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Agroforestry-pigs with tree leaves on the menu

If farmers are to plant more trees in the landscape, new techniques must be developed so the trees also benefit the farmer’s economy. Danish research in mixed farming is therefore testing a number of trees' potentials.

Pigs in agroforesty. Photo: Uffe Bregendahl, Organic Denmark

One option is to turn the trees into fodder. This work is part of a large EU project called “MIXED” with focus on the development of efficient and resilient mixed farming and agroforestry systems. The MIXED project includes a total of 14 farmer networks in 10 countries. Together with the farmers in the networks the project undertakes research, develops practices and exchange learnings. Recently, on 30 May, one of the Danish networks met to discuss experiences and share knowledge and practices on pig production in agroforestry systems.

Pregnant organic sows are subject to blind tasting

The animals do not know that one out of three fodder subjects in their troughs is made from leaves and new sprouts from willow trees, either pure or mixed with grass-clover and then ensilaged. But in the taste test, the sows actually eat leaves that have been converted into silage.

"They liked both types of silage, and it was quite clear that they gladly ate it. And they preferred the one kind of pure willow silage, rather than the carrots that were just left. With another type of silage mixture consisting of willow and grass-clover, they had taken the carrots but left the silage. So it was quite clear that it tasted different and that there were different preferences,' says senior specialist Søren Ugilt Larsen from Danish Technological Institute, who has contributed to the production of the willow silage.

The sows preferred the silage of leaves and new sprouts from pure willow rather than the silage of willow and grass-clover

"We had no idea if the pigs would eat it. So for us it's great that they seem to really like it," says an enthusiastic Anne Grete Kongsted, Senior researcher at Aarhus University and partner in the MIXED project.

Images and video from the blind tasting of the sows were shown to the group of researchers and farmers, who had gathered in Hotel “Skarrildhus” close to Herning in Denmark, to discuss the future possibilities of trees in agriculture and to look at the effect of a series of trials with trees under the theme mixed farming and agroforestry.

The blind tasting for sows is a pilot test but tests will be undertaken under more controlled conditions later and forms part of a larger study of, among other things, the nutritional value of the silage.

Trees are not just in the way

The Danish study, presented at the field workshop, is one of several that aims at finding ways to motivate farmers to plant more trees on the farm. In modern agriculture, large fields with monoculture are often preferred, where trees are perceived as something that just gets in the way of the big tractors and farming equipment.

"We want to give producers inspiration to develop or further develop their farms in the direction of mixed farming or elements of mixed farming. Getting more trees on the farms in a system that makes sense: Which trees should farmers plant to achieve the effects we want to achieve in terms of carbon sequestration, biodiversity, animal welfare and so on," explains senior researcher from Aarhus University Anne Grete Kongsted.

She presented, among other things, the result of the blind tasting, but also studies of, for example, the feed value of the willow silage, which may eventually become a valuable feed supplement for the pig farmers.

"The trees are already in the paddocks, where they provide shade for the pigs, but at the same time they can "capture" soil nitrogen in the paddocks. Grass can do this too, but the trees are more resilient, because when the pigs root, they can completely destroy a pasture. We wanted to see if we could also get some fodder out of the trees when they are placed in the field anyway”, says Søren Ugilt Larsen, who for the occasion had bags with taste and smell samples of the leaf ensilage. One of those who tried the samples was organic pig producer Bertel Hestbjerg, who is also one of the farmers and practical partners in MIXED.

It is specifically his pigs that have been used for the ensilage trials. He has many years of experience with poplar trees at the farm 'Hestbjerg Økologi' and markets his pigs as 'Poplar pigs'. However, he has entered the feed project with leaves with a bit of skepticism.

"I thought: 'This is going to be a long shot.' But then it's good that there are some researchers who insist that we try it. The sows want to eat it, but I just don't have the imagination to see how we can harvest the willow leaves in a profitable way, get it ensilaged and fed to our pigs”, he says.

Many challenges

Søren Ugilt Larsen also recognizes that there is a long way to go before his willow ensilage becomes a natural part of the feed for the pigs.

"We have harvested the willow at different heights so that it can still provide some shade. Perhaps we can harvest so high that the pigs will not take the new shoots after the trees are harvested. But there are many challenges – for instance with regard to whether you can get the  machines in to the paddocks. It is something that needs to be developed, but creative people like Bertel Hestbjerg are fortunately good at finding solutions that works in practice”, he says.

With silage made from the leaves, a new development project could be relevant, which in time can make it even more profitable to plant trees on farms. There are still a number of challenges in harvesting the leaves and getting them processed and into the troughs of the sows in a cost-effective way.  

"Perhaps we could make some three metre high and one metre wide hedges, so that we can harvest on a giant area. That will most likely work, but then we need to invent new machines, and that can probably also be done. That's how it is, when practice and theory meet; some crooked ideas come up, of which one or two per thousand makes sense and can make a change for future practices. So it's great when it is a success”, says Bertel Hestbjerg.

Long-term agroforestry

“Mixed farming is a discipline that often requires the farmer to be patient and set long-term goals.  It is important to think about the function of the trees well into the future in the operation of the farm and to include the financial perspective”, says Martin Jensen, senior researcher at the Department of Food, Aarhus University.

Another participant at the field workshop - organic pig breeder Brian Holm from Brørup in Denmark, has attempted a long-term strategy.

At his farm, long rows of poplar and a few willow trees have been included as a central part of the farm. First of all, to give his 650 sows and their piglets the opportunity to find shade in the paddocks. But also because the trees collect carbon and increase biodiversity in the area.

The trees on his farm – long narrow strips of trees each with four rows of poplar – are also part of his plan for the farm's finances in the long run.

"We want to plant more trees, and we have to find out how to do it in an appropriate way, so that we can use the trees for different purposes and opportunities that occur over time. For example, we put the trees we plant these days in rows, so that we can also harvest them for wood chips - that is, for bioenergy. But who knows what we can further develop it into? We have experimented with mixing chips from the trees in the deep litter and spreading it back to the field to get the carbon back into the soil. It may take 25 or 30 years before it will benefit the crops. We don't know what we don't know, and we want to look at the possibilities”, he says.

The many long rows of trees already planted in the fields are pollarded and cut at some point when they reach a certain height.

"Then they shoot again, and we can use them for many years. Maybe we can cut them every five years, and perhaps we can do that five or up to eight times. Then a few generations have passed with the trees, and who knows what the world will look like at that time?” says Brian Holm.

More widespread abroad

There are many possibilities for implementing trees into farms. In many places abroad, trees have long been an essential part of agriculture in ways that we can perhaps learn from in Denmark.

In England and France there are examples of several types of field crops planted between long rows of fruit trees.

"In Portugal and Spain they have Iberian pigs that forage under the cork trees, and here they produce a high-value air-dried ham. It is of course difficult to transfer this kind of method directly to the more intensive Danish agricultural production, but we can get inspired by foreign countries, which have many years of experience with mixed farming - especially in Southern Europe”, says Anne Grete Kongsted.

Now time will tell whether the ‘Danish practice’ will gain attention outside the country's borders.


Pig production in agroforestry is one of the mixed farming systems that MIXED is investigating and co-develops with farmers.

Each of the 14 farmer networks in the MIXED project are organising a field workshop and learnings will be shared among all networks.  

Read more about the MIXED project: https://projects.au.dk/mixed/

Read more about the Danish network in MIXED: https://projects.au.dk/mixed/networks-national-teams/denmark

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 862357