Often referred to as ‘Green Manures’, records of cover crops date back over 2000 years and with the introduction of SFI (Sustainable Farming Incentive), cover crops have once again become a very topical subject.
Cover crops are grown as cover for the protection and enrichment of soil, but they often offer more than that with regards to nutrient capture, wildlife habitats, and moisture retention, as well as improvement in local water quality.
Cover crops can however have a negative impact on the establishment and yield in the following crop. To help counter this, we have created a guide to help advise on which cover crops are best for each system and rotation.
WHAT ARE THE BENEFITS AND WHY SHOULD WE GROW THEM?
The benefits to growing cover crops have long been known, however an increase in research has led to a greater knowledge of their benefits and which species should be selected for which scenario.
Best results have come from long term use as well as the incorporation of other organic manures such as FYM, digestate, compost or sludge. Ultimately cover crops need to fit a system, and therefore deciding what you want to achieve by growing a cover crop needs to be the first question.
Aim: Reducing Nitrogen Leaching and capturing nutrients
A number of studies have shown that nitrogen leaching can be reduced through the capturing of nitrogen from cover crops. Work carried out by Origins showed where cover crops were used, an additional 38 kg N/ha had been retained in the soil and cover crop (Niab Tag, 2015).
This is important where spring crops are going to be planted, reducing the amount of nitrogen lost through the soil profile in the winter when heavy rainfall events take place.
Wessex water undertook a trial in Dorset and showed that early drilled brassica cover crops can reduce leaching by up to 70kg/ha. Dorset is entering a period of extreme nitrogen leaching measures due to pollution in Poole Harbour. Cover crops are now becoming a way to significantly reduce leaching totals, but only if they are established very quickly after harvest.
- Expert Tip: The highest reduction in nitrogen leaching comes from a brassica based cover crop established before the 20th August.
Although there is limited data, we have seen similar results with stubble turnips. On a couple of sites over 2 years, Turnips were established after harvest and grazed with sheep over the winter. On the sites where stubble turnips were grown, there was an average SMN of 97kg/ha after grazing compared to overwinter stubble that had an available SMN of 64kg/ha, a difference of 33kg N/Ha (Figure 1). It is worth noting however these were not fully replicated trials.
Figure 1 CCC field trials from 2017 showing the spring SMN levels between grazed stubble turnips and bare overwinter stubble
The use of Legumes as a cover crop can help to build nitrogen in a rotation, through fixing nitrogen in the plants through their Rhizobium. The fixed nitrogen will then be released once the crop is destroyed, leaving available nitrogen for the following crop and reducing the reliance on bagged fertiliser.
These will need to be sown early in the autumn for the chance of fixing nitrogen, as most rhizomes are formed in the spring and summer under active growth rather than through the winter. If peas and beans are grown in the rotation this may not be possible due to the risk of fusarium foot rot. However, more work is going into this as the PGRO see positive yield results from a legume in a cover crop before spring peas. This could be because the legumes don’t fix nitrogen straight away in the lifecycle and rely upon soil nitrate. What we are unsure of currently is the potential long-term problem with root rot due to having too many legumes in the system.
Aim: Reducing soil erosion
The risk of soil erosion on many farms is increased where soil is left bare as a bare slope can be eroded up to 1,000 times faster than one covered in vegetation (Intergovernmental Technical Panel on Soils, 2015). It is estimated that the UK loses 2.9 million tonnes of topsoil each year through erosion and deposition into rivers (Wynn, 2016). The use of a cover crop can reduce this risk substantially.
Phosphate, Potassium and Magnesium are predominantly lost on farms in the winter through soil erosion rather than leaching. By preventing soil erosion with cover crops and good soil management, the soil and its nutrients will remain available to the following crop.
- Expert Tip: Phosphate availability can also be increased through the addition of buckwheat in a cover crop as they exude organic acids from their roots which releases the phosphate from phosphate/metal complexes (Possinger, A. et al, 2013).
Aim: Soil Organic Matter, Carbon and Biology
The soil holds 3 times as much carbon as the atmosphere, so there is basis for the idea of trying to store carbon in the soil. In the UK soil contains about 10 billion tonnes of carbon, roughly equal to 80 years of annual greenhouse gas emissions (Defra, 2009). Soil contains an abundance of life – in just 1 gram of soil there can be as many as 1 billion bacteria (Natural England, 2015). This works out to about 5 tonnes of soil organisms per hectare, or the equivalent of 100 sheep (European Commission, 2008).
