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New project to look at the effect of growing cover crops

Scientists from Aarhus University will look at ways to reduce nitrogen leaching under different fertiliser regimes and soil types. The results are important for the new regulatory measures for land use being introduced.

[Translate to English:] Olieræddike er en af de efterafgrøder, der skal undersøges i det nye projekt. Foto: Janne Hansen

How much nitrogen will be leached if a farmer grows one hectare of land with a ryegrass cover crop at a standard fertilisation level? And what happens if you choose a different type of cover crop or apply a higher rate of fertiliser?

This is what the new VIRKN project that involves Aarhus University and the Knowledge Centre for Agriculture will be studying. The project will look at the effects of cropping system at different fertilisation regimes and soil types. The aim is to generate new data that can be used when the proposal of the Commission of Nature and Agricultural for new and more site-specific nitrogen regulation is to be implemented.

- One of the basic concepts behind the new nitrogen regulation is that farmers may be allowed to apply more nitrogen if they grow cover crops or if they use other measures that reduce nitrogen leaching. But in order to calculate the exchange ratio between the measure used and the extra bag of nitrogen this will translate into, we need to know the consequences of applying more fertiliser. That's basically what we are looking at, says project manager and senior researcher at the Department of Agroecology at Aarhus University, Ingrid K. Thomsen.

Large need for data

In the existing regulation it is already possible to increase the farm’s nitrogen quota by increasing the area growing cover crops, but according to Ingrid K. Thomsen there is a large need to update the conversion factor between the measure used and the size of the nitrogen quota.

The current estimated impact of cover crops is mainly based on field trials with ryegrass conducted in the 1980s and 1990s. Since then new catch crops have come to the fore that have not been studied to the same detail. Furthermore, the reference has changed since new regulations have been introduced for when the soil may be tilled. This means that there will be greater effect of weeds and volunteer seeds, which was not the case previously when ploughing could take place at any time of the year.

- It’s important that the crops are tested under the same conditions – that means in the same field and under the same climatic conditions. This is where we are really short of data, says Ingrid K. Thomsen, adding that possible residual effects of cover crops also need to be re-examined. Experiments will therefore also be conducted on the residual effect of different types of autumn vegetation – including the stipulated cover crops.

Drones to be used

More specifically, data will be generated through a series of field trials at Aarhus University’s facilities at Foulum and Flakkebjerg. In all experiments, the scientists will be measuring yield and nitrogen leaching. Two types of study will be undertaken. In the first study, a winter crop will be established – with the usual and an early sowing date – and a spring barley followed by a catch crop or by volunteer cereal seeds. There will also be a reference plot where the soil will be left bare in the autumn. The effect of the various measures will be tested by applying six different levels of fertiliser – from zero to about twice the present norm. Yield and nitrogen uptake will be determined in the grain and straw at harvest, while leaching will be determined in the leaching periods.

In the second study, the scientists will be looking at the residual effect of different types of autumn vegetation. Here spring barley will be followed by a cover crop, volunteer grain or bare soil and will be compared with a winter crop at two sites. The idea is to investigate the importance of the residual effect of a cover crop for the following year’s yield.

The project will also look at whether there is an easier way to determine autumn nitrogen uptake rather than the traditional harvest of biomass. Associate Professor René Gislum from Aarhus University will thus be sending drones across approximately 100 fields to test whether they can be used to measure nitrogen uptake and on this basis estimate leaching from individual fields. This is done together with the Knowledge Centre for Agriculture who will select the fields. The project will in other words be seeking to incorporate new technology to determine the effect of autumn vegetation.

Models to be adjusted

The project will run until 2018 and, according to Ingrid K. Thomsen, the results will be important for the establishment of new regulatory methods and for adjusting the models that predict the effects of these measures on a national level.

- We use modellers in the project and we compare their calculations with our data so that we can see how reliable the models are, says Ingrid K. Thomsen, who also stresses that the new project is unlikely to give a definitive answer.

- It is important to remember that we are only studying this for a four-year period, and we can only generate a limited amount of data. We can only hope that we get some good, average winters.


Financed through the Green Development and Demonstration Programme under the Ministry of Food, Agriculture and Fisheries

Project manager: Ingrid K. Thomsen, Senior Researcher at Aarhus University

Other partners: Knowledge Centre for Agriculture.

The project runs for the period 2014-2018.

For further information please contact:

Ingrid K. Thomsen, Senior Researcher

Department of Agroecology, Aarhus University

Telephone: +45 8715 7763

E-mail: ingrid.thomsen@agrsci.dk