Supporting biodiversity by occasionally abstaining weed control?

How much does weed biomass increase if one abstains weed control in a single year, and would that increase involve weed species that may be beneficial to pollinating insects? To answer these questions, data on weed biomass were analysed from untreated and chemically treated plots in nearly 2000 field experiments conducted in spring- and autumn-sown cereal crops in Sweden. About 50% of the weed biomass was of the 30 species classified as 'pollinator friendly' based on the amounts of pollen and/or nectar produced. Increases were slightly larger in autumn- than spring-sown crops. The increase in biomass varied somewhat geographically and due to soil type, while type of crop seemed less important. Most important for the increase was the overall abundance of weeds in a field. We conclude that applying a single-year no-treatment strategy would be most suited in fields with relatively low weed abundance. Considering the low cost, potential acreage covered and assumed biodiversity benefit, such a strategy might be a preferable environmental mitigation method. To further evaluate this potential mitigation methods, data on biodiversity and pollinator visits to weed flowers within arable fields would be welcome, as well as potential side effects, like weed control challenges in following years.