Pollination is important for reproduction of wildflowers as well as production of insect-pollinated crops. A large proportion of global agricultural production is dependent on pollinating insects. However, there has been a documented decline in the number of wild pollinating insects in both Europe and North America due to habitat loss, pesticide use, parasites, climate change and invasive species. According to researchers from Aarhus University, however, Danish farmers will be able to improve the conditions for the pollinating insects in Denmark by increasing the diversity of plants in their grasslands.

“Traditional grasslands typically consist of few but high-yielding plant species, which with 4 or 5 annual cuts provide very little floral food for insects. Therefore, we have investigated whether it is possible to maintain high yields and at the same time provide flower resources for pollinating insects by changing the composition of the grassland and the frequency of cuts,” Professor Jørgen Eriksen from the Department of Agroecology at Aarhus University explains.

It has to be beneficial 

Previous research shows that increased botanical diversity in the grassland can provide a stable yield because the plants complement each other. The diversity can also help to reduce the occurrence of weeds.

In addition, it has been found that a mixture of plant species can not only be used as feed for ruminants, but can also be used to extract protein for animal feed for instance for pigs.

“A higher number of plant species in the grasslands will increase diversity and thus also the number of pollinating insects, but we cannot expect all farmers to change their fields just for the sake of biodiversity. There has to be benefits for the farmer, and it cannot affect production,” Jørgen Eriksen explains. 

And that is exactly what the researchers set out to study on three different sites with varying surrounding landscapes.

Different mixtures and cuts

Four different grassland mixtures with 3, 5, 11, and 13 species, respectively, were included in the field trials at the three sites. Each mixture was tested with three different frequencies of cutting. None, two, and four annual cuts were carried out in the fields while the researchers recorded yield, flowering, and pollinating insects throughout the season. 

“The mixtures with three and five different species had many flowers under the two-cut strategy, especially due to the lucerne, and at four cuts primarily due to white clover. Mixtures with 11 and 13 species resulted in diversity in flowering species across the season. This was especially true when using a two-cut strategy, and we did not see a reduction in dry matter yield. In fact, here we did not see a difference in the flowering between two cuts and no cuts,” Senior Reseacher Yoko L. Dupont from Department of Bioscience says.

Furthermore, the researchers studied the pollination profiles of the plants, i.e. which functional groups of insects are attracted to the flowers. They found that the pollinator profiles were highly dependent on plant species.

“The pollination profiles varied only slightly among the three different study sites. We observed that the legumes especially attracted the large bees, honey and bumble bees, while the most frequent herbs attracted mainly other groups, including hoverflies,” Yoko L. Dupont says.

Diversity does not reduce yield

The study shows that it is possible to increase plant diversity of grasslands to benefit the pollinating insects, without reducing the yield of the grasslands. 

“Our results show that herbs in a grass and legume mix with two annual cuts can contribute to a significant increase in flowers, which are important for a wide range of wild pollinators. The study's conclusions can relatively easily be fitted into a modern agricultural practice with 4-5 cuts per year by systematically leaving uncut strips of multi-species mixtures at each cut. They can then be harvested in the subsequent cut. This will help to provide a significant increase in the food source for the pollinating insects on the cultivated area, and it will only marginally reduce quality and yield,” Jørgen Eriksen explains.

In other words, it will be possible to use a combination of different grassland mixtures and cutting strategies in an agricultural landscape to maintain a stable and high yield as well as ensure an increased population of pollinating insects as a result of better botanical diversity.

Behind the research