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Cover crops: Pros, cons and agronomic considerations

Whether you’re a small family farm or a large conglomerate you have probably been recommended from one source or another to integrate cover crops into your cropping system. So what are cover crops? Why use them and what could be the potential benefits and pitfalls?

As the name suggests one of the main functions of cover crops is to cover the soil, preventing the soil from being eroded by the wind and rain. There is no one ‘cover’ crop, indeed anything with leafy vegetation which provides the benefits you are looking for can be considered a cover crop, this can range from radishes to black oats. This method prevents the field being left fallow in-between sowings of the cash crop. So, let’s look a little closer at cover crops and the pros and cons of having them on your land.

Benefits

Cover crops are primarily used to improve the quality of the soil. We’ve already heard that biomass over the soil protects it from erosion, but what goes on below the soil can be just as important. Plants with deep roots can break up the soil reducing compaction and improving drainage, which in turn reduces runoff, leaching and flooding. Overall, this can improve soil structure and local water quality. This can be especially important if a plough pan has formed, or if soil has had to be tilled while wet causing compaction. Below ground, a rye cover crop can produce a large mass of roots down to a depth of 60 cm depth. Phacelia can also explore more of the soil for a given root biomass compared to other cover crop species and is considered to be important for soil structural improvement.

Cover crops can also be used as a ‘green manure’, through the addition of fresh biomass and returning organic material to the soil, improving soil health and function. Cover crops can be particularly good at capturing nitrogen, especially if they are legumes like red clover, these cover crops are grown to ‘catch’ the nitrogen (N) in the soil and prevent nutrient losses via run-off and leaching. Depending upon species, cover crops can typically capture between 30-50 kg N/ha (although up to 90 kg N/ha is possible over the winter period). Areas of low rain or drought where crops have not been able to utilise the available nitrogen may especially benefit from this. Cover crops scavenge this remaining nitrogen and capture it for the following crop.


Often a mix of different cover crop seeds are grown simultaneously to provide a range of benefits and different depth of roots. One option for this is a Ley, one of the most popular combinations being lucerne or red clover and cutting grasses. These help with suppressing weeds as they outcompete them, and can be grown for silage.


When the cover crop has reached the end of its planned cycle, it is usually tilled into the soil to incorporate carbon and nutrients. If adopting a no till system, cover crops are mowed, grazed or glyphosate is used to terminate them, with the cash crop drilled directly into it. However, cover crops have also been used as a ‘living mulch’ growing alongside the cash crop at the same time. This provides permanent soil cover, suppresses weeds, provides living roots and nitrogen to the soil. Mulch’s are then knocked back instead of removed. However, there are concerns that the ‘mulch’ can compete with the cash crop, possibly harming yields.

Certain cover crops can also prevent pests and disease, by providing habitats for pest predators, as well as some brassica cover crops emitting high levels of glucosinolates, a chemical which can inhibit nematodes. Some crops can also ‘trap’ nematodes, by releasing chemicals from their roots that convince the pest there is a host (what they are looking for is a tuber, usually a potato) nearby. The nematodes hatch and find that there is no host available and cannot complete their life cycle, making it safer to plant your cash crop in the next crop rotation. These pest solutions that don’t use chemicals are becoming more valuable as nematicides are revoked.


Cover crops are often an important component of regenerative agriculture which returns nutrients to the soil and aims to reduce overheads and machinery costs. If you’d like to find out more about regenerative agriculture check out our regenerative agriculture blog, and for more about the benefits of cover crops take a look at NIAB’s AgriEcology discussion: Video: Cover crops, living mulch and leys (2020) | NIAB.

Disadvantage

Nothing is perfect so what are the disadvantages of cover crops? Cover crops are often planted after the harvest of your cash crop, right at the busiest time of the year when labour is already stretched. This may put more pressure on an already stressful time of year. The planting and disposal of these cash crops, although minimal, will have costs. Make sure to weigh up the benefits of improved soil health and increased nutrients to the financial costs of seed, planting and harvesting. If not properly disposed of, the cover crop can end up contaminating and competing with the cash crop, which would be counter productive to yields and perhaps profitability.


As well as deterring pests with trap crops, cover crops can also attract them. Wireworm, the larvae of click beetles, often thrive in fields that have had high residue grass cover crops and can decimate potato yields. Carefully planned rotations are important, and it is recommended that cover crops are terminated three weeks before the planting of cash crops to stop ‘green bridges’ for pests.

So cover crops, yay or nae?

Cover crops are a great way to improve your soil health and structure, following a non-intensive and ELM’s friendly method of making nature work for you. NIAB has undertaken long term field trials with cover crops with positive results:

The research has demonstrated benefits in terms of enhanced soil characteristics, positive yield responses and improvements in financial margins over fertiliser input associated with the use of specific cover crop approaches.” (Stobart and Morris, 2014).

However, context is important in all aspects of agriculture not least when deciding how to use cover crops. The type of soil you have will determine how much of a benefit you will see from cover crops; if soil is degraded it’s going to be a more dramatic result. Use the context of your soil and what rotations you have to decide what kind of cover crops you want to use, as each crop will have different properties and associated risks to the following rotation.


If you need more advice on using cover crops on your land, get in contact with our FFRF team to make that the focus of your consultation visit.

References and Further reading

AHDB., (N.D). Grass leys for the arable rotation. Grass leys for the arable rotation | AHDB 25/1/23



Balkcom, K., Schomberg, H., Reeves, W., Clark, A., Baumhardt, L., Collins, H., Delgado, J., Duiker, S., Kaspar, T. and Mitchell, J., 2007. Managing cover crops in conservation tillage systems. Managing cover crops profitably, 3, pp.44-61.Managing Cover Crops in Conservation Tillage Systems (usda.gov)


Bhogal, A., White, C. and Morris, N. (2020) Maxi Cover Crop: Maximising the benefits from cover crops through species selection and crop management. AHDB Project RD-21140009. Project Report No. 620. Maximising the benefits from cover crops through species selection and crop management (Maxi-Cover crop) | AHDB


Dabney, S.M., Delgado, J.A. and Reeves, D.W., 2001. Using winter cover crops to improve soil and water quality. Communications in Soil Science and Plant Analysis, 32(7-8), pp.1221-1250. Using Winter Cover Crops to Improve Soil and Water Quality (usda.gov)



Stobart, R.M. and Morris, N.L., 2014. The impact of cover crops on yield and soils in the New Farming Systems programme. Asp. Appl. Biol, 127, pp.223-232. NAC_33_STOBART.pdf (niab.com)


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