Growers have long been advised that to ‘reset’ land with a heavy black-grass burden, the plough should be employed rotationally.
However, experts at Agrovista’s Lamport AgX trial site in Northamptonshire have been using lessons learned from their past eight years of work there to challenge conventional wisdom and leave soil undisturbed in the battle against black-grass.
An alternative approach was tested last season on a plot that had grown three wheats in succession.
In 2019, the third wheat was wiped out by black-grass and did not make it to harvest, despite receiving a comprehensive £150/hectare herbicide programme.
To investigate how best to tackle the heavy weed burden, the plot was split in half.
One half was set to be ploughed in mid- to late-October ahead of a spring-sown cash crop and the other half was planned to have a cover crop with a black oat and phacelia mix before direct drilling with a spring crop, using the principles learned at Lamport over the years.
Both plots would then return to winter wheat in autumn 2020.
The Sprinter Pro cover crop mix, which is designed to have an open growth habit and allow black-grass to actively grow in the cover, went in on September 5 at a seed rate of 15kg/ha using vigorous but shallow cultivations to stimulate weed germination without mixing seed through the soil profile.
Extreme The other half was left as a stale seedbed with two or three very shallow runs with a Vaderstad Cross Cutter.
This allowed the black-grass to chit, but in mid-October the extreme wet weather put paid to the idea of ploughing it.
Niall Atkinson, Agrovista’s farming systems research and development adviser, says: “We couldn’t get near it with the plough, but we had done successive cultivations and got all the black-grass growing and it was acting like a sponge.
“Water infiltration was virtually zero in the stale seedbed, while in the cover crop plot we had a lot of black-grass growing and the cover crop among it, and much better water infiltration.”
Agrovista’s head of soil health Chris Martin says the situation was not helped by the black-grass plants’ very shallow rooting.
“The high calcium:magnesium ratio, combined with the high silt content, means the soil at Lamport has a tendency to run and can become very tight as all the fines pack together.
“Where soil is colonised by shallow black-grass roots, the soil tends to form a pan around the 5cm mark and the soil surface becomes very dense and potentially anaerobic.
“It’s really important to encourage air back into this soil with a shallow low-disturbance tine.
It’s important not to go too deep with metal in this soil as it can de-structure it at depth allowing the fines to run deeper, exaggerating the problem.
Cultivation “Subtle, shallow cultivation should be employed alongside a cover crop with an extensive rooting system to encourage natural restructuring through biopores and roots.
“Everywhere there’s a root, there’s biology, and where there’s biology there’s a healthier soil.”
The autumn-established cover crop received two doses of glyphosate, one on February 6 and the other on March 25, to take out any remaining green material.
Spring oats were then direct drilled using a Weaving GD on March 27.
Mr Atkinson says: “We had a good crop of spring oats free from black-grass and harvested just short of eight tonnes/ha.
“In autumn 2020 a wheat crop went in, which now looks like it might yield nine-10 tonnes/ha and it has hardly any black-grass in.
This is a truly remarkable result.
“In the plot we were meant to plough, it was impossible to establish a spring crop, so instead we established a Sprinter Pro summer cover crop ahead of wheat sown in autumn 2020.
Soil structure “The cover crop alone only did a partial job of improving the soil structure, so the wheat was poor, had black-grass in and was patchy, due to a combination of wet weather and black-grass.
“When you dig down 10-12 inches, you’ll find it’s only the top 4-5in of this soil that is requiring a bit of loosening to open it up and let the roots through.
“If we go any deeper we’re going to do more harm than good [by cultivating it] – we are only going to smear the soil and exacerbate any problems.
A very shallow disturbance with a tine, coupled with cover crop roots, is doing a fantastic job and it’s more cost-effective.
“We all know delayed drilling of winter wheat for black-grass control, especially on more difficult soils where the weed tends to occur, is a high-risk strategy.
If you’ve got a large area of winter wheat to establish in autumn and you’re not starting until mid-October, you’re totally reliant on the weather.
“The black-grass burden at Lamport is as bad as you’ll see anywhere.
These results suggest that by using the right rotation and appropriate combination of cover cropping and cultivation, we could pull the wheat drilling window forward, returning to late-September sowing perhaps one year in three.
Restructuring “Before we carried out this work I would have generally agreed with rotational ploughing.
“Previously, as soon as the combine was out of the field, I’d have ripped up the whole lot.
But as we’ve learned more, you can see there’s nothing wrong with the field structurally, so why do that and restart? All we’re going to do is oblite rate any natural restructuring that’s gone on and damage soil biology.”
The importance of minimising soil disturbance at the time of drilling the cash crop has also been highlighted at the trials site.
In one ‘regenerative’ low-input plot lines of black-grass have emerged where the angled discs on the direct drill overlapped.
Mr Atkinson says: “If we’d gone through with a tined machine in spring, which creates even more soil movement, we would have really exacerbated that issue.
Regenerative “The regenerative plot also shows the importance of row spacing on drills in a black-grass scenario. Some newer direct drills are going to 200-250mm row spacing and where we’re looking for crop competition to help control black-grass, it’s just too wide. We’re finding 125- 170mm row spacing is much better in a grass-weed scenario.”