Blackgrass – Brampton goes west

A new trial site in Oxon puts soil at the forefront of the blackgrass battle. CPM went along to find out more.

Blackgrass and wet soil go together, so you have to sort out drainage first.

By Paul Spackman

Soil structure must be the primary focus when tackling burgeoning blackgrass populations, say organisers of a new farm trial in Oxon. With herbicide-resistant blackgrass populations exceeding 5000 plants/m², the new Hutchinsons’ blackgrass centre at Mollington, near Banbury, faces big challenges over coming years.

The firm took on four fields last autumn, varying in size from 4.5ha to 17.2ha, which will showcase a range of chemical and cultural techniques to reduce blackgrass numbers, based on findings from its Blackgrass Centre of Excellence at Brampton, Cambs.

 

“The aim is to apply the blackgrass management techniques developed over many years at Brampton, alongside other aspects specific to this site,” says trial manager and local agronomist Richard Hall.

 

Tricky soil conditions are the main challenge at Mollington and addressing underlying issues is essential for control strategies to be effective, particularly spring cropping on heavy land. The topsoil is primarily good quality clay silt loam, but it overlies less permeable clay subsoil that can inhibit drainage and exacerbate blackgrass, Richard Hall explains.

 

“Blackgrass and wet soil go together, so you have to sort out drainage first,” says Hutchinsons’ technical manager Dick Neale. However, this does not mean rushing in with the subsoiler, which can make matters worse by bringing fresh blackgrass seed to the surface and disturbing natural soil structure, he warns.

 

Solutions have to be tailored to individual fields. For example, closer inspection of waterlogging in one field at Mollington revealed the cause was runoff from a nearby yard and public road which was entering the field through a gateway, not subsoil compaction or high water table. Moving the gateway and reinstating the ditch should stop surface flooding without needing deep cultivations, although mole ploughing would be considered if subsoil clay is consistent enough, he comments.

 

Dick Neale believes restricting cultivations to the top two inches (50mm) is key to controlling blackgrass. Within this zone the grassweed can be encouraged to germinate then sprayed-off with a non-selective herbicide before drilling, and any seedlings will be germinating within the range of residual chemistry. Meanwhile, leaving deeper seed undisturbed allows natural depletion of the seed bank at 70% per year.

 

Shallow tillage, combined with good residue management further builds natural soil biology and structuring, which improves drainage, aids crop rooting and improves productivity, he adds.

 

“It may look like you’re hardly doing anything, but just moving the top two inches is enough to encourage blackgrass to germinate, while allowing the soil to aggregate naturally.”

 

At Mollington, shallow tillage is done with an on-farm-engineered version of the tine and press-based Cousins Surface cultivator.

 

Strong root growth is fundamental to improving soil structure and something that will be examined with cover crops on the site to improve natural drainage. “It’s easy to underestimate what roots will do,” says Dick Neale.

 

Crops that produce a strong tap root, such as legumes, oilseed rape, linseed, mustard or tillage radish, are best at penetrating deeper into the soil, but a range of three to four species should be sown to provide different rooting depths and spread risk, he advises.

 

Cover crops may have a role to play on heavy land by drying soil to depth and enabling spring cropping – a key tool in cultural blackgrass control. Two fields at Mollington are in spring barley this season, with a range of seed rates and drilling systems being compared.

 

Spring barley will only help reduce blackgrass if grown correctly, Dick Neale insists. This means getting a good flush of weeds prior to drilling, minimising soil disturbance at drilling, and sowing enough seeds to establish a crop that outcompetes blackgrass and delivers a decent yield.

 

“You must be realistic about how much seed will be lost, especially when sowing spring crops on heavy land. Yield is all about seed numbers in barley.”

 

Dick Neale says 22,000 barley seeds/m² are required for a 10t/ha crop, which based on 24-28 grains/ear, 915 ears/m² and three tillers per plant, equates to 310-320 established plants/m². At a seedbed survival of 70-75%, this means sowing 450-500 seeds/m².

Banbury blackgrass strategies:

• Cultivation and drilling techniques – shallow versus deep tillage
• Spring cropping
• Cover crops
• Variable seed rates in spring barley
• Herbicidal control in OSR
• Impact of varying flufenacet rates on pre-em control.

Cultural “stacks” key to blackgrass control

“Stacking” cultural techniques is essential for effective blackgrass control, according Hutchinsons’ Neil Watson. The concept is often applied to complex herbicide mixes, but has equal merit for the range of non-chemical options in the blackgrass armoury, he says.

 

Delayed drilling, spring cropping, higher seed rates, cover crops and cultivation techniques can all help reduce blackgrass pressure, but their impact is much greater when used in a coordinated way and integrated with chemical options, he says.

 

Attention often centres on “percentage control” but Neil Watson believes more focus must be given to plants that survive, as blackgrass has an impressive ability to compensate for lower populations by producing more tillers (up to 60 per plant) and larger ears typically containing at least 200 viable seeds per head.

 

This was clearly shown last season, when blackgrass tillered heavily and some ears were almost double the average size of 10cm, he notes. “In such cases there’s double the amount of seed return too.

 

“You might achieve 90% control of plants, but this is only equivalent to 45% control of heads given the ability of surviving blackgrass to compensate through tillering. From the very start it’s an uphill struggle.”

 

Blackgrass should therefore be hit hard with all available options to minimise seed return and deplete the seed-bank as quickly as possible, he adds.

 

Reducing numbers below 12 plants/m² allows a static population to be maintained, however research shows this amount can still result in a 5% yield loss, so a “zero-tolerance” approach may be better long term.

Building the cultural stack

• Soil – Improve structure and drainage to reduce waterlogging, improve crop growth, and facilitate spring cropping on heavy ground. Cover crops with different rooting characteristics could play a key role.
• Cultivations – Only work the top 25-50mm to keep blackgrass seed within a “kill zone” and allow deeper seed to deplete naturally.
• Delay drilling – Maximise stale seedbeds prior to sowing and drill after the main autumn flush. Bu this is only effective if a good flush of weeds can be established and sprayed off. Beware of delayed germination in high dormancy years such as 2016/17 and/or when soil conditions remain very dry.
• Cropping – Use spring cropping as the ultimate delayed drilling – spring barley remains most effective option. Maximise stale seedbeds and non-selective herbicides before spring drilling.
• Competition – Increase seed rates (up to 450-500/m²) to compensate for likely lower germination on heavy ground in spring and ensure crop outcompetes blackgrass.
• Chemistry – Focus chemical control on effective pre-emergence residual herbicides aiming for 70-80% control.
• Monitor – Tailor decisions to individual field and seasonal conditions. Give it time – there is no quick fix.