of weeds to be controlled from a flexible post-emergence timing.

“It has the broadest range of broad-leaved weeds but it controls volunteer cereals too,” she points out. “And its later application timing means that the results are less dependent on the seed-bed and surface trash levels than other products.”

She adds that Cleranda should ideally be applied when the weeds are between one to four true leaves and soil temperatures are still high.

“Autumn applications are best and it will take out the first flush of cereal volunteers if it goes on before they start to tiller.”

The management of Clearfield volunteers is easier if growers do as much work as they can at harvest time, she advises. “Aim for a quick germination, which will be helped by rain before cultivation, and then spray off those that chit. It is important to take more care and keep good records.”

That’s because Clearfield volunteers will have variable or little tolerance to other ALS inhibitor herbicides, she acknowledges. “But there are other non-ALS herbicide options. In winter cereals, you can use products such as Crystal (flufenacet + pendimethalin), Liberator (diflufenican + flufenacet), Stomp Aqua (pendimethalin) and Picona (picolinafen + pendimethalin) in the autumn, while there’s bromoxynil and mecoprop for spring use.”

For sugar beet growers, who may have been dependent on Debut, there are Betanal maxxPro programmes to use instead, she suggests.

Avoiding the application of Cleranda to non-Clearfield rape is essential, she stresses. “It will cause crop loss. There is a very distinctive herbicide label and the user guidance is prominent, but careful management and planning will be needed too.”

Mr Prince can see a place for the Clearfield system in certain situations. “We’re fighting a losing battle with weeds such as charlock and runch and there’s no indication that DuPont’s ethametsulfuron-based herbicide will be available in the near future.”

The Clearfield system allows oilseed rape to be grown where these weeds dominate, he comments.

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