By growing green biomasses such as grass and clover grass instead of cereals, it is possible to increase the biomass yield in the field, while reducing nitrogen leaching and the use of pesticides.

At the same time, the green crops are interesting because of their high protein content. Pigs and poultry cannot digest grass in large quantities, but with the help of new biorefining technologies, it is possible to extract protein from green biomasses in a quality that allows the grass protein to replace imported soy protein in feed mixtures for pigs and poultry.

The press residue can be used for cattle feed, while the remaining product, juice, can be used in biogas production or used as a basis for the production of various high-value products. Thus, the circle is closed with the utilization of all product flows and the recycling of nutrients as fertilizer in the field.

However, what biotechnological and economic challenges stand in the way of creating value around the utilization of green biomass in Denmark? Researchers from six different departments at Aarhus University and from the Department of Food and Resource Economics at the University of Copenhagen have taken a closer look at this in a new DCA report commissioned by the Ministry of Food, Agriculture and Fisheries of Denmark.

Potential for large yields of biomass

The comprehensive report highlights a wide range of aspects, including the resource base for green biomass production, the environmental impacts of production and the quality of end-products.

Uffe Jørgensen, head of the Aarhus University Center for Circular Bioeconomy, says:

- When looking at the supply options and the quality of the green biomass, the cultivation of pure grass crops and grass-clover mixtures, which are grown on arable land, shows a great potential in relation to delivering large yields of biomass and protein with an appropriate amino acid profile. In the case of pure grass, the protein yield increases significantly with increased supply of nitrogen fertilizer without compromising the protein quality, but in grass-clover mixtures nitrogen fertilizer has less effect on the yield.

New initiatives to increase the production of protein in particular and to ensure the durability of grassland crops over the years are in the making, but the results are not yet ready.

- Grass from unfertilized, permanent grasslands may prove to be an option if there is a focus on the fibre part of the grass, but if the focus is on the protein part, this will require fertilization of permanent grass with nitrogen, which can negatively affect other environmental conditions, says Uffe Jørgensen.

The report also looks at the possibility of using the catch crops of, for example, grass-clover or pure grass, which farmers already cultivate, to supply the biorefining. This is a good opportunity, but is only economically realistic if the yields of the catch crops can be increased. The report describes various possibilities for this.

Perhaps the most important effects of an increased cultivation of grass and clover grass in agriculture are seen in relation to Danish agriculture's environmental and climate impact:

- There is clear evidence that switching from winter wheat or maize to either grass-clover or pure grass results in reduced nitrogen leaching and greenhouse gas emissions - taking into account changes in the soil carbon stock. Only at very high nitrogen fertilization for long-term grasslands these benefits may disappear or become less pronounced, says Uffe Jørgensen.

Protein concentrate for feed - and much more

In the report, the researchers estimate that – with current biorefining technologies – it is possible to extract 40 % of the protein found in the green biomass and turn it into a protein concentrate with a protein content of about 50 % of dry matter. This corresponds to the protein content of soy cakes.

Based on laboratory evaluations, the protein concentrate is expected to be able to replace the traditional protein sources for pigs and poultry:

- The potential has been confirmed in several animal experiments where soy has been replaced, either partially or completely, without adverse effects on animal performance. However, a high content of unsaturated fat in the green protein has an effect on the connective tissue in meat and fat and can have a limiting effect on the amount of soy protein that can be replaced, says Uffe Jørgensen.

When biorefining grass, a fibre fraction is also produced, typically containing 15-18 % protein in dry matter. This can be used as feed for ruminants, for bioenergy production , for further biorefining into chemical building blocks, or for biomaterials for food packaging, coffee to-go materials etc.

Major uncertainties for the first biorefineries

At present, the establishment of the first biorefineries for green biomass for feed and bioenergy at an industrial scale is taking place in Denmark, but biorefineries for green protein for food are waiting:

- Further research is needed to evaluate protein quality for direct human use. Although there are already promising results when it comes to food functionality and thus usability in food, more knowledge is needed as well as a validation from the EU confirming that protein from green biomass can be used directly for human nutrition, says Uffe Jørgensen.

He also states that currently major economic uncertainties are associated with the establishment and ongoing costs of a full-scale biorefinery as well as major challenges regarding efficient and environmentally friendly transport and logistics. In addition, it is important for the sustainability to ensure that the energy consumption in the plant mainly comes from renewable energy. Some of the energy can be produced as biogas from the juice or from the fibre fraction created by biorefining.