Persevere with spring cropping to control blackgrass

Heavy-land wheat growers using spring cropping to control high populations of resistant blackgrass may have

to persevere for longer than they think

Returning to a winter wheat-based rotation after a couple of seasons growing an autumn cover crop/spring cereal is likely to be too soon,

even if blackgrass appears to have been almost completely controlled.

Latest results from Agrovista’s Project Lamport trials, where the background blackgrass population exceeds 2000 plants/sq m, shows too much viable blackgrass seed remains ready to infest winter crops, undoing the dramatic improvements delivered by the spring cropping programme.

For the past five years, Agrovista has been comparing 14 different rotational systems on farm-scale plots in Northamptonshire, to identify techniques that can help combat the weed. These include;

• Winter and spring cropping

• Autumn cover crops/spring wheat rotations

• Traditional fallows

• Hybrid rye for AD plants

• Winter wheat/OSR rotation

• Late drilled winter wheat
  

The site was ploughed before the trials commenced, to reset the blackgrass, and subsequent cultivations have left this undisturbed. The most successful programme for the duration of the project has been spring wheat following an autumn-sown cover crop.
The cover crop consists of black oats and vetch, which maintains an open habit early on to enable blackgrass to establish, before growing rapidly to condition soils and facilitate spring drilling on heavy land.

The cover crop, along with the ‘trapped’ blackgrass, is then sprayed off in the early winter. Spring wheat drilling is carried out using a direct drill to minimise soil disturbance and subsequent blackgrass chitting.

Over the past three very different seasons, this technique delivered 8.3-10.6t/ha of spring wheat, similar to yields achieved with a wheat crop with a moderate blackgrass infestation.

Blackgrass control has been impressive. Last season, just two heads/sq m were counted pre-harvest, in line with previous years and best of any plot on the site. The performance looks set to be repeated this year, with scarcely a blackgrass head to be seen. 

However, it might take longer to get on top of the blackgrass problem than many people think, says Agrovista technical manager Mark Hemmant.

“We often hear that a couple of years of spring cropping is enough, but results have shown us that won’t work here, and can result in real problems for a year or two afterwards.” 

After two successful years under the cover crop/spring cereal regime, in autumn 2015 the reset plot was returned to winter wheat after a deep cultivation, mimicking likely farm practice. 

However, blackgrass heads rose to 129/sq m pre-harvest 2016, compared with 13 the previous season, and yields slumped to around 5.7t/ha, down from 9.5t/ha in the previous spring crop. 

Further problems followed. To regain control, the autumn cover/spring wheat option was immediately reintroduced last autumn. “We didn’t want to plough because we would have turned up a huge number of viable seeds buried at the start of the project.”

However, so much seed had been returned in the winter crop that the resulting autumn flush almost out-competed the cover crop.

Despite being sprayed off in late winter to stop the blackgrass getting too big, this created a wet mulch which then turned into a crust. This shielded blackgrass from a pre-drilling glyphosate and so much grew in the following spring wheat it had to be sprayed off.

“On a farm scale this would have been a financial disaster,” says Mr Hemmant. “However, we put it back into a cover crop last autumn and the following spring wheat looks fantastic. That further reinforces my faith in the spring cropping approach.”

Winter rotation unsustainable

Compared with the autumn cover crop/winter wheat system, the farm’s traditional rotation of two winter wheats/oilseed rape looks unsustainable, says Mr Hemmant.

Last season, a first wheat in the trial harboured 500 heads/sq m and it yielded under 8.9t/ha. The headcount was double that recorded in a second wheat in July 2016, before the crop went into OSR.

The blackgrass count looks set to rise again in the current second wheat, with a forest of heads appearing above the crop in the past few weeks.

Farmers are concerned by growing a spring crop that they will lose out on yield,” says Mr Hemmant.

“This shows that a winter wheat crop with a moderate blackgrass infestation won’t perform any better, costs more to grow and is exacerbating the blackgrass problem – each year it is getting worse. Our rule of thumb is once head counts reach 40-50/sq m you are looking at serious yield loss, and you need to make significant changes. In this plot, the number has risen from 55 in harvest 2014 to 500 last year.”

Comparison of blackgrass control

Conventional rotation

Spring cropping (after autumn cover)

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