Good establishment conditions and a mild autumn has led to some sizable cover crops this season. But how can growers best destroy them to ensure a good start for the following spring crop? Abby Kellett reports.
Plenty of moisture at sowing and another open autumn means most cover crops are boasting substantial canopies. While this is good news for maximising soil conditioning, ground cover and light interception, it can make cover crop destruction challenging, particularly where there has been insufficient frosting, according to Agrii trials manager, Steve Corbett.
He says growers need to consider spray timing, cover crop species and the most appropriate spray practice before making a decision on how and when to kill cover crops this season.
“Do not leave cover crop destruction too late - drilling ‘on-the-green’ may be fine with catch crops in November, but three years of work with a variety of mixes at our Stow Longa Black-grass Technology Centre shows that covers are best sprayed-off 4-6 weeks before spring drilling.
“We cannot rely on the winter to do the job for us these days. At the same time, if the covers are not well-destroyed before spring planting we invariably see the sort of delayed and uneven crop establishment we always want to avoid.
“Bear in mind that even the best glyphosate formulations take more time than usual to work under colder late winter and early spring conditions,” he adds. “They need to be applied ahead of any hint of stem extension for the greatest efficacy too.”
For those farming heavier soil types, he says there is added benefit in spraying off cover crops in good time. “Cover crops will have done their job by February so there is no advantage in leaving them growing. In fact, heavy soils are likely to condition better for spring seedbeds if they have a good two weeks without a cover.”
Roundup technical specialist, Barrie Hunt warns that some cover crop species are much harder to destroy than others. “Our extensive trial work shows mustard, lucerne and oil radish can be problematic. Vetches are particularly difficult, really putting the pressure on glyphosate at a time of the year when low temperatures restrict herbicide uptake and translocation.”
As well as employing a rate of glyphosate suitable for the hardest-to-kill species in the mix, he advises using a formulation that works most effectively in challenging conditions.
“Modern Roundups work much better than other non-tallow amine formulations, getting more glyphosate into plants more rapidly and reliably at low temperatures and on the frost. They are also less affected by rainfall after application and are proven to minimise drift risk.”
Managing a large canopy
For the most effective penetration and coverage of tall, thick multi-species cover crop canopies, spraying specialist and Agrii regional technical adviser, David Felce says growers should angle spray nozzles, use increased water volumes and opt for a medium to course spray quality. He says: “Glyphosate may be translocated, but in more challenging early-February conditions, in particular, the best coverage of all the target surfaces will pay dividends.
“Overall, I would opt for a spray quality on the coarser side of medium, either with flat fan nozzles at less than 2.5 bar or pressure-responsive air induction nozzles at around 4.0 bar. Increasing water volumes from 100 to 150 litres/ha will give better coverage of larger, thicker canopies; and angling the nozzles alternately straight down and 30 degree forward will improve penetration.”
For very large or forward cover crops, growers may want to consider topping, flailing or rolling the cover to aid spring crop drilling.
Wright Resolutions director, Philip Wright says: “How the cover crop is managed will depend on the drill type – a disc based direct drill with adequate capacity to deal with high volumes of surface residue can then help establish the following crop. Rolling and crimping can lay the canopy over for the best action of the drill.
“If the drill is more conventional, or tine based, then flailing, topping, or a fairly extensive cutting and mixing cultivation, or even ploughing, may be needed to get the best establishment results.”
Opico managing director, James Woolway says use of a flail mower often means there is a lot of ‘wet mush’ left on the soil surface, which can make establishment of the following crop difficult.
Instead he recommends using either a cross cutter roller in front of a cultivator, or where growers want to minimise soil disturbance to prevent black-grass germination for example – a crimper roller.
“Our cross cutter roller can be fitted on the front of a disc cultivator – it has several slight curved knives that will chop the cover crop up – so if you are not bothered about disturbing the soil a little bit, then that is an option.
“If you are not wanting to disturb the soil, then we also sell a crimper roller, which is essentially a front press which has angles edges which damage and crimp the stem.”
Where cover crops have been grown for biofumigation, they can simply be ploughed or incorporated into the soil without any other form of destruction. However there is some evidence which suggests crimping first can improve performance of the biofumigant, according to Agrovista technical manager, Chris Martin.
Topping and incorporation are also popular ways of destroying cover crops, particularly where the field of use is as a green manure or where cover crops are being used to improve soil organic matter. “Care must be taken with this technique as over cultivation could potentially undo a lot of the benefits provided by the cover crop,” says Mr Martin.
“Crimping rollers can also be employed to leave a nutrient rich, weed suppressing mat to drill into, and this technique has been successfully used in organic situations.
“This does however limit your drill choice due to smearing and the ability to cut through trash without blocking up. The organic mulch can also immobilise nitrogen reducing establishment, and it can also provide the perfect environment for slugs.”
Some cover crops can be sufficiently destroyed through hard grazing by sheep. However Mr Martin says this technique should be avoided if the purpose of the cover crop is to aid grass-weed control as the vibration from the animals feet are enough to ‘wake up’ grass-weed seeds.
“Compaction also needs to be carefully monitored with this system, but this system can be very successful in improving soil organic matter in particular if done correctly as has been shown by mob grazing throughout the world,” adds Mr Martin.