Put cultivations in context for better blackgrass control

Using the plough strategically and better understanding the impact of post-harvest cultivations could dramatically improve blackgrass control across farms infested with the grassweed.

Many growers will be tired of reading the endless media coverage of blackgrass trials, which often portray the same old messages, just from a different source.

The common view is that ploughing is key to reducing blackgrass, combined with other cultural methods such as delayed drilling and spring cropping to maximise its success.

Many trials’ datasets also vilify direct drilling and label anyone doing it “criminally insane” in the context of blackgrass control.

See also: Blackgrass-infested fens grower turns to cultural control methods

However, results when adopting these cultural practices can be variable and direct drilling doesn’t always exacerbate grassweed problems where used.

John Cussans

For this reason John Cussans, weed biologist at Niab, believes this “rigid orthodox thinking” is flawed and industry needs a better grasp of the principles behind what is happening in the field.

“This simplistic view misses the fact that cultivations happen in different places or circumstances in the rotation and often have a different effect, depending on where they are deployed.”

Explaining variation

Mr Cussans gave an example of one Niab trial, carried out in partnership with Bayer, that showed an 80-90% reduction in blackgrass numbers after ploughing.

As usual, direct drilling performed poorly, with huge numbers of blackgrass plants emerging in the crop. Min-till was somewhere between the two.

However, in the same dataset, using the same treatments, there was a 30-40% variation for ploughing and direct drilling.

In context, the trial was carried out in two blocks – one following a heavily infested winter wheat crop, the other after a spring crop where seed return would be minimal.

“In the winter wheat scenario, the seed pressure would be on the surface and you would expect inversion tillage to be effective,” explained Mr Cussans.

He added that after the spring crop, there would be less seed on the surface, but on a bad blackgrass site you would expect a significant seed bank in the soil profile.

Therefore, ploughing would be less effective and by reviewing the complete set of original trials’ data, this variation can be explained.

“That is why the simplistic view that rotational ploughing and building everything else around it needs to be nuanced.

“We need to think about cultivations in context to understand how they effect grassweed populations.

“Ploughing should be a strategic tool, not rotational, and only used when absolutely needed.”

Post-harvest management

Another area where a fresh perspective is needed is post-harvest management, said Mr Cussans.

Current thinking is based on the model that you have a weed seed bank that naturally declines about 70-80% year.

You then encourage blackgrass to germinate with cultivations and kill it with glyphosate, further contributing towards seed losses.

However, independent research into three post-harvest regimes found no consistent link between killing high numbers before drilling and having less in the crop.

“There is an obsession with the thought that if you get everything to ‘green up’, you are doing a good job and I believe it is dangerous.

“Colleagues elsewhere in Europe dealing with problems such as loose silky bent and blackgrass look at us like we are from Mars when we talk about post-harvest control,” said Mr Cussans.

In countries such as Denmark they do as little as possible for as long as possible, taking into account naturally occurring direct seed mortality encouraged by leaving the seeds on the soil surface.

In UK conditions, existing research linking post-harvest management with blackgrass in the following crop shows that in dry conditions, leaving seed on the soil surface between August and mid-September promotes direct seed mortality and reduces persistence.

However, there are differences between seasons and sites. For example, without cultivations, direct mortality increased in a dry autumn and blackgrass numbers were cut in the following crop, but in damper conditions the influence of direct mortality is reduced.

Mr Cussans said that exploring weed biology in this area could help uncover why direct drilling can work much better than predicted.

“Abandon the rigid orthodoxy, really think about where your seed is and what you are doing with cultivation strategy to influence it.”

John Cussans was speaking at the recent Association of Independent Crop Consultants annual conference, near Towcester, Northamptonshire