Aarhus University is now entering the second round of a major field experiment aimed at determining whether farmers can harvest catch crops for biogas without increasing nitrate leaching. At the same time, the effect of early cereal harvest is being examined and is already showing promising results in the first trial year. The work is part of an Interreg project called Accel AgroBiogas.

Initial measurements from 2025 show that catch crops establish significantly better when cereals are harvested earlier than full maturity. “We can see that the catch crops simply have more time to establish when they get a head start. This results in more autumn biomass, which can potentially be utilised in biogas plants,” says senior advisor Søren Ugilt Larsen from the Department of Agroecology.

Leaching measurements underway

The key question, however, is whether harvesting the catch crops in autumn increases nitrate leaching. Soil water samples from the winter period have now been collected and are ready for analysis, with final results expected later in the year.

“If we can document that leaching does not increase with autumn harvesting, farmers will be able to use the biomass without jeopardising their nitrogen balance. That would be an important breakthrough,” says Søren Ugilt Larsen.

Biogas plants need new feedstocks

The biogas sector is currently facing a need for new, sustainable feedstocks as energy crops such as maize are phased out. Catch crops, which must be grown anyway for environmental reasons, could therefore become a key resource.

For this reason, the methane potential of straw, green crops and ensiled mixtures is also being investigated. “We are testing whether ensiling acts as a form of pretreatment that can increase gas yields,” says Søren Ugilt Larsen.

2026 trials strengthened with greenhouse gas measurements

The new Accel AgroBiogas trials in 2026 build on experiences from 2025 but feature a more robust design: two harvest timings, more types of catch crops, and straw/biomass harvesting on half of the plots.

The novelty in 2026 is that nitrous oxide (N₂O) emissions will also be measured in selected plots, in collaboration with the Climate Smart Research (CSR) project led by Professor Lars J. Munkholm at Aarhus University. This enables researchers to assess the overall climate impact of different cropping strategies.

Does early cereal harvest pay off?

Undersown catch crops may slightly reduce grain yields, but if the autumn biomass can be sold to biogas plants, the overall calculation may still be positive for farmers. The project is therefore developing a calculation tool to illustrate the economic consequences of different strategies based on real field data.

Results expected later in the year

Final results on leaching, methane potential and residual effects are expected later in 2026. Researchers hope the trials will provide much clearer guidance on how catch crops can play a larger role in the green transition of Danish agriculture.

More information

Read more about the Accel AgroBiogas project here or on the international website here.

Contact

Communications Advisor Camilla Brodam Galacho
Department of Agroecology, Aarhus University
Tel.: +45 9352 2136
Email: brodam@agro.au.dk