The impact of changing the season in which cereals are sown on the diversity of the weed flora in rotational fields in Denmark.
The weed flora of the unsprayed field margins of 19 fields in arable crop rotations was studied in Denmark in 1988-92. The farmer determined the crop rotation and the crops grown included spring barley and winter wheat or winter barley. Plant and species densities and accumulated species richness were lower with winter than with spring cereals. Floristic similarity was greater in spring cereal fields and between spring and winter cereals within the same fields than among winter cereal fields. Species that occurred with unequal density in spring and winter cereal fields occurred at higher densities in spring cereal fields; these species germinated mainly in the spring. Plants that are important food resources for arthropod herbivores occurred at higher densities and abundances with spring than with winter cereals. Change in land use from spring to winter cereals involved an immediate reduction of >25% in the density of plants and species, and altered and made less predictable the composition of the weed flora.