Methods of cover crop destruction vary considerably. The best approach depends on the soil type, cover crop choice and the establishment system for the following crop.
Use this information as a guide to help you identify what is most likely to work on your farm. Be aware that a trial-and-error approach will still be needed to achieve good results.
Although the methods described support reductions in herbicides, they will still be needed at some stages of termination.
The cover crop planting date significantly impacts the amount of biomass produced.
Figure 1 shows results from a cover crop mix (phacelia and black oats) planted at three planting dates (mid-August, end of August and mid-September) and three termination dates (end of November, January and March). Both planting dates and termination dates had a significant impact. However, planting dates were by far the most influential – earlier planted cover crops are associated with far greater biomass production.
Figure 2 shows the effect of three planting dates (mid-August, end of August and mid-September) on cover crop biomass of single species of phacelia, black oat and clover (all terminated in December). Once again, earlier planting was associated with significantly greater biomass production (for all three species).
Generally, aim to destroy cover crops as far ahead of the following crop, unless ‘drilling on the green’. An often cited rule of thumb is to destroy a cover crop six to eight weeks before the intended drilling date of the following crop to avoid a yield penalty.
Very early termination (pre-Christmas) provides the longest time for the cover crop residue to die back. This will allow easier cultivating/drilling in spring. Always ensure termination dates are compatible with rules detailed in any agreements (such as countryside stewardship or SFI options).
It is also important to consider soil moisture, as it takes land longer to dry under a cover crop canopy. Light/medium soils are generally more forgiving. On heavier land, wet soil can persist, which can contribute to drilling delays for the following crop. On heavy soils, consider terminating cover crops earlier to allow the soil surface to dry. This is particularly important in cover crops with dense (and wet) canopies.
A longer window between termination and the cash crop also allows more time for cover crop residues to break down and release nutrients back into the soil.
One of the main factors that influence the dynamics of mineralisation of the nitrogen is the carbon to nitrogen (C:N) ratio of cover crop residues.
Cover crops are rarely taken to maturity. Typically, cover crops are terminated four months after establishment (when still green). This helps to maintain an optimum C:N ratio of 24:1, which facilitates the greatest availability of nitrogen to the following crop (leaving little nitrogen susceptible to leaching and volatilisation).
Anything terminated within six weeks of planting the following cash crops is considered late. In such scenarios, there is a strong possibility of yield losses in the following crop.
This may* be due to:
*The mechanisms are not fully understood.
Consider including species in a mix that are not frost tolerant, especially on difficult-to-traffic land or when labour availability could limit opportunities to terminate the cover crop.
While some species, such as buckwheat, are reliably killed by frost, others, such as phacelia, are not.
Information on sensitivity to frost is available on the species selection page.
(Image: David Purdy Soils)
Use of rolling to terminate cover crops tends to work best in the UK when there is a frost.
The approach can be very effective on larger plant species.
In general, it does not terminate grass species as well, which are likely to require a glyphosate application.
When rolling, a temperature of -4oC is generally required to achieve termination, preferably for several days prior to rolling, according to Innovative Farmer field lab trials. Such conditions allow frost to penetrate the whole cover crop (helping to shatter stems). Well-frozen soil also reduces compaction risks.
Such conditions are often found very early in the morning (between 2am and 7am), with the heaviest frost usually present before sunrise.
Discs tend to deal with cover crop residues better than tines, although either work when set up well.
Weed pressures may also inform termination plans. For example, it can be difficult to target black-grass shaded under a dense cover crop. In such scenarios, consider methods that destroy the bulk of the cover crop (such as light grazing or crimper rolling) and reveal black-grass foliage (which can be hit with glyphosate).
(Image: David Purdy Soils)
Well-managed livestock can do a fantastic job of reducing the bulk of a cover crop and have the additional benefit of starting the process of breaking down organic matter (and adding to it, via manure).
By starting the breakdown process, livestock help balance C:N ratios and make the nutrients available to the following crop quicker. They also provide income from the grazing, either as meat to sell or through grazing payments, as well as giving the grazier’s grass a rest from grazing.
Many favour grazing by sheep to terminate cover crops (the ‘golden hoof’ treatment). Cattle are also used, although their extra weight can increase poaching risks, particularly on heavy soils.
As poaching can occur rapidly, it is important to monitor soil and weather (risk of runoff) to ensure conditions are suitable. Grazing (in any form) should be ‘fleet of foot’, particularly on heavy soils.
Be aware that some cover crop species are potentially toxic to livestock.
If in doubt, consult your vet, especially before grazing unfamiliar covers.
(Image: AHDB)
When bringing livestock onto arable farms, also consider:
Chemical termination, usually with glyphosate alone or with 2,4-D, is often the most reliable and typical route for cover crop destruction.
When cover crops are used to manage pernicious weeds, such as black-grass, or where mixes include brassicas, more than one glyphosate application may be needed. For example, one to terminate the ‘upper canopy’ and another to kill the ‘under canopy’ before cultivation/drilling.
Typically, cover crops need at least 4.5l/ha of 360g/l glyphosate.
Higher rates may be beneficial in drier conditions (when the crop growth is slow) or to terminate bigger cover crop stands.
Always follow crop drilling interval requirements.
Take extra care when destroying cover crops in front of broadleaf crops, because available in-crop herbicides may not cover wide weed spectrums and may not kill cover crop species when initial destruction is not completely successful.
In general, the earlier the cover crop is sprayed off the better the results, as there is less residue to interfere with the drill/cultivator. Early termination also reduces issues with allelopathy and nitrogen lockup from the cover crop.
Aim to spray six to eight weeks ahead of drilling, leaving plenty of time for a follow-up spray application.
If a crop has been grazed pre-glyphosate application, ensure adequate regrowth to take up the herbicide on application.
(Image: AHDB)
The most commonly used online resources for cover cropping have been reviewed by a team at Newcastle University, with the following recommended for this topic.
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