Selecting the right species is critical to the success of autumn cover crops when tackling blackgrass in badly infested fields.

The system developed by Agrovista then enables growers to profit from a relatively clean crop of spring wheat, as demonstrated by Project Lamport.

The trial site in Northamptonshire has a severe resistant blackgrass problem and back in 2011. It had a population of 2,000 plants/sq m, explains Agrovista’s national trials co-ordinator Niall Atkinson.

See also: Plan cover crops with care to avoid disease pressure

“The grower was typically only seeing 40-50% control with pre-emergence residuals plus Avadex (tri-allate) and was returning a substantial number of seeds back to the soil,” he says.

In the wet autumn of 2012, the oilseed rape failed and then the site was fallowed a year, being ploughed followed by a series of stale seed-beds.

“Therefore, the site had received some cultural control before we started the trials.”

Unsustainable

The area not being used for trials went back to winter cropping, receiving a full herbicide programme.

“This wheat crop followed a full year of fallow plus a failed oilseed rape crop and two autumns of residuals. And after having spent £1,000/ha on the wheat, as a farmer I would now be considering glyphosating it off,” says Mr Atkinson.

He believes the reliance on autumn residuals means that if farmers can’t apply them at the right time or it is a dry autumn, they’ve had it.

“It is a risk which is becoming too great with the increased growing costs and prices where they are.”

Therefore, there is a need to look at a new system and this is what spawned project Lamport, explains Mr Atkinson. “The aim is to tackle blackgrass while keeping the system in profit.”

Agrovista technical manager Mark Hemmant adds: “We know from Stephen Moss’ [at Rothamsted] work that spring cropping, ploughing and delayed drilling are the key ways of reducing blackgrass numbers.

However, the challenge at Lamport in not just establishing spring cropping on heavy land, but also direct drilling it to avoid disturbing soil and causing blackgrass flushes.

“It is not easy land to plough and we are reluctant to plough for the next five to six years, so we are looking at cover crops instead,” says Mr Hemmant.

Mr Atkinson explains the main consideration for using cover crops is to suck moisture out of the soil. “So when spring comes, you can travel and drill into these heavier soils. The roots also helps minimise soil disturbance when drilling in spring.”

Results

Now into its second cropping year, the results have been impressive producing a clean crop of Willow spring wheat, despite only having received Herold (diflufenican + flufenacet) at one-quarter rate, 30g/ha of diflufenican and 1,000g/ha of pendimethalin.

“It received hardly any chemistry and we’ve seen blackgrass numbers come down hugely. This shows it is possible to go from a full infestation of 2,000/sq m to a clean spring crop.”

Winter wheat with a robust herbicide programme is giving 90% control, but there is still a significant level of blackgrass heads being seen.

Add in stale seed-beds and spring wheat and you are getting 96% control and with a cover crop, you are nearly at 100% control with fewer than two heads/sq m.

“Cover crops are giving you an extra 3.6% control so are going from the seed bank continuing to increase to a level of control where it is reducing,” says Mr Atkinson.

Economics

Last year, the spring wheat averaged 8.6t/ha and gave a more attractive return than sticking with winter wheat (see table).

“Assuming the blackgrass infestation here at Lamport has reduced winter wheat yields down to 7t/ha and with a price of £120/t, you are looking at a return of £600/ha over the cost of herbicides and fungicides.

“If you are getting 7t/ha for the spring wheat (average at Lamport was 8.6t/ha), you will get a return of £720/ha plus £20/t premium for spring wheat.”

Mr Atkinson calculates a return of £860/ha compared with £600/ha for winter wheat struggling with a blackgrass infestation.

There are other benefits like spreading the workload and soil health improvements.

As for the cost of establishing the cover crop, Mr Hemmant points out that growers would be going in anyway to create the stale seed-bed. Cultivation work is done in the autumn, when put cover crop in.

Seed costs are about £39 to £42/ha, depending on the mix used.

What cover crop?

