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AHDB funded review uncovers cover crops

Growers can access the scientific theory behind cover crops, thanks to the results of a comprehensive review funded by AHDB Cereals & Oilseeds.

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AHDB review uncovers cover crops #clubhectare #arable #covercrops

The nine-month review, by ADAS and NIAB TAG, provides the most comprehensive analysis of cover crops to date and acts as a practical reference source to aid with cover crop species selection and management, according to AHDB.


Ensuring cover crops are established early – with clear objectives in mind – and considering benefits over an appropriate time frame are among the review’s key recommendations.

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Review conclusions

1. Nitrogen fixation and release


  • Different legume species vary in their ability to fix N, ranging from 15 - 325 kg N/ha/yr
  • N fixation is positively correlated with the total biomass of the cover crop
  • N fixation between late summer and winter was generally between 30 and 100 kg N/ha/yr, but could be as much as 150 kg N/ha/yr
  • The most active reported soil temperature range for N fixation is between 7 degC and 20 degC, but the effects of temperature on N fixation varies between species and varieties
  • Mixtures of legumes and non-legumes can encourage greater N fixation and lower N leaching risk compared to a straight legume cover crop
  • Uptake of N by cover crops sown in late summer/autumn ranges from 30 to 120 kg N/ha/yr before spring
  • Depending on the species and cover crop biomass, destruction method and timing, 10 – 100 kg N/ha/yr can be expected to be released in the first year of cash cropping following the cover crop. However, in some cases there is a potential negative effect where cover crops such as rye deplete soil N
Weed suppression

2. Weed suppression


  • Cover crops suppress weeds by physical competition for which early emergence, high seedling vigour, rapid growth and early canopy closure increase competitiveness. However it is often difficult to separate physical competition from allelopathic effects in which chemicals released from cover crops suppress weed growth
  • A number of cover crops have been reported to have in-field allelopathic effects which suppress weed growth

See also: Where do cover crops fit in the weed control strategy?


  • The release of allelochemicals from cover crop residues is affected by plant age, vigour and environmental factors and the effectiveness of allelochemicals on weed suppression can be affected by soil texture, organic matter, temperature, light and microbial breakdown

3. Soil organic matter


  • The effect on cover crops on the total soil organic matter (SOM) content of a soil is variable and difficult to detect because effects take several years to accrue. Some studies have 73 reported increases in SOM or soil organic carbon (SOC) ranging from 0.3 per cent to 42 per cent relative to treatments without a cover crop, while other studies have reported no change in SOM or SOC
  • No study reported a decline in SOM

4. Environmental gain


  • The roots of cover crops create biopores in the soil, and can break up compacted soil layers, which can improve subsequent crop root growth
  • If sufficient canopy cover is achieved (30 per cent or more) over winter cover crops have been shown to decrease soil erosion and run off. This was one of the most consistent benefits from cover crops


Yield and gross margins

5. Yield and gross margins


  • Mean yield response of 0.36 t/ha from autumn cover crop use ahead of spring barley (five years of data). This gave an economic benefit of £43/ha in spring barley at N doses typical to farm standard (excluding cost of cover crop seed and establishment)
  • Typical margin responses of £50 – 75/ha have been recorded in winter wheat within the NFS study lead by NIAB (excluding cost of cover crop seed and establishment) from the use of legume and brassica cover crops
  • The most important agronomic factor for achieving benefits for cover crops is to establish early (late summer/early autumn)


  • All content adapted from AHDB Cereals and Oilseeds Research Review No. 90
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