Soil organisms such as earthworms play an important part in maintaining the structure and functioning of soils. The casts of earthworms act as channels allowing water and air to penetrate the ground. The encouragement of earth worms is often found to be higher where radish and deep rooting brassicas are grown (Crotty & Stoate, 2019). Earthworms also help to incorporate organic matter from the surface into the soil and redistribute nutrients. Results from a recent on-farm earthworm survey showed that most fields have some earthworms present, but 42% of fields may be overworked, as indicated by an absence or rarity of earthworms. Tillage had a negative impact on earthworm populations, and organic matter management did not mitigate tillage impacts (Stroud, J.L, 2019). Therefore, trying to use cover crops to “till” the ground rather than a cultivator will significantly improve earthworm and other soil organism numbers.
Aim: Improving soil structure
One of the key benefits to cover cropping is through the improvement of soil structure. Cover crops with a large, vigorous root system can help to open up soil and improve anaerobic conditions as well as soil structure. A study carried out by Niab tag found that where cover crops with a bulbous or fibrous rooting structure such as Radishes are used, the soil structure assessment was as good as that of the spring cultivated ground. If deep rooting cover crops are not necessary then the rye and vetch mixture was equally as effective, however, knowing where structure needs to be improved is key to selecting the right species. This may give farms the opportunity of cutting down cultivation and machinery costs if soil structure allows. If a level of compaction is severe or deep, then a subsoiler should not be overlooked. Anaerobic conditions and waterlogging caused from compaction are likely to be more damaging to the soil and the environment then the use of a subsoiler or inadequately alleviating this from poorly established cover crops. For example in anaerobic conditions Nitrous Oxide gas is produced which is 300x more polluting than Carbon Dioxide. Therefore, by having good soil structure it will reduce the chance that the soil is anaerobic. Alternatively, a long-term ley with the inclusion of deep rooting species such as Chicory and plantain could be used, however correct management is essential.
Aim: Habitat creation
Many cover crops can offer winter cover and habitat for insects, birds and small mammals. Not only do they offer small mammals and insects cover from predators, but they can also help to increase beneficial insects within the farm rotation. Beneficial predators such as spiders, rove beetles and ground beetles may all flourish in cover crops (Fox et al, 2016) however this often depends on the establishment methods. Hoverflies are often associated with smaller flowering plants such as black meddick, whilst forage legumes have been found to attract birds such as partridge, and bees. Not only do these cover crops offer a habitat for wild mammals and insects, destruction through grazing can offer valuable forage for livestock, whilst returning nutrients in an available form for the following crop.
Intermediate Soil Standard
We now know that if farmers want to apply for the intermediate soil standard, then 20% of the area applied for must have a multi species cover crop of at least 2 different species. The mix of species is up to the farmer, but the guidance does say a cereal or grass, legume, brassica or herb, which is actually a much more flexible set of requirements than we were expecting.
- Expert Tip: If you already growing forage crops like stubble turnips, you can use a legume like berseem clover or vetch as a cost effective way of achieving the guidelines. The rules do say you can graze the covers, but a level of cover must remain so it might mean doing 2 light grazes rather than one heavy one. Hybrid rape might be a better option than stubble turnips as the rape will regrow well after grazing.
ROTATIONAL PLANNING AND OTHER CONSIDERATIONS
Selecting the right cover crop is key to the success of the following crop. As a rule of thumb, avoid selecting a species closely related to the following crop. If peas or beans are likely to be planted, avoid a legume as a cover crop, if a brassica is going to be planted, avoid having a brassica in the cover crop mixture. If a spring cereal is likely to be planted, try to avoid having a cereal in the mixture.
Soil Fungi and Diseases to be Aware of:
Take-all carry over – If planting any cereal in a cover crop mixture it is likely there will be a take-all carry over except from Oats. Volunteer cereals from the previous crop are also likely to host take-all, so controlling volunteers before establishing the cover crop is essential.
An interesting paper found that a chemical (2-propenyl ITC) released from mustard (Cruciferous) roots helped reduce take-all by up to 40% preceding wheat (Couëdel, A. et al, 2019). Hopefully more research will show if this is significant. If so, it could work well as a catch crop between a 1st wheat and a 2nd wheat.
Club Root – Club root is a soil borne fungus that occurs in all brassica species, although there are now several clubroot tolerant Oilseed rape varieties. Symptoms often take hold after 6 weeks when soil temperatures are above 15°C (AHDB, n.d). Brassica cover crops sown in July and August are going to be a high risk, and if oilseed rape rotations are close then this will be potentially detrimental to the oilseed rape crop. Once the pathogen takes hold, it can survive in the soil for up to 15 years. Agricultural lime products, which are associated with a spike in pH and available calcium at drilling, can significantly reduce clubroot infection. High doses of lime (applied at 8 t/ha) can reduce clubroot severity by 25% (AHDB, n.d). Boron also has some activity against clubroot, so correct any deficiencies in the soil (AHDB, n.d.).
