Improved soil structure, nutrient retention and enhanced soil organic matter levels are just some of the suggested benefits associated with cover cropping, but does the strategy directly impact on weed populations?
At this year’s British Crop Production Council (BCPC) Weed Review, growers and industry experts shared their thoughts on the value of cover crops in the fight against weeds, in particular black-grass.
According to Kent farmer Andrew Barr, the use of cover crops alongside spring cropping means the direct value of the cover crop itself is unclear.
He said: “We know we are getting some control just from spring cropping. So are we getting weed control incidentally because we are spring cropping? How much are we actually getting from cover crops?
In some cases, he said evidence suggested cover crops were successfully ridding fields of black-grass.
“A couple of years ago we found black-grass dying in cover crops and we thought we had cracked it. But it was not happening in every field and the following year it did not happen at all.”
But while the ability of cover crops to mitigate weeds in one given season was questioned, the importance of cover crops in preventing weed pressure over a longer period was well recognised.
Mr Barr said: “Having a growing root in the ground does have an effect on weeds. If you think about black-grass, if the soil is waterlogged and badly structured, that favours the weed, so by getting your soil structure right you are helping control it.”
He indicated while creating a stale seedbed may be effective in combating weeds short-term, longer term it may actually encourage weed establishment.
“In fields where we went for multiple stale seedbeds, spraying off the fields with glyphosate and rolling them repeatedly, we killed a lot of black-grass.
“But over winter soils became completely waterlogged and there was a complete lack of soil structure, which is exactly what black-grass loves,” said Mr Barr.
Equally, by elevating soil organic matter levels it was suggested improved drainage could allow for earlier establishment of spring crops, thus improving competition against weeds.
For Hertfordshire farmer Ian Pigott, one of the most important features of cover cropping is improved soil structure, which allows for minimal soil disturbance when drilling in spring.
He said: “Prior to spring beans, we found when we used a cover crop containing cereals, the cereal bound the soil together which meant we created less disturbance when we drilled into it, which meant we had less incidence of black-grass.”
But where growers were opting for alternative establishment methods, the value from cover crops in terms of weed control might be more moderate, according to Mr Barr.
“I think the more tillage you do, the less benefit you get from cover crops and the less tillage you do, the more you need cover crops.”
However, when the speakers were challenged about the possibility of using cover crops on heavy soil types, it became clear succeeding in these conditions was more difficult.
Although most of the discussion at the event hinted at the potential to smother out weeds, Niab weed biology specialist John Cussans proposed an alternative approach.
“One of the claims which has been made about some cover crops is they could, through allelopathy or chemical exudation, reduce the recruitment of the weed you are trying to control.
“But actually, if you are thinking about the cover crop as a trap crop, you want the maximum amount of weed seed recruiting into the cover crop before you destroy the crop itself,” said Mr Cussans.
Since evidence suggests greater ground cover reduces weed recruitment, if the target is to maximise germination, Mr Barr added sowing cover crops later might be helpful in encouraging weed growth.
Mr Barr said: “On heavy ground, cover crops in spring can be an issue. I find the more cover you have, the wetter the soil becomes.
“Some people say if you get the right cover crop, it will suck the water out, which it can, but not until later on when the crop starts growing.
“With disc drilling especially, I have found it is easy to smear the soil; I find you get much better success with a narrow tine.”
Whether used in summer or when preceding a spring crop, cover crops have proved successful in shading out emerging weeds on Mr Barr’s farm.
“Where I have had success is when I have been able to drill early and get a big thick cover crop.
“Last season, we planted beans at a particularly high seed rate on a bad black-grass field and they grew really well.
“Initially, the black-grass grew and tillered but as the canopy got bigger it got shaded out and died,” he added.
“After harvest, the only place you could find black-grass was on the tramlines.
“So is it a case of using cover crops or crop cover?”
During trials on his farm, where both wheat and oats were drilled in narrower rows, weed pressure was reduced in most instances, emphasising the need for a dense crop canopy.
Crop residue was identified as another means of covering black-grass on the soil surface, which could be provided by spreading straw from the back of the combine.
“We got some really good residue cover this year where we got masses of barley straw from a crop of hybrid barley, Volume.
“But it can be difficult getting that much residue without impacting on yield and spreading it evenly. Also, drilling the following crop into that residue is not always straightforward.”
To mimic this shading effect, Mr Barr implied there was a potential need to apply fertiliser to cover crops in order to boost canopy size.
“I do not apply fertiliser to my cover crops but some people do to help try and build that cover and maybe we may need to do this some years.
“But I think if we are going to do that, we have to put it in the row and just feed the cover crop and not the weeds,” said Mr Barr.