Understanding the role of Cover Crops

Cover crops have certainly made it into the mainstream in the last few years. What was known as a “Green Manure” and was seldom seen outside organic farms, is now much more widespread thanks in part to an increasing awareness of soil health and the role that maintaining a cover can have in limiting erosion and run off.

Knowing where to use cover crops and how to manage them is key.

Cover crops are typically planted immediately after a cash crop is harvested, and grown through the fallow period to improve soil structure, build organic matter and improve soil biology, before being terminated by spraying off with glyphosate or mowing prior to planting or land preparation.

Typical concerns about cover cropping include:

  • The cost of seed and planting
  • Moisture loss during fallow period
  • How to plant the following crop into the residue left by the cover crop

It is important to see the cost as an investment in your soil. I have commonly seen cover crops holding over 80kg of Nitrogen in the soil in a Deep Nitrogen soil test, compared to a bare soil fallow. The soil structure benefits also make tillage easier and promote deeper rooting. Work by NIAB TAG in the UK demonstrated the soil pan-busting ability of Daikon Radish cover crops for the following crop.

Caution should be exercised in dry areas. I have successfully grown – and grazed! – cover crops on farms with <650mm annual rainfall. However, the positive effect of increased bulk density of soils and water holding capacity must not be outweighed by the cover crop sucking out too much moisture. Selecting the right species helps greatly.

If this balance is right, the benefits can be very useful in building organic matter, maintaining ground cover and regulating soil temperature. Just be flexible, and don’t be afraid to terminate the cover crop earlier than planned.

In my experience a surprising large number of drills – including tines – can plant through cover crops, provided that the correct species and planting density are used to avoid blockages. Single discs are however preferable and more reliable. Species such as Phacelia and Linseed provide good root systems and are far less likely to cause problems with tine planters.

In some cases (plenty of surface moisture) seed can be broadcast to reduce the cost and time of establishment.

Selecting, or avoiding, the wrong species for the cover crop mix is also important. I avoid Brassicas such as Mustard in Canola rotations to limit the build-up of soil borne diseases such as Olpidium and Clubroot.

Grazing of cover crops is a hotly debated subject. The aim is not to strip the field bare, but to just open the cover enough to help the planter. I am not a fan of cattle grazing cover crops due to the effect on the soil structure, but a low stock rate of sheep in dry conditions is a useful option to have.

Weed control is an area that needs particular care – be sceptical of the claims of lower weed pressure after cover crops. Having searched high and low for high quality research on this, and conducted my own trials, weed control is not a benefit or indeed a reason to cover crop.

Ultimately, trialling small areas on farm a step at a time is the most cautious way to proceed and to understand how to manage them effectively.


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