Fusarium foot rot – In the UK at present we have 3 species of Fusarium foot rot; fusarium spp., Didymella Pinodella and Aphanomyces Euteiches. These species can infect Clover, Lucerne, Peas, Field Beans as well as a variety of other legumes. The closer these are grown in the rotation, the more likely it is for infection to occur with heavy yield penalties. It is therefore best to avoid Legumes in cover crops if Peas or Beans play a significant role in your rotation.
Mycorrhizal Fungi (MF)– Mycorrhizal Fungi have become a bit of a buzz of late, helping to access nutrients in the soil previously unavailable to the plant. Crops like Buckwheat and Legumes will help to support Mycorrhizal Fungi. In contrast, the addition of any Brassicas in the rotation will possibly have a negative effect on MF as they are not hosts and therefore inoculum is reduced the longer a brassica is grown without a host species. This is one reason why companion crops are grown alongside OSR so that the MF have a host through the summer and Autumn, therefore reducing the time without a host (Ryan, M.H. 2001)
ESTABLISHMENT AND DESTRUCTION:
There are many ways to establish cover crops, from auto-casting through to conventional establishment. When using mixtures with different seed sizes it may be best to use different types of establishment methods.
Direct Drilling or broadcasting tends to be the cheapest form of establishment, but this is only worth doing if soil conditions are good before doing so. If there is soil compaction, the use of a deep tine may help to mineralise nitrogen and get increased rooting, improving soil structure through the winter and reduce the intensity of cultivations needed in the spring.
One thing many farms have struggled with is the excessive trash in the spring. Some drills are capable of drilling successfully through the trash, however there may still be an allelopathic effect from the trash and roots of the cover crop breaking down and releasing phytotoxins that inhibit seedling germination (Shinners et al, 1994).
The most effective way of removing the excess trash is to either graze with livestock, or to ensure the cover crop is sprayed off and topped early enough for the biomass to begin to break down. If farms have been cover-cropping for a long period of time or keep incorporating large biomasses and other forms of carbon, there may be a reduction in nitrogen mineralisation. The incorporation of plant biomass and therefore Carbon, can increase the Carbon:Nitrogen ration in the soil. If the carbon proportion becomes unbalanced, microbes decomposing the trash will utilise the nitrogen in the soil, reducing the amount of nitrogen available to the following crop (Jarvis and Woolford, 2017). By grazing cover crops or ensuring early destruction, the risk of increasing the C:N ratio into an unbalanced proportion will be reduced.
Cover crops have been used in Semi-arid areas for years to help preserve soil moisture and reduce soil temperatures in the summer months. One of the detriments of growing cover crops in the UK can be that cover crops preserve too much moisture and take longer to dry out and warm up compared to bare stubble in the spring. This needs to be taken into consideration on heavier ground and managed accordingly (i.e. destroying early or grazing early to allow the wind to dry the soil).
COVER CROP PROS AND CONS
Cover Crop Family
Mustard, Tillage Radish, Stubble turnips
Rapid growth, large root systems, good to reduced nitrate leaching, improving soil structure and reducing soil erosion.
Club root risk if OSR in rotation. Restricts Mycorrhizal fungi, Increase slugs pressure, large biomass, Some mustard and varieties of radish can bolt in winter (Important to prevent seed return)
White clover, Red Clover, Burseem Clover, Crimson Clover, Black Medick (Yellow Trefoil) Lucerne, Vetch
Potential to fix N (if sown early), good at improving biodiversity. Predominantly prostate ground cover. Little management required Black Medick is arguably the quickest to establish.
Slow to establish and develop. N unlikely to be fixed in autumn cover crops. Fusarium foot rot risk where peas and beans in rotation. Relatively expensive additions.
Rye, Barley, Oats
Quick easy establishment, offer good surface rooting and ground cover if sown early. Easy management. Oats are good nitrogen scavengers. Can often farm save seed (PVR protection rules apply)
Rye and Barley can host take all (Effecting following cereals). Better in mixtures rather than single species. Can host aphids and other diseases that could act as a vector to neighbouring crops
Rye Grasses and Westwolds, Fescues (undersown maize)
Similar to cereals, and good at improving surface soil structure. Offer good ground cover and reduced soil erosion. Vigorous if sown early. Westwolds may perform better in the later sowing slot. Good grazing options.
Can be an issue in the following crop. Won’t help deeper lying compaction. Can be relatively expensive. Seed return can be a major problem as it can lead to resistant ryegrass issues.
Unrelated to most commercial crops. Rapid establishment. Good flowering nectar source, easy to establish, frost susceptible so may get winter kill
Doesn’t like compaction, can be difficult to control in following crops if set seeds.
