How to destroy cover crops

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 be needed to achieve good results.

Although the methods described support reductions in herbicides, they will still be needed at some stages of termination.

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).

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).

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).

Biofumigant or trap crops may have specific destruction requirements.

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.

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.

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: 1. Progressive supply of nitrogen. 2. Reduced above-ground biomass permitted a more effective use of herbicide. 3. Partial decomposition of the cover crops eased residue flow through machinery.”

Tom Storr, Research Agronomist, Dyson Farming.

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 -4^oC 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.

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).

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.

Herbicide resistance management guidance must be followed to minimise resistance risks.

Always follow crop drilling interval requirements.

Take extra care when destroying cover crops in front of broad-leaf crops.

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.

AHDB Research Review 101a (2025) considered research on cover crop destruction methods, biomass breakdown after cover crop destruction and the risk of cover crops becoming a weed burden in the following crop.