A Guide to growing Cover Crops

Due to recent government legislation, a shift in farming practices and a greater concern from the public over the protection of the environment and soils, there has been a resurgence in the use of cover crops. Often referred to as ‘Green Manures’, records of cover crops date back over 2000 years ago (Pieters, 1927). Cover crops, by definition are, a crop 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 (Dabney et al., 2007). Cover crops can however come at a cost, and for all the benefits, there also many negatives, often at the expense of poor establishment and yield in the following crop. To help counter this, we have created a guide to help advise on what cover crops are best for each system and rotation to ensure cover crops can be a benefit rather than a negative.

What are the benefits and why should we grow them?

Benefits to growing cover crops have long been known, however an increase in research of cover crops has led to a greater knowledge in 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.

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. This is important where spring crops are going to be planted, reducing the amount of nitrogen being lost through the soil profile in the winter when heavy rainfall events take place. 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 compare to where no cover crop was used (Niab Tag, 2015). Although having limited data, we have seen similar results with stubble turnips capturing nitrogen and making this available in the following crop. On a couple of sites over 2 years, Turnips were established after harvest and grazed with sheep over the winter returning nutrients in an available form for the following crop. On the sites where stubble turnips were grown, we had an average SMN of 97kg/ha after grazing compared to overwinter stubble that had an available SMN of 64kg/ha, an increase in 33kg N/Ha. It is worth noting however these were not fully replicated trials. At todays Ammonium Nitrate price, 33kgN/ha would equate to £23.75/ha worth of fertiliser for the following crop.

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. As the price of nitrogen continues to go up, this may be a method of reducing 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 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.

Reducing soil erosion

The risk of soil erosion on many farms is increased where soil is left bare. It is estimated that the UK loses 2.9 million tonnes of top soil 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, soil and its nutrients will remain available to the following crop. Phosphate can also be fixed through the addition of buckwheat in a cover crop, however little is known about how available this is in the following crop.

Improving soil structure – One of the key benefits to cover cropping is through the improvement of soil structure and increasing microorganism activity.  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 structures such as Radishes are used, the soil structure assessment was as good as that of the spring cultivated ground (fig 2). 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. Alternatively, a long term ley with the inclusion of deep rooting species such as Chicory could be used, however correct management is essential.

Figure 2 The impact of different cover crops compared to cultivation on VESS (Visual Evaluation of Soil Structure) pre and post winter on a Kellogg’s OriginsTM site (Niab Tag, 2015)

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. The encouragement of earth worms is often found to be higher where radish and deep rooting brassicas are grown (Crotty & Stoate, 2019). Hoverflys 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.

Rotaional 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.

Things to be aware of:

Soil Fungi and Diseases

Take all carry over – If planting any cereal in a cover crop mixture there is likely there will be a take all carry over except from Oats. Volunteers cereals from the previous crop are also likely to host take all, so controlling volunteers before establishing the cover crop is essential.

Club Root – Club root is a soil borne fungus that occurs in all brassica species, although there are now some 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.

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 – 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 however the addition of any Brassicas in the rotation will possibly have a negative effect on these soil pathogens due to the release of Isothiocyanates (Pellerin et al, 2007).

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 in good conditions 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 reducing the amount 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 N 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.

Environmental and EFA options

EFA- With changes to the EFA requirements in 2018, many people saw cover crops and catch crops as a practical alternative compared to other options. Catch crops are crops grown in the space between two main crops, or at a time when no crops are being grown. For the EFA requirement catch crops must be sown by the 20th August and maintained until the 14th October. Cover crops under the EFA requirements must be sown by the 1st October and maintained until the 15th January. Under the EFA rules, catch crops and Cover crops have to be made up of one cereal crop (Rye, Barley or Oats) and one non-cereal crop (Phacelia, Vetch, Mustard, Radish or Lucerne). After these dates have passed they can be sprayed off or grazed and another crop established.

Country side Stewardship- If your farm is in the Mid or High tier Scheme: (SW6) Winter cover crop is an option – On ground that is prone to nitrogen leaching or where land drains run directly into water courses an over winter cover crop can be used. Species such as Rye, Barley, Vetch, Phacelia, Mustard, Radish and Rye Grass can be used. If this scheme is entered into the cover crop needs to be established by the 15th Sept and maintained until the end of Jan.

Water Companies – Recently, water companies have started to financially incentivise the use of cover crops to try to reduce the amount of Nitrogen leaching. At present this is only available in high risk areas but may get rolled out further. With the financial incentive from water companies covering the cost of the seed and potentially establishment cost it would be well worth taking up. This is also available to farms that are not in Environmental schemes at present and payment is often swifter.

Cover Crop Pros and Cons

Cover Crop Family Crop examples Pros Cons
Brassica Mustard, Tillage Radish, Stubble turnips Rapid growth, large root systems, good to reduced nitrate leaching, improving soil structure and reducing soil erosion.

Grazing options

Club root risk if OSR in rotation. Restricts Mycroorhizal fungal, Increase slugs pressure, large biomass, Some mustard and varieties of radish can bolt in winter (Important to prevent seed return)
Legumes White clover, Red Clover, Burseem Clover, Crimson Clover, Black Meddick Lucerne, Vetch Potential to fix N (if sown early), good at improving biodiversity. Predominantly prostate ground cover. Little management required Black Meddick 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.
Cereals 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
Grasses 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.
Boraginaceae Phacelia 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.
Polygnum Buckwheat Unrelated to most commercial crops, can potentially fix Phosphorous. Can be killed by frost Needs to be sown early, short lived and not very winter hardy. Very expensive so usually sown as a companion crop
Asteraceae Chicory Deep tap root may help alleviate compaction. It also has livestock worming characteristics. Only suitable in long 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

Cover Crop Seed Rate (kg/ha)

Approximate

 Seed Cost (£/kg)

Approximate

Total Cost (£/ha) at lower seed rate

 

Black Meddick 7-12 £11.00 £77
Buckwheat 50-60 £1.50 £75
Burseem Clover 10-15 £2.50 £25
Chicory 10-15 £12.0 £120
Crimson Clover 10-15 £2.50 £25
Lucerne 15-20 £9.00 £135
Mustard 10-15 £2.00 £20
Oats (Spring or Winter) 30-70 £0.70 £21
Oats (Black) 30-40 £2.00 £60
Phacelia 8-10 £5.75 £46
Radish 6-10 £2.15 £13
Forage Rye 30-70 £0.60 £18
Stubble Turnips 4-10 £2.50 £10
Vetch 50-75 £1.50 £75
Westwolds (Rye grass) 20-30 £2.00 £40
White Clover 10 £7.00 £70
Rye and Vetch 40-60 £0.90 £36
Oat and Phacelia 30-50 £1.30 £39
Oat and Mustard 30-50 £1.00 £30
*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. The mixed cover crops are suggested EFA cover crops and a guide price only. Where straights are suggested, this is as a single species only, seed rates and costs can be reduced if grown in mixtures.

Is it really worth it?

Overall its clear to see there can be many benefits to growing cover crops from improving soil structure, to creating over winter habitats for wildlife. However, with the cost of seed and establishment in many cases exceeding that of cash crops, it is often asked, “Is it worth it?”. It is very 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. 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 crops. Available: Clubroot management in crops. Last accessed 08/01/2020.

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.

Pellerin, S,. Mollier, A., Morel, C., Plenchette, C.. (2007). Effect of incorporation of Brassica napus L. residues in soils on mycorrhizal fungus colonisation of roots and phosphorus uptake by maize (Zea mays L.). European Journal of Agronomy. 26 (1), 113-120.

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. Last accessed 08/01/2020