Cover Crops

Quick Facts

Cover crops, catch crops or green manures are normally grown between successive production crops to provide ground cover, to capture soil nutrients and to improve soil characteristics or benefit the following crop.
There are a large number of species that are grown either on their own or as mixtures, many with different costs, characteristics and benefits.
The species selected can impact on the sustainability of crop rotations as cover crops are often of the same family as break crop s in the rotations.
Growing cover crops are increasingly supported measures in farmer support schemes (e.g GLAS and as a ‘greening’ measure).
The benefits to crop production that cover crops bring are frequently small and variable so growers must be careful in selecting them.

A range of cover crops types can be used either alone or as mixtures.

Grasses or cereals can be used. In a predominantly cereal rotation, these can carry over pests and diseases. They can also present weed challenges in the subsequent crop. They are generally less suitable for use with reduced tillage. They are a potential source of forage for livestock.
Brassicas are easily established and can be fast growing. They act as a good winter break when grown in cereal rotations. However as they are of the same plant family as oilseed rape and other brassicas, they can cause problems in rotations with these crops. If they grow tall, they can be challenging to plough without chopping.
Legumes have the potential to fix nitrogen and reduce fertiliser in the subsequent crop. They act as a good winter break in a cereal rotation. They may not prevent leaching as well as other crops. Legumes are expensive and can be difficult to establish.
Phacelia fits in well as it’s a different family to other crops. It is expensive and can be difficult to establish. Phacelia can supress weeds quite well and is easier to incorporate than brassicas.

Cover crops are not primarily grown for a market, however some of them (rape, turnips, grasses etc) can be used for winter grazing.
A market issue that must be considered by the selection of cover crop species type is whether it will restrict the growing of other crops within the rotation and impacting on the capacity to produce for markets for those crops.

The range of cover crops listed will grow with various degrees of biomass accumulation in Ireland rendering them broadly suitable for Ireland.
The suitability of individual species depends on the benefits being sought such as: reduced nutrient loss; reduced disease, pests and weeds, prevention of soil erosion, stabilising/improving organic matter, improving soil structure, N fixation and a potential forage source.

The break crop benefits are limited by the fact that cover crops are in the ground and growing for a relatively short time at a time of year when growth is poor.
While the concept of growing cover crops to improve soil organic matter levels is valid, the amount of change achieved is small.
Some of the cover crops can imact on soil aggregate stability, soil structure and soil water and temperature.
Weed suppression effects are related to crop type and growth rate.
Cover crops can reduce nitrate leaching on light textured soils prone to leaching.
The impact of non-legume cover crops on N sparing and yield in the subsequent crop is generally small and variable.

Research on cover crops in Ireland has been limited relative to the complexity of the impact that cover crops may have on the soil and crops. In particular the impact of long-term use of cover crops on soils of different texture and cropping histories is incomplete. Similarly the effect on organic matter fractions and its subsequent impact on productivity needs to be more clearly defined.
More precise information concerning the agronomy of these species is needed.

Select cover crop species and species mixtures carefully with regard to the functionality required (nutrient capture etc) and the impact they may have on crop rotation being practiced.
Generally sow as early as possible post harvest to ensure good early growth is achieved as relatively small changes in sowing date in August can impact hugely on production.
Ensure that the correct balance between cost effective establishment and appropriate seedbed conditions for the cover crops being used, is achieved.
Consider destruction carefully and ensure it is carried out in a timely fashion to avoid creating problems in subsequent soil cultivation.