The cultivation of a single crop is common practice in agriculture today, i.e., crops such as wheat or corn are usually grown as pure cultures. However, as monocultures without versatile crop rotation, they are not always environmentally friendly. They use nutrients in a very one-sided way and are more susceptible to pests, which means that fertilisers are not used optimally and pesticides have to be applied regularly. This damages both groundwater and soil, leaving the soil less protected from weather extremes and erosion. Biodiversity also demonstrably suffers from the consequences of one-sided and usually intensive cultivation. Not only fields and meadows, but also forests are struggling with the consequences of monoculture. New forms of cultivation are needed to secure yields and protect the environment at the same time.
Genotypes tested for mixed cultivation
A promising alternative to this is so-called mixed cultivation. The cultivation of different plant species on one area was therefore the focus of the joint project IMPAC3. From 2015 to 2020, researchers from the Georg-August University of Göttingen, together with two plant breeding companies, Norddeutsche Pflanzenzucht Hans-Georg Lembke KG (NPZ) and Deutsche Saatveredelung AG (DSV), tested novel genotypes for mixed cultivation. The project received funding of around 3.5 million euros from the German Federal Ministry of Education and Research as part of the "Integrated Plant Breeding in Cultivation Systems - IPAS" funding program.
"It is well known that greater diversity can make better use of resources and increase yields. But which varieties and genotypes are best suited for mixed cropping not yet," explains Ulf Feuerstein, research manager for seed technology at DSV. The IMPAC3 project was about finding the ideal combination of mixing partners in terms of productivity and stability and integrating them into modern land use systems.
Legumes alongside wheat, grasses and poplars
Mixed cropping was studied at two sites near Göttingen - Reinshof and Deppoldshausen - for three different land use systems: Arable land, grassland and forest. Two crop types were cultivated at each site. "One of them was always a legume, because it can bind nitrogen from the air with the help of bacteria and release it to the neighboring plants," explains Feuerstein.
The joint cultivation of winter wheat and winter field bean was tested on the arable land, a combination of white clover, German ryegrass and forage chicory on the grassland, and the mixed cultivation of poplar and black locust on the forestry land. In parallel to the two large-scale field trials conducted by the Göttingen researchers, the NPZ tested mixed cropping on arable land and the DSV tested combined cropping on grassland in order to later compare the results of the large-scale trial. Factors such as root growth, nitrogen accumulation and soil moisture were measured using modern technologies, such as drones.
Higher yield, better nitrogen supply
The results of the field trial are promising: on the field, for example, the nitrogen supply to the wheat was increased by 40% compared with pure cultivation, thus reducing the need for fertilizer. In addition, "The yield from the mixed cultivation was up to 34% higher here." But it's not just the wheat that benefits from mixed cropping, says Ulf Feuerstein. "The wheat in turn serves as a support for the field bean. In mixed cultivation, the field bean gets better stability."
Co-cultivation of legumes and non-legumes was also quite successful on grassland and in the forest: while grasses such as the ryegrass used in the trial root shallowly, white clover roots deeper and could thus supply the neighboring grasses with nutrients over a longer period, especially during drought. In mixed cropping, chicory - a deep-rooter - proved particularly successful. "In the case of white clover, a genotype that grows particularly towards the light was decisive," Feuerstein reports. The "forest experiment" with poplar and the legume robinia also yielded new findings: "It is essential that the tree species planted together grow in a well-synchronized manner, otherwise one species will outgrow the other," says Feuerstein. Accordingly, the black locust should be planted three years after the poplar, for example, because it grows very quickly and casts too much shade on the poplar if sown at the same time.
Making mixed cropping attractive for farmers and foresters
The findings are important for plant breeding: "For mixed cultivation, genotypes must be selected that have a high degree of stability and harmonize with the growth of the other plant species," summarizes the DSV expert. In addition to saving fertilizers and improving yields, the project partners hope that joint cultivation will also add value to mixed cultivation, making it attractive for agriculture and forestry. "The acceptance of the mixture must be clarified to make mixed cultivation worthwhile, because the plants are harvested together," Feuerstein emphasizes. Mixed crops grown in meadows and pastures have long been used for forage. For arable crops, joint use has not yet been established.
Author: Beatrix Boldt
