1. Effective weed control in agricultural crop fields increases yields, but simultaneously reduces floral resources for pollinators because many weed species provide pollen and nectar. Consequently, efforts to enhance crop yields on organic farms by using effective weed control methods risk compromising positive effects of organic farming on pollinating insects. Thus, it is important to find management strategies that alleviate the trade-off between crop yields and flowering weeds on organic farms.
2. We investigated the relationship between cereal yields, flowering weeds and bumblebees on organic and conventional arable land. We also investigated the potential of adjusting crop sowing density to benefit flowering weed species richness and floral resources to bumblebees without affecting crop yield.
3. Floral resources and species richness of flowering weeds were higher in organic compared to conventional fields and were negatively related to crop yield in organic but not conventional fields (where the variation of floral resources and flowering weed species richness was comparatively low). Bumblebee species richness was higher in organic compared to conventional fields, and abundance was twice as high in organic as in conventional fields, but not significantly so. Yields in organic fields were two thirds of those in conventional fields. When simultaneously testing the effect of farming type (organic vs. conventional), crop yield and floral resources, only floral resources were related significantly to bumblebee abundance and species richness. A lower sowing density of the crop increased floral resources without negatively affecting crop yield.
4. Synthesis and applications. We show that organic farming practices in cereals benefit bumblebees by allowing more flowering weeds, but at a cost in terms of lower yields. However, adjusting crop sowing density provides an opportunity to attain increased floral resources without negatively affecting crop yields. Thus, by increasing floral resources, adjusting crop sowing density may contribute to supporting high bumblebee densities, which in turn sustain pollination services to wild plants and insect-pollinated crops, such as oilseed rape and field beans, in agricultural landscapes. We suggest that sowing strategies have the potential to contribute to ecological intensification by supporting organisms that provide ecosystem services to agriculture.

Study design

We conducted the study on organic and conventional farms in southern Sweden. We surveyed two cereal field on each of ten organic and nine conventional farms. Both the organic and conventional cereal fields were predominantly sown with barley. To ensure similar landscape context between the two farming types, we selected both organic and conventional farms along a gradient of semi-natural grasslands in the surrounding landscape.

Data collection

We surveyed bumblebees and flowering weeds five times between May and August in 2017. Fields were surveyed along a 1 × 100m transect within the central parts of the fields. We estimated floral resources for pollinators based on the percentage of ground surface area that was covered by flower corollas (flower cover) in the transects: <2%, 2-6%, 6-10%, 10-20%, 20-25% or >25%. We also recorded the flowering weed species in the transect. We surveyed bumblebees by walking along the same transect for 10 minutes while recording individuals and species within one meter on each side of the transect. We also visited each field once in the last week of June to collect measures of realised crop sowing distances to investigate their association with flower cover, flowering weed species richness and crop yield. We surveyed crop sowing distances along a 100-meter-long transect (different from the flowering weed and bumblebee survey transect) placed randomly within the central parts of the fields. We placed a 0.5 × 0.5m frame above the crop plants at five points along each transect. Within the frame, we measured the distance between crop plants within the same row (sowing distance within rows) and between crop plants in different rows (sowing distance between rows). We obtained crop yield information for each field (estimated as kg/ha for the entire field) from farmers at the end of the season.