How to destroy cover crops

Cover crops are usually killed before or during soil preparation for the next cash crop. Methods of destruction vary markedly and depend on growth (canopy size and type of growth), equipment and objectives.

Methods of destruction

Typically, autumn-established cover crops are killed off or die prior to incorporation and establishment of the following crop in the spring.

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.

Mechanical destruction

Mowing and ploughing into the soil or rolling and crimping can be used to mechanically destroy cover crops.

Herbicides

In general, non-selective herbicides can kill cover crops and weeds. However, herbicide resistance management guidance must be followed to minimise resistance risks.

Winter injury (frost)

Some cover crops species are sensitive to frost and die naturally over winter.

Grazing

Grazing cover crops can maximise the return by increasing livestock numbers or receiving an additional income stream from renting out the land to graziers.

Note: Food safety standards do not allow cover crops to be grazed off immediately prior to growing leafy salads.

A note on biofumigants

Biofumigant or trap crops may have specific destruction requirements.

Timing of destruction

The timing of cover crop destruction affects soil temperature, moisture, nutrient cycling, tillage and drilling operations, and levels of toxic chemical compounds (associated with some cover crop species).

The best approach is often farm-specific.

However, there are several advantages of destroying a cover crop relatively early as it:

• Increases soil-warming rate
• Reduces potential phytotoxic effects of residues on the following crop
• Reduces disease inoculum survival
• Speeds up crop residue decomposition
• Decreases interference with drilling operations
• Helps the soil surface dry out ahead of drilling

The last point is particularly important on heavier soils. Cover crop biomass can act as a blanket and reduce the natural drying process.

Establishment of the following crop

It is often beneficial to leave a week or two between cover crop destruction and the establishment of the main crop.

This delay allows decomposition to start.

Drilling cash crops later may also reduce nitrogen immobilisation issues, minimise allelopathic effects*, avoid seed decay, reduce damping-off diseases (especially under wet conditions) and lessen issues with cutworm and wireworm.

Typically, it also produces a better seedbed for small-seeded crops.

However, establishing a good seedbed may be difficult because of the lumpiness caused by the fresh residues.

Good suppression of vetch in a no-till system has been obtained using a modified rolling stalk chopper.

A crimper-roller ahead of the tractor may also deliver good cover crop suppression and allow no-till planting of the main crop at the same time. Although not recommended for most direct-seeded vegetable crops, this works well for many other crops.

In other scenarios, single-pass drilling through residues is used.

*Sudan grass (Sudex) cover crop is relatively allelopathic, so tomatoes, broccoli and lettuce should not be planted for six to eight weeks after destruction.