By David Jones, Broad Acre Agronomist
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. Cover cropping has become a highly debated subject however with many having a strong opinion on the merits or disadvantages of planting a cover crop in the fallow period.
What is a cover crop expected to do?
One of the most important questions to answer before looking at cover cropping is what you are trying to achieve. Cover crops are typically planted immediately after a main 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 for the next crop.
Typical concerns about cover cropping include:
- The cost of seed and planting
- Moisture loss during the fallow period
- How to plant the following crop into the residue left by the cover crop
A long term investment
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. Research by NIAB TAG in the UK demonstrated the soil pan-busting ability of Daikon Radish cover crops for the following crop.
Below is an example of a cover crop sown after canola harvest, allowed to grow for 8 weeks, then sprayed off with glyphosate and planted with wheat:
Understand your rainfall
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 lowering 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 and seed rate helps greatly.
The graphs below from Washington State University Extension help to simplify this story; the green line shows how much soil moisture was at each depth of the profile at the start, and the red line shows where soil moisture finished after 90 days.
Retain crop residues and terminate early
However, the works also showed that at 60 days after sowing, cover crops were still RETAINING as much moisture as the residue cover!!!
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 – 60 days was the message here.
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 – chose your cover crop accordingly.
Reducing the cost
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, and adds value to the livestock enterprise with worm-free grazing.
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 proven benefit or indeed a reason to cover crop, yet!
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.
If you want to know more about cover crops and Conservation Agriculture, www.no-tillfarmer.com is a fantastic resource and news site.
Also, search for Gabe Brown, Jake Freestone and Blake Vince on Youtube to see practical examples of cover crops on farms in other parts of the world.
Some fascinating work from Australia on the effects of long fallow periods on soil biology and nutrient availability:
Have a look at this recent research into the benefits of cover crops on soil biology:
See you next week!
David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.