Traditionally when you mention cover crops, many growers think of mustards, as they rapidly produce cover. But Mr Hemmant highlights that this is also when blackgrass germinates and leaves no light for the blackgrass, which doesn’t germinate.

“Therefore, we are looking for something that establishes rapidly to give a sparse, open cover, allows the blackgrass to germinate and then fills in and kills blackgrass later in the autumn.”

The crop also must be suitable to drill into in the following spring, will not lead to volunteers in spring wheat and has sufficient rooting to hold soil together when drilling in spring.

“We tried phacelia and again it created too much cover and did not allow blackgrass to germinate. This resulted in more of it coming up through spring wheat.”

“Many farmers have wheat and rape rotations so we ruled out brassicas.”

The result is two different mixes, both of which contain black oats, as it gives huge biomass above and below the soil and is hardy, being grown in the Hebrides for forage.

However, he adds that there is a lot of confusion over black oats, as there are significant differences between the species. “Avena strigsoa is the species to go for as it is a later variety. Some earlier maturing varieties such as A pratex are not suitable for autumn planting, as they put up a seed head.

The two mixes then either contain berseem clover (Chlorofiltre N-structure) or common vetch (Chlorofiltre 25) to give a range of rooting depths, the latter being Environmental Focus Area (EFA) compliant.

“Berseem clover is the jackhammer of rooting, which helps break up the soil. Oil radish is also good rooting, but we found it left holes where the root rots in spring and, consequently, the blackgrass germinated in these holes,” says Mr Hemmant.

Cover crop establishment

When establishing the cover crop, the aim is to produce a good seed-bed for both the cover crop and blackgrass. “We use a combi drill to encourage the blackgrass by disturbing the surface as much as possible.”

However, if starting out for the first time, Mr Atkinson urges growers carry out some cultural controls and consider ploughing, as at Lamport, to reduce numbers to enable the cover crop to establish itself before the autumn blackgrass flush.

“If you can’t plough, there are two options. Deep cultivation using low disturbance legs or direct drill the cover crop. Once established, then go through with a straw rake at the 3-4 leaf stage to help encourage blackgrass to germinate.

Timing wise, Mr Atkinson says late August/early September although the clover mix is less suitable for later sowing being a small seed. Both mixes need to be in by the first week of September.

The aim is to establish 70 black oats/sq m with a seed rate of 25kg/ha for the vetch mix and 17.5kg/ha for berseem clover. However, if are following oilseed rape where there is more residual nitrogen, he advises cutting back to 20kg/ha and 15kg/ha, respectively.

Spring wheat establishment

“In the following spring, the black oats are easy to kill with glyphosate, so there is no risk of creating a new weed problem. Residuals also take it out and by that time the frost has already taken out the clover, says Mr Hemmant.

The resulting soil is drier at depth and the residue keeps the surface wetter, therefore, providing moisture for the spring wheat seed. In contrast, the plots with no cover crop were very dry at the top while wetter at depth – less ideal conditions.

Two weeks before drilling, a low-dose glyphosate kills the black oats and then spray again to take out the blackgrass a few days later. Last year, the spring wheat was drilled on 18-19 March.

The spring wheat is drilled at a rate of 450-500 seeds/ha to compete out blackgrass and also to drive up yield, as spring wheat tillers less than its winter counterpart.

One tip is to put fertiliser on at drilling or the day before, as is important to get crop away, says Mr Hemmant.

In conclusion, Mr Atkinson sees it as a viable system that offers blackgrass control in severe infestations, while offering viable returns. He sees the system involving two spring crops in every six years.

Project Lamport is supported by Bayer CropScience and will run for a full rotation with being monitored for soil health, soil organic matter and nutrient levels as well as blackgrass populations and yields.

 Economics of cover crop/spring wheat system

(Assumptions: wheat £120/t, quality premium of £20/t, winter herbicide + fungicides = £240/ha, spring herbicide + fungicides = £120/ha)