Which new compulsory catch crops are most suitable?

Researchers at Aarhus University are investigating and documenting the effect and efficiency of a wide range of potential compulsory catch crops in relation to the ability of the catch crops to reduce nitrogen leaching.

2018.01.10 | Janne Hansen

Mallow is one of the plant species that researchers from the Department of Agroecology at Aarhus University are investigating as a potential new catch crop. Photo: Hardy Plants, Wikipedia Commons

Researchers from Aarhus University are investigating potential new catch crops for their ability to reduce nitrogen leaching. Photo: Elly Møller Hansen

What will we be seeing as compulsory catch crops in Denmark in the future? Will the fields be filled with sunflowers, chicory, buckwheat or borage after the summer harvest and until the fields start to green with the next spring crop? Or will the catch crops be white mustard, grass or fodder radish?

Catch crops are one of the measures that farmers can use to reduce leaching of nitrogen from agricultural fields to the aquatic environment. The question is if new catch crops can be found that are more efficient at absorbing nitrogen from the soil – and that is what a research project at Aarhus University aims to elucidate.

Researchers from the Department of Agroecology at Aarhus University are presently investigating a series of new catch crops with regard to their nitrogen uptake efficiency. They are also looking at how well catch crops that are undersown can compete with the main crop – or if they cope too well and out-compete the main crop. 

Potential new catch crops     

The researchers began investigating potential new compulsory catch crops in the spring of 2017 at two different locations with different soil types. Selection of the catch crops to be tested took place following receipt of proposals from a wide range of stakeholders in the agricultural sector, including advisory services and seed companies.

In the screening trials the researchers compare sunflower, mallow, common corn-cockle, linseed, buckwheat, borage, black oat and common oat with three of the present catch crops approved for use as compulsory catch crops in Denmark, namely fodder radish, spring barley and mountain rye. These species were sown after a spring barley harvest.  

Catch crop dilemmas

The study also includes the legumes common bird’s foot, sainfoin, lentil and white clover in two grass mixtures and red clover in one grass mixture.  These mixtures were all undersown in spring barley in the spring.  

- There is a great demand for new compulsory catch crops. Present compulsory catch crops include cruciferous catch crops such as white mustard and fodder radish. The problem with these is that there is a risk that they will maintain or even propagate the serious fungal disease clubroot, explains Senior Researcher Elly Møller Hansen from the Department of Agroecology at Aarhus University. 

Clubroot can lie in wait in the soil as a dormant danger for years and prevent the farmer from growing rapeseed for many years. Another challenge among the present compulsory catch crops is with the legumes. 

- Legumes are in demand because they can bind nitrogen from the atmosphere and thus enrich the soil with nitrogen. However, the compulsory catch crops are first and foremost supposed to reduce nitrogen leaching – not act as green manure. If you get a strong legume catch crop, such as vetch, then you risk greater leaching afterwards than if the catch crop had not been a legume. This is because legumes will contribute additional organically bound nitrogen which can be released outside of the following crop’s growth season, says Elly Møller Hansen. 

A significant part of the studies will comprise investigations of how much nitrogen each of the new catch crops can absorb and to compare these results with how much the present compulsory catch crops absorb. Following this there will be a selection process. The best of the new catch crops must be able to cope with the main crop without out-competing it and they must absorb sufficient amounts of nitrogen. The most promising of the catch crops will be included in leaching studies in 2018. 

The Danish Agricultural Agency will in all likelihood set up a committee to deal with selecting the most promising new species for the leaching trials.   


For more information please contact:

Senior Researcher Elly Møller Hansen, Department of Agroecology, email: elly.m.hansen@agro.au.dk, telephone: +45 8715 7732


Sustainable Nutrient Management is one of the research areas in which the Department of Agroecology is particularly strong and from which results are delivered in line with national and global societal challenges and goals.

Plantekongres, Agro, DCA, Crops