Green manures control weeds, boost biodiversity and improve crop yields

New study confirms the multiple benefits of cover crops such as ryegrass and clover - and they can boost the sustainability of non-organic farms

Grace Maher

Grace Maher

Cover crops have become increasingly popular in arable production, mostly due to the fact that it is supported under the GLAS scheme as a greening measure.

Cover crops, green manures or catch crops are the terms given to a wide range of crops that are generally grown between successive crops to capture and provide nutrients, provide ground cover, suppress weeds and improve soil conditions. With our temperate climate, a broad range of cover crops are suitable for conditions in Ireland. They have been a feature of organic production for many years and are gaining more attention in non-organic farming.

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Research on cover crops in Ireland has been limited, mainly because of the complexity of the impacts on both the soil and successive crops and the difficulty in monitoring them. So it is with interest that results emerging from the Maximising Organic Production Systems (MOPS) EIP-Agri project on green manures are being received.

MOPS is a European Innovation Partnership (EIP) project, co-funded by the Department of Agriculture and the European Commission

The research is being conducted on an organic farm outside New Ross, Co Wexford, owned by Desmond and Olive Thorpe, which has been certified by the Irish Organic Association for 32 years. The farm is mixed and includes field-scale vegetable production, cereals and livestock.

As part of the MOPS project, a three-year trial is being carried out to quantify the effects of different green manures on parameters such as soil organic matter content, soil nutrient content, weed control and beneficial insects.

Organic farmers use green manures for a variety of reasons, including: capturing nutrients; covering the soil to prevent soil erosion; controlling pests and weeds; improving soil aggregates and structure; forage supply; and as a source of organic nitrogen.

In addition, this trial is identifying how green manure crops can achieve particular effects for subsequent cash crops.

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Weed control

As organic farmers cannot use artificial herbicides, the management of weeds is an important aspect of production.

For the purpose of this trial there are four plots: ryegrass/phacelia, clover/ryegrass, phacelia/buckwheat and a control.

Each was measured for weed pressure in 2018, and the three green manures achieved significant reductions in annual weed populations, compared to the control plot.

This is a positive result especially in fields with a high weed burden, which can impact crop production.

Beneficial insects

With a growing focus on biodiversity and the urgent need to encourage species habitats on farm, knowing what is in your soil is an important starting point.

Measuring the density of invertebrates in the different trial plots was done by using pitfall traps, with ground beetles the main beneficial insects recorded - these feed on pest species such as slugs and insect larvae.

When analysed the green manure plots also displayed greater populations of beneficial soil bacteria, again important for long- term organic farming systems.

Cash crops

Following incorporation of the green manures, cabbages and onions were sown as cash crops and monitored for weeds, beneficial insects, crop quality, yield and harvest dates.

Overall crops performed well, and it is this aspect which is of particular interest to farmers, because one of the criticisms of green manures is the cost of seed and establishment. So conducting a cash-benefit analysis of using green manures both on the soil and subsequent cash crop is important.

While the initial results of this trial are promising, farmers certainly need more guidance on how best to improve their individual farms with the use of cover crops.

Choice of green manure crops is important as some are in the same plant families as the crop, so farmers - particularly organic farmers - must be mindful of this when planning rotations and green manures.

While grasses and cereals can be used, they also have the potential to carry over pests and diseases.

In non-organic farming the rising true costs of inputs, both economically and environmentally, are forcing farmers to look at alternatives - and green manures certainly are an option for many.

One of the attractive aspects of green manures is that they are multi-functional, which can justify the sowing costs.

If farmers are being asked to farm smarter then a crop that delivers several beneficial results is certainly worth investigating.

One of the problems when farmers convert to organic production is the limited body of research available on organic methods in an Irish context.

There is a need to expand the research sector in tandem with conversion and market demands. Farmers are coming under pressure to become more sustainable, but viable solutions need to be offered to enable this to become a reality.

For more information on the trial see

Grace Maher is development officer with the Irish Organic Association,

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