Interannual variation in weed biomass on arable land in Sweden

Data were analysed on weed biomass from untreated plots in 2672 field experiments conducted in spring‐ and autumn‐sown cereal and oilseed crops in Sweden 1972–1992. The coefficient of variation (CV) among years in the biomass of annual weeds per square metre was 29% in autumn‐sown crops. In spring‐sown crops, which had less weed biomass, the corresponding value was 49%. The biomass of summer annuals varied most (105%). Low winter temperatures seemed to increase the biomass of weeds in autumn‐sown crops. In spring‐sown crops, weed biomass production was higher in years with high rainfall during late spring. Partial ordination (pCCA) of weed data indicated that the interannual variations in autumn‐sown crops were mainly manifested by the occurrence of summer annuals. Results from similar ordination of data from spring‐sown crops were partly explained by the occurrence of winter annuals.