February 1, 2018
The Essentials of Effective Cover Crop Destruction
Effective cover crop destruction will be more important than ever ahead of spring crops this season to avoid compromising establishment and performance, insists Agrii trials manager, Steve Corbett.
Plenty of moisture at sowing and another open autumn means most cover crops are well-established with substantial canopies. As a result, they will need especially careful spraying-off wherever there is insufficient frosting; especially where they contain harder-to-kill species like vetches, oil radish and lucerne.
Key considerations in this respect include the best-timed spraying, matching treatment to species and the most appropriate spraying practice.
“Don’t leave cover crop destruction too late,” he warned. “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 convinces us covers are best sprayed-off 4-6 weeks before spring drilling.
“We can’t rely on the winter to do the job for us these days. At the same time, if the covers aren’t 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,” Steve Corbett added. “Equally, they need to be applied ahead of any hint of stem extension for the greatest efficacy.
“Cover crops will have done their job by February. So there’s 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. Not having one also gives you the best chance of dealing with any early weed growth with pre-planting or pre-em glyphosate.
“So the best and safest strategy is to spray-off your covers in late January or early February.
That way you’ve cleared the decks for the most rapid and cleanest spring crop establishment.”
Regardless of how well they’re established, some cover crop species are much harder to destroy than others.
Providing spraying is not left until stem extension is underway, the cereal components of cover crop mixes are unlikely to present much of a challenge. Phacelia is also relatively easy to destroy in his experience.
However, extensive Roundup trial work shows that mustard, lucerne and oil radish can be more problematic. And vetches have proved particularly difficult, really putting the pressure on glyphosate at a time of the year when low temperatures restrict herbicide uptake and translocation.
Steve Corbett stresses that you need to be right first time with cover crop spraying because you won’t get a second chance. As well as employing a rate of glyphosate suitable for the hardest-to-kill species in your mix, this means formulations that work most effectively in challenging conditions.
To deal with any grass and broad-leaved weeds emerging following cover crop removal he advises a further glyphosate treatment ahead of spring crop drilling or an approved formulation in the pre-em.
At the same time, to ensure the most effective penetration and coverage of tall, thick multi-species cover crop canopies, everyone should spray with as much care and attention as they would in-crop.
“A mix of species and development means there are several layers within most cover crop canopies,” explained Steve Corbett. “What’s more, there can be a good amount of black-grass growth below them. “So it’s important to get sufficient canopy penetration as well as maximising both canopy and individual leaf coverage.
“Glyphosate may be translocated, but in more challenging late-January/early-February conditions, in particular, the best coverage of all the target surfaces will pay dividends.
“Overall, we recommend 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 30o forward will improve penetration.
“Set the boom about 50 cm above the canopy and take enough time to do the best possible job,” he suggested. “We always find spraying at 10 km/hour gives the best penetration. It also really improves boom stability. This may seem slow but it’s well worth holding back on the throttle to get the quality of kill you need.
“For any follow-up glyphosate spray a few days ahead of drilling to target blackgrass that may have been sheltered under the cover crop a finer spray quality at 100 litres/ha will ensure the best coverage of smaller leaves.”