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02 Mar 2022

Are cover crop volunteers weeds of the future?

As featured in Arable Farming Magazine

Are cover crop volunteers weeds of the future?

by Arable Farming Magazine February issue

Volunteer cover crops are creeping in as an emerging weed issue on some farms, but trials are looking at the best way to keep on top of them. Alice Dyer reports.

Cover crops now play an integral role in many farming systems, but with sky-high glyphosate costs, tight application windows and often unfavourable conditions, getting them controlled ahead of the following crop can be a challenge.

With so many different species potentially being grown, controlling them in other crops in the rotation may not be straightforward, says Adam Espir, commercial technical manager at FMC.

Problems People are having real problems controlling cover crops such as phacelia in following cereal crops. Once you put these cover crops in the rotation, they will become weeds. We really see these as weeds of the future.

In trials at Bright Seeds headquarters in Fovant, Wiltshire, Mr Espir has looked at the use of different herbicides to control volunteer cover crops such as fodder radish, rye, berseem clover, phacelia, vetch and chicory.

In the trials, Centium 360CS (clomazone) at 0.2 litres/hectare was applied pre-em of the cover crops and controlled chicory and vetch, but not radish or kale.

Centium can be used in spring peas and beans, so this information is useful if volunteers are expected in these spring pulses, Mr Espir says.

A range of sulfonylurea (SU) herbicides typically used in cereal crops was also trialled on volunteer cover crops four to eight inches tall and results three weeks later showed that all performed well.

Of the products tested, Ally Max SX performed the best with its tribenuron component adding significantly to the weed control.

While effective against some cover crops, Jubilee SX and Harmony M did not control some of the species as well as Ally Max SX, adds Mr Espir.

Despite trials being undertaken during the autumn on a growing cover crop, Mr Espir believes that when the herbicides are applied in the spring onto volunteers in a cereal crop, control is likely to be even stronger.

In a spring barley crop, for example, volunteers would be sprayed in April or May when the weeds are actively growing, not October when there are declining temperatures.

Also, with potentially less glyphosate available in spring to spray off over-wintered cover crops, growers may need to rely more on the SUs, so having this information about controlling volunteer cover crops is very important.

Actives mentioned

  • Centium 360CS Clomazone
  • Ally Max SX Metsulfuron methyl + tribenuron methyl
  • Jubilee SX Metsulfuron methyl
  • Harmony M Metsulfuron methyl + thifensulfuron methyl

Variety choice

Cover crop variety choice will also be a key consideration going forward, according to Ben Dolbear, of Bright Seeds.

Late-maturing cover crops will reduce the risk of volunteers, he says but he warns there is huge variation between varieties.

Take fodder radish for example quite often the market can be flooded with maybe a cheaper variety which draws people in, but it could be very early-maturing, so it is going to seed quicker.

This means we have got more seed to deal with, which could become volunteers later down the line.

A later maturing fodder radish will hold back from going to seed and stay at more of a growing stage which is then easier to kill in spring.

Frost sensitive Richard Barnes, of Kings Crops, says frost-sensitive species such as buckwheat, phacelia, black oat and mustard have greater potential to naturally break down.

If you are destroying the cover mechanically during a frost with a crimper roller, roller or drilling into it, the fleshy species go down quite quickly because as soon as you bruise or snap the stem and the frost gets in, it is much more of a terminal intervention.

However, one thing to be mindful of is that many farmland bird species are responding well to cover cropping in the wider landscape.

If you are aware of ground-roosting species such as grey partridge in your field, you need to consider your option choice.

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