THE GROWING of Clearfield oilseed rape varieties has gone ‘mainstream’ as growers see the benefits of growing them to control problem weeds.

Once viewed as a niche crop, the Clearfield types have grown up and some 61,000 ha were grown last year, with an expectation of further growth in 2019. There are now 14 varieties available from a number of different breeders.

It is anticipated that more will try out the system this year to rid themselves of difficult weeds such as charlock, hedge mustard and runch, or as a prophylactic approach to making sure the seed sample does not exceed the necessary erucic acid levels. These currently stand at 5% in oil but are likely to be reduced to 2% in the near future.

However, industry experts remain worried that some farmers are still reluctant to try it out because they think yields are compromised. “That may have been a valid worry in the early days but not now. The modern generation of Clearfield varieties have shot up in terms of yield,” pointed out Neil Groom, technical director for Grainseed – one of a number of breeders involved in such types.

“The earlier Clearfield varieties did struggle to compete with non-Clearfield ones on yield but this is not the case for the newer ones such as Es Decibel CL. In NIAB/TAG trials, our variety Es Decibel CL was the top yielding Clearfield out of 16 under test, averaging 106% (4.41 tonnes/ha) across seven trial sites harvested in 2017 and 2018.

“If growers want the advantages of the Clearfield system plus yield, then they should be growing this variety,” argued Mr Groom, who said it was a unique system for winter oilseed rape that combined hybrid varieties with the use of high quality BASF herbicides – Cleranda (imazamox + metazachlor) or Cleravo (imazamox + quinmerac).

Using conventional breeding techniques, varieties have been bred to develop tolerance to approved herbicides. By applying these herbicides post-emergence, all weeds and volunteer rape can be controlled. “This means growers can use its as a management tool, bringing fields back into rotation and allowing rape to be grown profitably where it would have been difficult or impossible to do so.”

Mr Groom said that this season reliance on bifenox to control charlock in winter oilseed rape had been risky because the long, mild winter meant that crops had taken a while to wax up effectively. Bifenox is a strong contact acting product and this year charlock has come through this herbicide treatment and are now in flower.

These weeds will pollinate and produce seed high in erucic acid (charlock has 31.7% erucic acid) and seeds could act as a contaminant. “The solution to this problem is to move to Clearfield oilseed rape,” said Mr Groom.

“By applying post-emergence Clearfield herbicides to Clearfield rape, difficult cruciferous broad-leaved weeds or volunteer rape are killed but the rape left unaffected to grow. Some cruciferous weeds have high levels of erucic acid and need to be controlled because they can affect levels in the sample at the crush.

“Currently, UK and EU legislation sets a limit of 5% erucic acid in foodstuffs but this is likely to be reduced to 2% before next season’s crop is crushed.

“When it comes to growing any Clearfield variety, it is important to get the timing and application of the post-emergent herbicide right. It is recommended to use at least 100 l/ha and also to add 1 litre/ha of the non-ionic surfactant, Dash, to improve leaf cover and uptake of the herbicide.

Normally, the best time to apply the herbicide is when weeds have 1-4 true leaves and are actively growing (typically September to October).”

It is important to make sure that early weeds don’t get too big whilst waiting for the others and he added that the only disappointing results were seen when the Clerando or Cleravo herbicide was applied late to weeds which were too large or when an adjuvant had been omitted.

Es Decibel CL has an 8 rating for resistance to lodging and 6 for stem stiffness. It has good oil levels of 44.1%, an excellent disease resistance rating of 8 for phoma stem canker and a 5 for light leaf spot, plus excellent verticillium tolerance.