Unrelated to most commercial crops, can potentially fix Phosphorous. Can be killed by frost
Needs to be sown early, short lived and not winter hardy. Very expensive so usually sown as a companion crop and in mixtures
Deep tap root may help alleviate compaction. It also has livestock worming characteristics.
Suitable in short and medium term leys as a slow developer. Needs to be closely managed due to its excessive spring growth. Can be difficult to kill
SEED RATES AND GUIDE COSTS
Seed Rate (kg/ha)
Seed Cost (£/kg)
Total Cost (£/ha) at lower seed rate
Oats (Spring or Winter)
Westwolds (Rye grass)
*Savings could be made from FSS. All prices are a rough guide and not actual quotes. Please call crop advisors for actual prices. Prices will often vary on quantity ordered. Where straights are suggested, this is as a single species only, seed rates and costs can be reduced if grown in mixtures.
Overall, it is clear to see there can be many benefits to growing cover crops from improving soil structure to creating over winter habitats for wildlife. However, the cost of seed and establishment in many cases exceeds the cost of the benefits you are receiving.
It is easy to get caught up in multiple specie mixtures at great cost and for very little benefit, therefore the correct species need to be selected at the right price. A cement mixer will be a lot cheaper as you can mix it yourself.
While there is no denying the benefits cover crops can bring, ultimately it is the following cash crop that brings in the income. Ensuring the correct cover crop species are selected, the correct establishment methods are chosen, and the correct destruction of the cover crop will ultimately determine how profitable cover crops can really be for your system.
REFERENCES AND FURTHER READING:
AHDB. (n.d). Clubroot management in Oilseed Rape: Field Crops. Available: Clubroot management in crops. Last accessed 11/01/2023.
Crotty, F.V., & Stoate, C. (2019). The legacy of cover crops on the soil habitat and ecosystem services in a heavy clay, minimum tillage rotation. Food and Energy Security, 8
Dabney, S.M., Delgado, J.A., Reeves, D.W.. (2007). USING WINTER COVER CROPS TO IMPROVE SOIL AND WATER QUALITY. Communications in Soil Science and Plant Analysis., p1221-1250
Fox, A., Kim, T., Bahlai, C., Woltz, J., Gratton, C., Landis,D.. (2016). Cover crops have neutral effects on predator communities and biological control services in annual cellulosic bioenergy cropping systems. Agriculture, Ecosystems & Environment. 232 (1), 101-109.
Jarvis, P., Woolford A.. (2017). The contributions of organic additions on soil quality.
Niab Tag. (2015). Cover Crops A practical guide to soil and system improvement. Available: https://www.agricology.co.uk/sites/default/files/NIABTAG%20Cover%20Crops_lowres.pdf. Last accessed 08/01/2020.
Pieters, A.J. (1927). Green manuring principles and practices. John Wiley & Sons, Inc., New York, N.Y
Shinners, K.J., Nelson, W.S., Wang, R. (1994). Effects of residue-free band width on soil temperature and water conten. Transactions of the American Society of Agricultural Engineers. 37 (1), 39–49.
Wynn, S. (2016). Soil management – what can be done to secure a sustainable food supply chain?. Available: https://www.adas.uk/News/soil-management-what-can-be-done-to-secure-a-sustainable-food-supply-chain
Couëdel, A., Kirkegaard, K., Alletto, L., Justes, E., 2019. Crucifer-legume cover crop mixtures for biocontrol: Toward a new multi-service paradigm. Advances in Agronomy.
Ryan, M.H, 2001. The effect of Brassica crops on the level of mycorrhizal inoculum in soil. CSIRO Plant Industry, Canberra, ACT. ryan.pdf (agronomyaustraliaproceedings.org)
Possinger, Angela & Byrne, Loren & Breen, Nancy. (2013). Effect of buckwheat (Fagopyrum esculentum) on soil-phosphorus availability and organic acids. Journal of Plant Nutrition and Soil Science. 176
Intergovernmental Technical Panel on Soils (2015). Status of the world’s soil resources, FAO. http://www.fao.org/3/a-i5199e.pdf
Defra (2009). Safeguarding our soils. A strategy for England. https://www.gov.uk/government/publications/safeguarding-our-soils-a-strategy-for-england
3 Natural England (2015). Summary of evidence: soils. Access to Evidence Information Note EIN012. http://publications.naturalengland.org.uk/publication/6432069183864832
European Commission (2008). The soil is alive! Protecting soil biodiversity across Europe. http://ec.europa.eu/environment/archives/soil/pdf/handouts_bonn.pdf
Stroud, J.L, (2019). Soil health pilot study in England: outcomes from an on-farm earthworm survey. PLoS One 14:e0203909. https://doi.org/10.1371/journal.pone.0203909
Wessex Water Cover Crop Trials, 2018. Wessex Water Cover Crop Trials | Agricology