Article Taken from the Agronomist & Arable Farmer
Getting a cover crop off to the right start is vital if it is to perform properly, says Agrovista’s technical manager Mark Hemmant
Cover crops can be very effective tool on heavy land when integrated with spring cropping to help control resistant black-grass. However, choosing the right mix and establishing it correctly is vital.
“Spring cropping offers the best control of all the non-chemical options,” says Mr Hemmant. “The trouble is on heavy soils it’s often impossible to establish a crop at the optimum time.
The cover crop’s key role is to draw enough moisture from depth over winter and improve structure, enable spring wheat to be direct-drilled into a good seed-bed in early spring, he explains.
The cover crop plays no direct role in black-grass control. Indeed, the aim is to encourage as much black-grass to germinate in the autumn as possible, then spray it off with the cover crop before the spring wheat is drilled.
The theory is being proved in Agrovista’s large-scale trials at Lamport, Northamptonshire and on an increasing number of farms. At the trials site, where untreated black-grass populations reach 2000 plants/sq m, over 99% control of black-grass was achieved both last season and this using a cover crop consisting of the black oat Avena strigosa and common vetch, followed by spring wheat.
By following a few guidelines, growers will be able to repeat this on their own farms, says Mr Hemmant.
The first step is to establish a standard stale seed-bed to encourage an early flush of black-grass to reduce competition with the young cover crop and provide initial control, Mr Hemmant advises.
At Lamport, stubbles were given a light discing soon after harvest and were sprayed off with glyphosate a few days before final seed-bed cultivation.
Choosing the right cover crop species and variety is critical to encourage a further flush of black-grass during the autumn, says Mr Hemmant.
Agrovista offers two mixes, both containing Avena Strigosa, a black oat specifically chosen for its slow initial growth, which helps maintain an open cover crop structure during the peak germination period for black-grass. It then accelerates away in late winter, creating plenty of biomass above and in the soil to help improve soil structure.
One of the mixes, Chlorofiltre 25, also contains common vetch, and the other Chlorofiltre N-structure, contains berseem clover, offering a range of rooting depths.
Cultivation technique is also important to maximise the autumn black-grass flush. “The soil surface should be disturbed thoroughly during seed-bed preparation, he explains. “For most growers, this is the key message when it comes to establishment – any cultivator that mixes the top couple of inches well pre-drilling will do.”
There are exceptions to this rule, Mr Hemmant advises. Very high populations of black-grass can smother the young cover crop, so rather than encouraging the weed to grow early in the first year on heavily infested fields, the aim should be to restrict germination as much as possible until the cover crop is established, he explains.
Ploughing is likely to be the best option, especially where soil structure is compromised. “This will help alleviate the problem and will help black-grass control especially on fields that have not been ploughed for several seasons – as at Project Lamport.
“Farmers who are adamant they won’t plough could put a deep cultivator through, but they should use low-disturbance legs to minimise black-grass germination. Alternatively they might be able to direct drill the cover crop to minimise soil disturbance. In both cases they could then run a straw rake through the cover crop once it has established to encourage black-grass to chit.”
After two or three years, once black-grass seed is exhausted in the upper surface layer and soil structure has improved, Mr Hemmant believes direct drilling alone might be more appropriate to establish cover crops. “It might be a more reliable option – we will be looking at this in more detail this coming season.”
Whatever technique is used, the cover crop mix should be drilled at a low seed rate towards the end of August and no later than mid September, says Mr Hemmant. No fertiliser is needed, but slugs should be monitored.
The aim is to establish 70 black oats/sq m, which will be achieved using 25kg/ha of Chlorofiltre 25 or 17.5kg/ha of the Chlorofiltre N-structure. However, when following oilseed rape, or where soil N levels are high, he suggests cutting back to 20 and 15kg/ha respectively.
Done correctly, regular cover cropping, followed by direct-drilled spring wheat, can control the most severe of black-grass infestations and offer good returns, Mr Hemmant concludes.