Cover crops are a love-hate topic for some farmers who have seen yields fall or land struggle to dry out as a result of their introduction.
But work at Agrovista has shown success can be dictated by two crucial factors, according to head of soil health Chris Martin.
“Number one for success is date of establishment – the earlier the better. For cover crops, it’s all about biomass. The daylight and temperatures in August offer huge potential, but when you get to September it drops off a cliff.”
For those further north, where getting a cover crop in early might not seem viable, there are options available.
“Blowing it into the growing crop could be an option depending on the herbicides used.
You can then drill some black oats directly into that [post-harvest].
A September drilled black oat might look quite moderate above ground, but what it’s doing below the surface can still be very beneficial,” he says.
Unless a cover crop is being used to target a specific problem on-farm, such as black-grass, a diverse mix of four or five complementary species is a good place to start.
“In a perfect world, the more diversity the better, but in practice not all species will be able to grow and it can be quite expensive, in our experience.
Extensive rooting “I would always base the mix on a black oat as my grass species because it’s like an iceberg with really extensive rooting.”
Other choices should depend on cash crops in the rotation.
“I like a legume such as vetch or clover for nitrogen fixing, but one of the problems is in a dry season you may get pea and bean weevil.
“There’s evidence that some vetch and clovers are beneficial in a pea or bean rotation because they will prime the bacteria and help the beans fix nitrogen.
We’ve had lots of beans after vetch cover crops with no problems.
Phacelia “If you haven’t got brassicas in your rotation they are great, but if you grow OSR I wouldn’t include a brassica. If you want a more robust plant, phacelia is good too.”
The other key to success is to destroy the cover crop early in December or January.
This is not only to allow the soil to dry out ahead of drilling, but because the organic acids produced as part of the decomposition process as plants break down can have a negative impact on establishing the next crop.
“Whereas if you destroy the crop eight weeks before drilling, they are out of the system,” Mr Martin adds.
“Immobilisation is also key, because when something breaks down, it needs nitrogen to enable that process, so if you spray off earlier it is already broken down and the nutrients are available for the crop.
If you break the cover crop down on the day, it’s immobilising, rather than mineralising nitrogen.
“That nitrogen won’t be mineralised until six to eight weeks down the line, which is after the critical period for establishment.
This is why having the right species is key.
“A black oat has the root mass to keep the soil stabilised and help the decomposition process and soil biology when there’s nothing technically growing above ground.”
Work at Agrovista’s Lamport AgX trial site, on heavy clay soils, has demonstrated that as long as a cover crop is destroyed in December or January, it has no detrimental effect on the following crop.
For those drilling ‘on the green’, directly into a cover crop, choosing species such as vetches which have a low carbon:nitrogen (C:N) ratio means immobilisation does not occur as quickly because the plants break down much quicker.
“Something like a mustard with a high C:N can take up to a year for that nitrogen to be available, whereas vetches is just a matter of weeks.”