Updating cover crop guidance: destruction approaches and soil health impacts (scoping review)

This desk-based project collated cover crop research, identified key knowledge gaps and suggested ways to address them.

The research used a rapid evidence assessment (REA), which is a structured and transparent way to collate, review and appraise academic and non-academic (grey) literature. REAs are considered more robust and reliable than traditional literature reviews but quicker and less costly than full systematic reviews.

The REA included research that was conducted in the UK or in temperate farming systems with similar characteristics to the UK. It considered winter-, spring- and summer-sown cover crops.

Searching and screening of the literature was conducted concurrently for both parts of the REA:

• Optimum destruction of cover crops (RR101a final project report)
• The long-term soil health impacts of using cover crops (RR101b final project report)

It resulted in 16,168 articles screened at title and abstract for inclusion.

Due to time and resource limitations, the process (coding) only used the abstracts of the included studies with no quality appraisal conducted. However, where evidence was published as a meta-analysis or systematic review, it was generally assumed that the weight of evidence was stronger than individual primary research studies.

Recommendations made by the authors of the included studies should be interpreted with care.

Optimum destruction (Part A)

Part A considered research on cover crop destruction methods, biomass breakdown after cover crop destruction and the risk of cover crops becoming a weed burden in the following crop.

A total of 70 articles were included for this REA.

Primary research was identified for the following destruction methods:

• Mechanical only (28 articles)
• Comparison of destruction methods (26 articles)
• Grazing only (eight articles)
• Chemical only (six articles)
• Frost-killed only (six articles)

25 articles were identified that assessed destruction timing of cover crops.

Four articles mentioned cover crops becoming a weed burden in the following crop.

Three articles mentioned what soil type the experiment was conducted on.

Only two primary research studies were found that were conducted in the UK:

• One study surveyed UK use of cover crops and which destruction method was used (81% of participants used herbicides to terminate cover crops)
• The other study demonstrated frost-sensitive cover crop species could not be reliably terminated under a temperate climate.

This REA identified that research with a focus on destruction methods may not be widely available in the public domain. Much of the research included in the REA had destruction method as part of the experiment and not the focus.

Key knowledge gaps identified included the need for:

• UK-specific research on destruction methods – including termination timing effects
• Information on the efficiency of destruction methods on different soil types
• Data on cover crops other than winter cover crops
• Research on the risk of cover crops becoming a weed burden in following crops (especially in the UK)

Table 7 in the final project report (101a) provides practical guidance for the various destruction methods (in isolation and in combination).

Soil health impacts (Part B)

Part B considered research on long-term soil health impacts of cover cropping, including cover crops grown multiple times in a rotation. It examined how cover crop species impact soil biodiversity and how various cover crop species or mixtures impact the rotation.

A total of 95 articles were included for this REA.

Primary research was identified for the following long-term soil health impacts:

• Soil biodiversity (including soil microbiome, fungi and earthworms, as well as general impacts on soil taxa)
• Soil structure and water regulation (including water retention, runoff and erosion and general soil structure impacts)
• Soil organic matter (including soil organic carbon and other soil organic matter findings)
• Nutrient retention (mainly reducing leaching)

Only four primary research studies were UK-based, which mainly focused on nitrogen leaching, earthworm diversity and soil physical properties.

This REA concluded that more long-term studies are required to measure most impacts of cover crops on soil-health indicators. These knowledge gaps in the UK may be filled by ongoing, long-term experiments at research institutions. However, studies that purely focus on cover cropping impacts, rather than rotational impacts of systems, are required to realise the impacts of cover cropping alone. This includes more research into rotational impacts of cover cropping. More research around cover cropping strategies beyond winter cover cropping is also required.

Table 9 in the final project report (101b) provides practical guidance associated with each soil health topic area.

Providing answers to your questions

This research project was funded via a levy-payer-led commissioning process