While harvest is only really starting, it’s time to think of planting winter oilseed rape.

While late harvesting may delay planting, most growers now use winter barley as the lead-in crop for timely rape planting and this should help this year unless straw is still in the way.

The first decision with rape is where to sow it, then what variety to use. In mid-July we published the winter oilseed rape recommended list (Saturday 20 July), which includes seven varieties.

It is preferable to leave at least four years between consecutive winter oilseed rape crops; five is even better.

But some fields do not suit rape due to the high presence of brassica weeds like charlock, hedge mustard or volunteer rape.

One new option to tackle these problems is the use of Clearfield technology, which was developed by BASF.

Clearfield uses a non-GM genetic trait which allows the use of a herbicide to kill brassica weeds but not damage a Clearfield variety, denoted by a CL prefix after the variety name.

This technology is now licensed to many plant breeders and there are many varieties available commercially.

Some breeders now offer CL varieties such as Phoenix CL, DK Impression, DK Impressario and PT279 CL. These are not yet on our recommended list, but some are now in-trial.

What is Clearfield?

Clearfield is the name given to a form of specific herbicide resistance in oilseed rape to a specific active ingredient. The technology is produced using non-GM breeding of hybrid varieties.

The genetic resistance confers tolerance to the ALS (acetolactate synthase) inhibiting active, imazamox.

This active would normally kill a brassica crop but it will not kill a Clearfield variety, allowing its use on such varieties as a selective herbicide.

At a recent seminar, Steve Dennis of BASF explained that the technology uses two genes to provide the resistance to imazamox.

One of the fears around this type of technology is the risk that it could produce a new volunteer problem with resistance to ALS (SU) herbicides elsewhere in the rotation.

However, Steve said that this has not been an issue so far in the UK where it has been used now for a number of years.

Firstly, the resistance is very specific to imazamox and is much less effective against robust rates of other ALS herbicides.

Secondly, the fact that two genes are required increases the chance that volunteers beyond the first generation are less likely to have both genes present to provide the resistance.

While out-crossing with other brassicas is likely, research indicates that only 25% of the trait is likely to remain after two years.

While the Clearfield trait can still be tackled by other ASL herbicides, the technology seems to be very useful against residues of ALS actives in the soil from previous crops.

This was an unexpected benefit and has often resulted in increased establishment vigour. This additional seedling vigour may be very useful for flea beetle tolerance, especially in non-inversion establishment systems.

The actual herbicide used is called Cleranda, which contains imazamox in combination with metazachlor (the active in Butisan).

The strengths

Brassica weeds like charlock and runch are the main targets of Clearfield technology.

Other important targets are hedge mustard and volunteer rape, which can be a big issue in hybrid seed production. Control of volunteers could also be useful as a tool to control plant population.

It is a post-emergence herbicide applied from the cotyledon up to the eight-leaf stage of the crop.

But the earlier the better because shading can be an issue, but this must be balanced against the need for full weed emergence.

Steve emphasised that imazamox has mainly contact activity, needs good foliar uptake and preferably a dry leaf.

So earlier treatment is better and the preferable timing is somewhere in the one- to four-leaf stage of the crop, depending on the problem being targeted, in September or early October.

However, he emphasised that imazamox is not glyphosate and that there is a limit to what it can do.

Cleranda has a very broad spectrum of weed control and it is also effective against grass weeds, but this activity is much slower.

Where grasses are being targeted, the adjuvant Dash must be included (1.0l/ha) with Cleranda (2.0l/ha).

Grower experience

Martin Bourke of Teagasc used Clearfield technology this year and was very happy with its performance in a situation where stubble turnip volunteers had made rape untenable.

He now intends to push from 19ac in 2019 to 50ac of Clearfield for 2020.

Asked if the use of this herbicide could be a risk to subsequent catch crops, the advice was that it will be largely gone from the soil in less than six weeks.

Variety performance

While the yield level of CL varieties lags behind the best of the standard varieties, they will considerably outperform them in fields where the Clearfield technology is needed.

But the newer varieties are a significant step up in performance from the initial ones and this improvement will continue.

Tim O’Donovan of Seedtech told us that greater than 98% of the seeds in a bag will carry the Clearfield technology and imazamox tolerance.

He recommended that Phoenix CL be drilled at 35-45 seeds/m2. He also said that Phoenix CL has good autumn vigour and pod shatter resistance, and suits mid-early to late sowing dates.

It can be weak-strawed so autumn growth regulation can be useful on advanced crops. Seedtech now also have a spring Clearfield variety called Click CL.

Husbandry

Lizzy Matthews of Bayer also spoke about varieties and indicated that DK Impression appears to be the variety that best suits Irish conditions from that breeder.

She said that breeders are now bringing multiple traits in new Clearfield varieties. Conventional RLm7 phoma resistance is starting to break down and so multiple gene stacking is around the corner.

Lizzy stated that the presence of the pod shatter resistance gene can give up to a month of shatter resistance post-ripening. Pod shatter was historically a major cause for concern about growing rape.

Indeed, recent research found that seed yield actually increased past the normal cutting time as later-developing pods had matured. She also said that growers have been burning off winter crops too early.

Change of name

BASF’s new man on the ground, David Leahy, told us that Caramba is being replaced by Juventus, a higher concentration of metconazole.

Applied in autumn this can decrease aerial growth, increase prostrate growth, improve tap root growth and increase root collar diameter.

BASF recommends the use of Juventus on strong crops at the four- to six-leaf stage at 0.4-0.6l/ha. The advice recommends the application of 0.08l/ha/leaf for growth regulation.