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Nutritional benefits from over-winter cover crops are heavily influenced by destruction method, suggests new research. The advantages of over-winter covers are well-accepted – and... Cover crop benefits ‘depend on destruction method’

Nutritional benefits from over-winter cover crops are heavily influenced by destruction method, suggests new research.

The advantages of over-winter covers are well-accepted – and include soil health, structure, fertility, weed suppression and reductions in nutrient losses. Other benefits include biodiversity – and some financial return if grazed by livestock.

But questions remain over the best destruction method and the quantity of nitrogen captured and when it might become available to the following crop, according to research by ADAS and two water companies.

Without the answers, farms struggling to produce a profitable spring cash crop may miss out on the benefits, said ADAS principal research scientist Anne Bhogal at last month’s Association of Independent Crop Consultants (AICC) annual conference.

Working with Affinity Water and Portsmouth Water, the two-year project measured nitrogen release from cover crops during the 2021-22 cropping season and again between 2022 and 2023.

Nitrate leaching

On sites in Hertfordshire and West Sussex, researchers looked at the effect of cover crop type on over-winter nitrate leaching and assessed the amount of nitrogen captured by the cover crops.

The fate of captured nitrogen was then explored following two different destruction methods (chemical or mechanical), with soil mineral nitrogen (SMN) and spring crop performance measured post-destruction.

Ms Bhogal said that the benefits of cover crops to nitrate capture were shown at both sites in 2021-22, keeping concentrations in drainage water well below the EU Water Framework Directive limit of 50mg/litre, while a stubble did not.

Ground cover and species mix were factors in the amount of nitrate leaching, with a well-established cover (80-94% groundcover) and oil radish achieving the greatest reductions.

The spring nitrogen balance was calculated at cover crop destruction and on the deeper soils at the West Sussex site. It was estimated that up to 70kg/ha of nitrogen was in the cover crop before the spring barley cash crop was drilled.

“We wanted to understand how the different destruction techniques might impact how that nitrogen was released,” said Ms Bhogal.

Monitoring

After cover crops were destroyed, SMN measurements were taken at fortnightly intervals to track nitrogen release during the early part of spring.

Looking at the West Sussex site in isolation, there was an overall trend of slightly greater SMN after cover crops, as expected.

Perhaps more interesting was the observation of greater nitrogen availability where glyphosate was used to destroy the cover crop relative to mechanical destruction with a topper.

Weeds also started to appear in the spring barley crop following mechanical destruction, particularly where oil radish was in the mix as it regrew.

Grain yields were up to 1 t/ha higher following a winter cover crop relative to a weedy stubble, but only where it had been destroyed chemically.

Mechanical destruction of winter cover resulted in a 0.7-1t/ha yield reduction, the study found.

Spring oats

At the Hertfordshire site, no-till spring oats were grown instead of barley and yield was variable due to heavy blackgrass infestation, so results were not as reliable.

However, a similar overall pattern was observed, including more nitrogen uptake and a higher specific weight after cover crops were destroyed chemically.

“Specific weight is controlled by nitrogen to a certain extent, and particularly early nitrogen.

“We are hypothesising that where you chemically destroy cover crops, you have that early nitrogen supply, which is really important for subsequent crop development.”

Margin analysis was undertaken for each treatment. At West Sussex, any yield increase did not cover the additional cost of establishing and destroying the cover crop, which may put growers off.

But water company incentive schemes – and agri-environment scheme options like Countryside Stewardship SW6 or Sustainable Farming Incentive SAM2 – offer financial support for over-winter cover crops, making the practice economically viable.

Apex Agronomy agronomist and AICC member Chris Nottingham says such incentives and benefits mean cover cropping is here to stay – so long as challenges can be overcome, including pest and disease build-up and biomass and residual nutrients after destruction.

“The research is providing practical information on this and it’s nice to put some facts and figures on aspects like destruction methods, nutrient capture and release and financials,” said Mr Nottingham.

Future work

Glyphosate is clearly the easiest, simplest way for destruction in the short term, he adds, but farms are trialling alternatives.

“It’s more challenging without it and growers need to be aware of the potential downsides of non-chemical methods in the following crop.”

Regarding nitrogen supply, Mr Nottingham says he has seen plenty of anecdotal evidence of useful release to the following crop – but he urges caution when it comes to reducing rates too far.

“We are reducing inputs after cover crops, but the industry needs a better handle on exactly what’s there through soil testing and factoring in conditions post-destruction,” said Mr Nottingham.

The second part of the project, will take things further in 2023-24. This part of the project will look again at destruction method, further validate findings, and compare early and late crop destruction.

Water companies are keen for farmers to keep cover crops alive for longer, says Ms Bhogal.

“We will also have a full nitrogen and reduced rate programme, depending on the cover crop, and see if farmers can reduce nitrogen fertiliser without impacting yield.”