Termination
Cover crops are rarely taken to maturity. Typically, autumn-established cover crops are killed off or die prior to incorporation and establishment of the following crop.
There are various ways to terminate cover crops. The best approach depends on the soil type, cover crop choice, growth (canopy size and type of growth), equipment, objectives and the establishment system for the following crop in the spring.
In general, options and opportunities are greater on light and medium soils.
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.
Biomass production
Cover crop planting and termination dates significantly affect biomass production.
This can be seen in the results of a trial that compared cover crops (a mix of phacelia and black oats) at three planting dates (mid-August, end of August and mid-September) and three termination dates (end of November, January and March).
Results (based on dry-matter production)
- Earlier planted cover crops produced far more biomass (with biomass production dropping off relatively rapidly at the latest sowing date)*
- Later terminated cover crops also produced more biomass
- As a result, the cover crop that was established the earliest and terminated the latest produced the most biomass (about 14,000 kg/ha)
* A similar effect was observed in another trial that tested three single species (phacelia, black oat and clover).
Termination timing (best)
Always balance biomass production with the need to establish the following crop:
- Aim to destroy cover crops as far ahead of the following crop, unless ‘drilling on the green’
- As a rule of thumb, terminate cover crops 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 cover crop residues to die back, allowing easier cultivating/drilling in the spring
- Ensure termination dates are compatible with rules detailed in any agreements (such as countryside stewardship or SFI options)
Why earlier destruction is often best:
- Increases soil-warming rate
- Reduces disease inoculum
- Provides more time for crop residue decomposition
- Reduces potential phytotoxic effects of residues (associated with some cover crops) on the following crop
- Decreases interference with drilling operations
- Helps the soil surface dry out ahead of tillage and drilling operations
The last point is particularly important on heavier soils and for cover crops with dense (and wet) canopies (cover crop biomass can act as a blanket and reduce the natural drying process).
The interaction of soil type, cover crop growth (canopy size and type of growth) and sowing system affect the best method of destruction.
In general, options and opportunities are greater on light and medium soils.
Notes on nutrition
A longer window between termination and the cash crop allows more time for cover crop residues to break down and release nutrients back into the soil.
The carbon-to-nitrogen (C:N) ratio of cover crop residues is one of the main factors that influences the dynamics of nitrogen mineralisation.
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).
Visit the AHDB web page on this topic
A note on biofumigants
Biofumigant or trap crops may have specific destruction requirements.
Termination timing (late)
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:
- Nutrient lockup (especially of nitrogen when cover crop residues break down)
- Localised acid production (created by soil biology breaking down the residue)
- Wetter soils (causing waterlogging and anaerobic conditions), which is more likely on heavier soils
*The mechanisms are not fully understood.
A note on drilling on the green
Drilling straight into green cover has its risks (see earlier points on why earlier destruction is often best). Additionally, the lumpiness caused by fresh residues creates a sub-optimal seedbed. A crimper-roller ahead of the tractor may suppress cover crops and allow no-till planting of the main crop at the same time.
Weather considerations
Always consider the frost tolerance of species in a mix, 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.
Frost-sensitivity information (winter hardiness) is published on the species selection page.
Because of the variation in the duration and severity of frosts in the UK, other destruction methods are likely to be required to finish the termination job.
A quote from related research
“In East Anglia, frost only partially terminated the cover crops. However, the gradual and partial termination of the cover crop had several benefits:
- Progressive supply of nitrogen
- Reduced above-ground biomass permitted a more effective use of herbicide
- Partial decomposition of the cover crops eased residue flow through machinery”
Tom Storr, Research Agronomist, Dyson Farming.
(Image: David Purdy Soils)
Use of machinery
Rolling and crimping can mechanically destroy cover crops.
It tends to work best when there is a frost and 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.
Timing is key. 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).
Although cover crops can be destroyed by ploughing, it is important to consider the potential impact on soil condition.
(Image: David Purdy Soils)
Use of grazing
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 compaction and 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’ to limit the amount of time spent grazing one location, particularly on heavy soils.
Be aware that some cover crop species are potentially toxic to livestock.
- As red clover has high levels of phytoestrogens it can cause fertility issues. Do not feed it to breeding ewes six weeks before and after tupping
- Some linseed can release cyanide when grazed, which can be toxic to animals
- Buckwheat can be toxic to livestock (in high concentrations)
If in doubt, consult your vet, especially before grazing unfamiliar covers.
(Image: AHDB)
When bringing livestock onto arable farms:
- Prepare a manure management plan* that shows the calculated Livestock Manure Nitrogen Farm Limit and a risk map with all watercourses and no-spread zones
- Allow time after grazing for the cover crop to ‘green up’ before spraying off
- Consider using a strip till drill (rather than a disc direct drill) to help take out compaction towards the top of the soil
*You do not need to account for manure ‘deposited’ by grazing livestock in the N-max for the following crop. Although it is difficult to estimate the level of crop-available nitrogen, it is likely to be relatively low (in terms of spring crop nitrogen allowance). Manure distribution is also likely to be uneven. However, a deposition figure does need to be calculated within manure management plan calculations.
Resources
The most commonly used online resources for grazing and animal feed have been reviewed by a team at Newcastle University, with the following recommended.
The benefits of sheep and arable rotations
Connecting arable farmers with livestock farmers (carbon dating)
Use of herbicides
Chemical termination is often the most reliable and typical route for cover crop destruction.
In general, non-selective herbicides can kill cover crops and weeds.
Herbicide choice depends on the cover crop species:
- Grass cover crop species are usually controlled with glyphosate
- Control of legume cover crop species with a single active ingredient can be variable
- Glyphosate combined with 2,4-D may be the most effective approach across cover crop species
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.
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.
Herbicide resistance management guidance must be followed to minimise resistance risks.
Read the Weed Resistance Action Group (WRAG) guidance
(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.
