Weed vegetation (wild flora) of long established organic versus conventional cereal fields in Denmark


*E-mail: abh@dmu.dk


The vegetation of weeds was sampled in matched pairs of long established organic and contemporary conventional cereal fields in summer 1987, and in spring and summer 1988, with an extended Raunkiaer circle at distances up to 50 m from the field boundary. In spring 1988, the number of plants were also counted. Species density, plant density, a proxy measure of plant density (accumulated Frequency Sum), crop and weed biomass, and occurrence of plants assigned to functional groups, are compared between farming systems. The plants were assigned to functional groups according to: (i) herbivore associations to broad-leaved taxa and (ii) flower location in the canopy, i.e. visibility and availability to flying insects. Furthermore, crop margin and mid-field were compared concerning species density and accumulated Frequency Sum.

All variables that differed among the two farming systems had highest values - often several times higher - in the organic system with four exceptions: total biomass, biomass of crop, proportion of a single broad-leaved taxon and of grasses. The differences between the two systems were largest mid-field due to a pronounced pre-herbicide spraying gradient in species and plant density from crop margin to mid-field in conventional fields. A similar gradient was not found in the organic fields.