The stewardship option AB15: two-year legume sown fallow is intended to create areas of abundant food for insects, improve soil health and fertility, all while helping growers get on top of bad black-grass areas.
But trials at Agrovista’s Lamport AgX trial site in Northamptonshire are exploring ways to adapt the stewardship option, after identifying flaws in its ability to
tackle the heavy grass-weed infestations.
Under current Countryside Stewardship (CS) rules, AB15 should be established in early autumn, as soon as possible after harvest, ideally by the end of August in year one and year three of the agreement and must be present until the end of the second summer after sowing (years three and five).
It should be cut at least twice in the first year following the autumn sowing to control emerging black-grass and growers are told to prepare for subsequent cuts to be as close as three weeks apart as black-grass re-heads more rapidly after each cut. Because the aim of this option is to provide an abundant supply of pollen and nectar-rich flowers, it cannot be grazed, fertilised or sprayed.
Lamport AgX, which was initially established to find sustainable solutions to blackgrass control on heavy soils, has been investigating how AB15 works in practice, by mirroring CS rules on one plot and adapting it slightly using lessons learnt from the long-term experiment on another.
Niall Atkinson, Agrovista’s farming systems research and development adviser, says: “This option is sold to farmers as a way of improving soil health and controlling black-grass. It pays £522/hectare which, as a headline figure, can look attractive.”
However, the fact the rulebook states the legume mix should be established before the end of August sounds alarm bells, he says.
“From a black-grass point of view, we know we should do everything we can to delay drilling and take out black-grass in the autumn, yet this would mean planting a legume mix into a potentially very bad black-grass situation.”
Putting the theory into practice, in August 2020 an AB15 legume mix of alsike clover, bird’s-foot trefoil, black medick, common vetch, lucerne and red clover was
drilled on August 27 after winter wheat, which had had a high black-grass burden. However, very little of the legume crop established after a ‘carpet’ of black-grass smothered it out.
As of mid-July 2021, the plot had been mown four times and the legume was sparse while black-grass was abundant, with heads close to the ground Mr Atkinson says: “In the first year of the scheme you’re told to mow it several times. However, you don’t want to start mowing too early because when you cut
the seed head, the plant goes into survival mode and throws out another seed head but much closer to the ground. You get to the point where your mower just
can’t take out the seed heads low enough.
“Looking at the plot now makes you wonder what it’s done for soil health to date because black-grass has a root system that’s very close to the surface and doesn’t get down to any depth. It holds water close to the surface, so from a water infiltration and soil health point of view it’s doing nothing and from an environmental point of view it’s been pretty much a disaster with all that mowing.
“Anyone that seems to have success with AB15 has had low levels of black-grass. If you can get the legumes established, then it’s been successful. However, if you don’t have the black-grass issue, at £522/ha you could earn more doing something else. It’s only attractive to me if it sorts out your black-grass problem, while improving overall soil health and margin.”
Exploring how AB15 could be adapted to improve grassweed control, in 2018/19, rather than establishing the legume mix post-harvest, Sprinter Pro, a cover crop of black oats and phacelia designed to specifically target black-grass control, was planted in a separate plot.
Mr Atkinson says: “It’s not a dense cover crop and is open with space and light, allowing germination of black-grass.
“At the time of cover crop establishment, we carried out quite a lot of shallow soil movement to encourage black-grass to chit and allow it to grow amongst the cover. We then desiccated the cover and black-grass early in the new year, applying a second spray close to establishment of the AB15 mix to hit any shielded black-grass or anything that had emerged since.”
The AB15 legume mix was drilled in the spring and no successive mowing was needed through the summer.
In August 2020, the legume mix was sprayed off and winter wheat was direct drilled in mid-October. It looks set to yield well with very low levels of black-grass present.
Spring drilling of AB15 is not allowed under CS rules but will be an option under the new Sustainable Farming Incentive.
However, it is unclear how long it needs to be in the ground, says Chris Martin, head of soil health at Agrovista.
“Legumes tend to establish best in the spring anyway, but the question is if the scheme starts on January 1, and you’ve effectively got to keep it for two years, then will the only option be to follow it with a spring crop?
“From a soil health point of view, the cover crop in the autumn is doing a fantastic job and we didn’t need to do successive mowing through the summer.”